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@ -4769,9 +4769,10 @@ better suited to running multiple things in a sequence.
<h4 class='subsectionHead' id='tasklets'><span class='titlemark'>14.1 </span> <a id='x1-5400014.1'></a>Tasklets</h4>
<!-- l. 1765 --><p class='noindent'>Here is an example tasklet module. The
<code> <span class='ectt-1000'>tasklet_fn</span>
</code> function runs for a few seconds and in the mean time execution of the
</code> function runs for a few seconds. In the meantime, execution of the
<code> <span class='ectt-1000'>example_tasklet_init</span>
</code> function continues to the exit point.
</code> function may continue to the exit point, depending on whether it is interrupted by
<span class='ecbx-1000'>softirq</span>.
</p><!-- l. 1 --><p class='indent'>
</p>
<pre class='fancyvrb' id='fancyvrb77'><a id='x1-54004r1'></a><span class='ecrm-0500'>1</span><span id='textcolor2403'><span class='ectt-0800'>/*</span></span>
@ -4818,7 +4819,7 @@ better suited to running multiple things in a sequence.
<a id='x1-54086r42'></a><span class='ecrm-0500'>42</span>
<a id='x1-54088r43'></a><span class='ecrm-0500'>43</span><span class='ectt-0800'>MODULE_DESCRIPTION(</span><span id='textcolor2447'><span class='ectt-0800'>"Tasklet example"</span></span><span class='ectt-0800'>);</span>
<a id='x1-54090r44'></a><span class='ecrm-0500'>44</span><span class='ectt-0800'>MODULE_LICENSE(</span><span id='textcolor2448'><span class='ectt-0800'>"GPL"</span></span><span class='ectt-0800'>);</span></pre>
<!-- l. 1770 --><p class='indent'> So with this example loaded <code> <span class='ectt-1000'>dmesg</span>
<!-- l. 1771 --><p class='indent'> So with this example loaded <code> <span class='ectt-1000'>dmesg</span>
</code> should show:
@ -4830,23 +4831,23 @@ Example tasklet starts
Example tasklet init continues...
Example tasklet ends
</pre>
<!-- l. 1777 --><p class='nopar'>Although tasklet is easy to use, it comes with several defators, and developers are
<!-- l. 1778 --><p class='nopar'>Although tasklet is easy to use, it comes with several defators, and developers are
discussing about getting rid of tasklet in linux kernel. The tasklet callback
runs in atomic context, inside a software interrupt, meaning that it cannot
sleep or access user-space data, so not all work can be done in a tasklet
handler. Also, the kernel only allows one instance of any given tasklet to be
running at any given time; multiple different tasklet callbacks can run in
parallel.
</p><!-- l. 1782 --><p class='indent'> In recent kernels, tasklets can be replaced by workqueues, timers, or threaded
</p><!-- l. 1783 --><p class='indent'> In recent kernels, tasklets can be replaced by workqueues, timers, or threaded
interrupts.<span class='footnote-mark'><a href='#fn1x0' id='fn1x0-bk'><sup class='textsuperscript'>1</sup></a></span><a id='x1-54092f1'></a>
While the removal of tasklets remains a longer-term goal, the current kernel contains more
than a hundred uses of tasklets. Now developers are proceeding with the API changes and
the macro <code> <span class='ectt-1000'>DECLARE_TASKLET_OLD</span>
</code> exists for compatibility. For further information, see <a class='url' href='https://lwn.net/Articles/830964/'><span class='ectt-1000'>https://lwn.net/Articles/830964/</span></a>.
</p><!-- l. 1788 --><p class='noindent'>
</p><!-- l. 1789 --><p class='noindent'>
</p>
<h4 class='subsectionHead' id='work-queues'><span class='titlemark'>14.2 </span> <a id='x1-5500014.2'></a>Work queues</h4>
<!-- l. 1790 --><p class='noindent'>To add a task to the scheduler we can use a workqueue. The kernel then uses the
<!-- l. 1791 --><p class='noindent'>To add a task to the scheduler we can use a workqueue. The kernel then uses the
Completely Fair Scheduler (CFS) to execute work within the queue.
</p><!-- l. 1 --><p class='indent'>
</p>
@ -4883,36 +4884,36 @@ Completely Fair Scheduler (CFS) to execute work within the queue.
<a id='x1-55062r31'></a><span class='ecrm-0500'>31</span>
<a id='x1-55064r32'></a><span class='ecrm-0500'>32</span><span class='ectt-0800'>MODULE_LICENSE(</span><span id='textcolor2476'><span class='ectt-0800'>"GPL"</span></span><span class='ectt-0800'>);</span>
<a id='x1-55066r33'></a><span class='ecrm-0500'>33</span><span class='ectt-0800'>MODULE_DESCRIPTION(</span><span id='textcolor2477'><span class='ectt-0800'>"Workqueue example"</span></span><span class='ectt-0800'>);</span></pre>
<!-- l. 1795 --><p class='noindent'>
<!-- l. 1796 --><p class='noindent'>
</p>
<h3 class='sectionHead' id='interrupt-handlers'><span class='titlemark'>15 </span> <a id='x1-5600015'></a>Interrupt Handlers</h3>
<!-- l. 1797 --><p class='noindent'>
<!-- l. 1798 --><p class='noindent'>
</p>
<h4 class='subsectionHead' id='interrupt-handlers1'><span class='titlemark'>15.1 </span> <a id='x1-5700015.1'></a>Interrupt Handlers</h4>
<!-- l. 1799 --><p class='noindent'>Except for the last chapter, everything we did in the kernel so far we have done as a
<!-- l. 1800 --><p class='noindent'>Except for the last chapter, everything we did in the kernel so far we have done as a
response to a process asking for it, either by dealing with a special file, sending an
<code> <span class='ectt-1000'>ioctl()</span>
</code>, or issuing a system call. But the job of the kernel is not just to respond to process
requests. Another job, which is every bit as important, is to speak to the hardware
connected to the machine.
</p><!-- l. 1803 --><p class='indent'> There are two types of interaction between the CPU and the rest of the
</p><!-- l. 1804 --><p class='indent'> There are two types of interaction between the CPU and the rest of the
computer’s hardware. The first type is when the CPU gives orders to the hardware,
the order is when the hardware needs to tell the CPU something. The second, called
interrupts, is much harder to implement because it has to be dealt with when
convenient for the hardware, not the CPU. Hardware devices typically have a very
small amount of RAM, and if you do not read their information when available, it is
lost.
</p><!-- l. 1808 --><p class='indent'> Under Linux, hardware interrupts are called IRQ’s (Interrupt ReQuests). There
</p><!-- l. 1809 --><p class='indent'> Under Linux, hardware interrupts are called IRQ’s (Interrupt ReQuests). There
are two types of IRQ’s, short and long. A short IRQ is one which is expected to take
a very short period of time, during which the rest of the machine will be blocked and
no other interrupts will be handled. A long IRQ is one which can take longer, and
during which other interrupts may occur (but not interrupts from the same
device). If at all possible, it is better to declare an interrupt handler to be
long.
</p><!-- l. 1814 --><p class='indent'> When the CPU receives an interrupt, it stops whatever it is doing (unless it is
</p><!-- l. 1815 --><p class='indent'> When the CPU receives an interrupt, it stops whatever it is doing (unless it is
processing a more important interrupt, in which case it will deal with this one only
when the more important one is done), saves certain parameters on the stack and
calls the interrupt handler. This means that certain things are not allowed in the
@ -4924,10 +4925,10 @@ heavy work deferred from an interrupt handler. Historically, BH (Linux
naming for <span class='ecti-1000'>Bottom Halves</span>) statistically book-keeps the deferred functions.
<span class='ecbx-1000'>Softirq </span>and its higher level abstraction, <span class='ecbx-1000'>Tasklet</span>, replace BH since Linux
2.3.
</p><!-- l. 1824 --><p class='indent'> The way to implement this is to call
</p><!-- l. 1825 --><p class='indent'> The way to implement this is to call
<code> <span class='ectt-1000'>request_irq()</span>
</code> to get your interrupt handler called when the relevant IRQ is received.
</p><!-- l. 1826 --><p class='indent'> In practice IRQ handling can be a bit more complex. Hardware is often
</p><!-- l. 1827 --><p class='indent'> In practice IRQ handling can be a bit more complex. Hardware is often
designed in a way that chains two interrupt controllers, so that all the IRQs
from interrupt controller B are cascaded to a certain IRQ from interrupt
controller A. Of course, that requires that the kernel finds out which IRQ it
@ -4944,7 +4945,7 @@ need to solve another truckload of problems. It is not enough to know if a
certain IRQs has happened, it’s also important to know what CPU(s) it was
for. People still interested in more details, might want to refer to "APIC"
now.
</p><!-- l. 1835 --><p class='indent'> This function receives the IRQ number, the name of the function,
</p><!-- l. 1836 --><p class='indent'> This function receives the IRQ number, the name of the function,
flags, a name for <span class='obeylines-h'><span class='verb'><span class='ectt-1000'>/proc/interrupts</span></span></span> and a parameter to be passed to the
interrupt handler. Usually there is a certain number of IRQs available.
How many IRQs there are is hardware-dependent. The flags can include
@ -4954,16 +4955,16 @@ How many IRQs there are is hardware-dependent. The flags can include
<code> <span class='ectt-1000'>SA_INTERRUPT</span>
</code> to indicate this is a fast interrupt. This function will only succeed if there is not
already a handler on this IRQ, or if you are both willing to share.
</p><!-- l. 1841 --><p class='noindent'>
</p><!-- l. 1842 --><p class='noindent'>
</p>
<h4 class='subsectionHead' id='detecting-button-presses'><span class='titlemark'>15.2 </span> <a id='x1-5800015.2'></a>Detecting button presses</h4>
<!-- l. 1843 --><p class='noindent'>Many popular single board computers, such as Raspberry Pi or Beagleboards, have a
<!-- l. 1844 --><p class='noindent'>Many popular single board computers, such as Raspberry Pi or Beagleboards, have a
bunch of GPIO pins. Attaching buttons to those and then having a button press do
something is a classic case in which you might need to use interrupts, so that instead
of having the CPU waste time and battery power polling for a change in input state,
it is better for the input to trigger the CPU to then run a particular handling
function.
</p><!-- l. 1847 --><p class='indent'> Here is an example where buttons are connected to GPIO numbers 17 and 18 and
</p><!-- l. 1848 --><p class='indent'> Here is an example where buttons are connected to GPIO numbers 17 and 18 and
an LED is connected to GPIO 4. You can change those numbers to whatever is
appropriate for your board.
</p><!-- l. 1 --><p class='indent'>
@ -5112,14 +5113,14 @@ appropriate for your board.
<a id='x1-58284r142'></a><span class='ecrm-0500'>142</span>
<a id='x1-58286r143'></a><span class='ecrm-0500'>143</span><span class='ectt-0800'>MODULE_LICENSE(</span><span id='textcolor2584'><span class='ectt-0800'>"GPL"</span></span><span class='ectt-0800'>);</span>
<a id='x1-58288r144'></a><span class='ecrm-0500'>144</span><span class='ectt-0800'>MODULE_DESCRIPTION(</span><span id='textcolor2585'><span class='ectt-0800'>"Handle some GPIO interrupts"</span></span><span class='ectt-0800'>);</span></pre>
<!-- l. 1852 --><p class='noindent'>
<!-- l. 1853 --><p class='noindent'>
</p>
<h4 class='subsectionHead' id='bottom-half'><span class='titlemark'>15.3 </span> <a id='x1-5900015.3'></a>Bottom Half</h4>
<!-- l. 1854 --><p class='noindent'>Suppose you want to do a bunch of stuff inside of an interrupt routine. A common
<!-- l. 1855 --><p class='noindent'>Suppose you want to do a bunch of stuff inside of an interrupt routine. A common
way to do that without rendering the interrupt unavailable for a significant duration
is to combine it with a tasklet. This pushes the bulk of the work off into the
scheduler.
</p><!-- l. 1858 --><p class='indent'> The example below modifies the previous example to also run an additional task
</p><!-- l. 1859 --><p class='indent'> The example below modifies the previous example to also run an additional task
when an interrupt is triggered.
</p><!-- l. 1 --><p class='indent'>
@ -5293,19 +5294,19 @@ when an interrupt is triggered.
<a id='x1-59330r165'></a><span class='ecrm-0500'>165</span>
<a id='x1-59332r166'></a><span class='ecrm-0500'>166</span><span class='ectt-0800'>MODULE_LICENSE(</span><span id='textcolor2713'><span class='ectt-0800'>"GPL"</span></span><span class='ectt-0800'>);</span>
<a id='x1-59334r167'></a><span class='ecrm-0500'>167</span><span class='ectt-0800'>MODULE_DESCRIPTION(</span><span id='textcolor2714'><span class='ectt-0800'>"Interrupt with top and bottom half"</span></span><span class='ectt-0800'>);</span></pre>
<!-- l. 1862 --><p class='noindent'>
<!-- l. 1863 --><p class='noindent'>
</p>
<h3 class='sectionHead' id='crypto'><span class='titlemark'>16 </span> <a id='x1-6000016'></a>Crypto</h3>
<!-- l. 1864 --><p class='noindent'>At the dawn of the internet, everybody trusted everybody completely…but that did
<!-- l. 1865 --><p class='noindent'>At the dawn of the internet, everybody trusted everybody completely…but that did
not work out so well. When this guide was originally written, it was a more innocent
era in which almost nobody actually gave a damn about crypto - least of all kernel
developers. That is certainly no longer the case now. To handle crypto stuff, the
kernel has its own API enabling common methods of encryption, decryption and your
favourite hash functions.
</p><!-- l. 1869 --><p class='noindent'>
</p><!-- l. 1870 --><p class='noindent'>
</p>
<h4 class='subsectionHead' id='hash-functions'><span class='titlemark'>16.1 </span> <a id='x1-6100016.1'></a>Hash functions</h4>
<!-- l. 1872 --><p class='noindent'>Calculating and checking the hashes of things is a common operation. Here is a
<!-- l. 1873 --><p class='noindent'>Calculating and checking the hashes of things is a common operation. Here is a
demonstration of how to calculate a sha256 hash within a kernel module.
</p><!-- l. 1 --><p class='indent'>
</p>
@ -5373,20 +5374,20 @@ demonstration of how to calculate a sha256 hash within a kernel module.
<a id='x1-61124r62'></a><span class='ecrm-0500'>62</span>
<a id='x1-61126r63'></a><span class='ecrm-0500'>63</span><span class='ectt-0800'>MODULE_DESCRIPTION(</span><span id='textcolor2766'><span class='ectt-0800'>"sha256 hash test"</span></span><span class='ectt-0800'>);</span>
<a id='x1-61128r64'></a><span class='ecrm-0500'>64</span><span class='ectt-0800'>MODULE_LICENSE(</span><span id='textcolor2767'><span class='ectt-0800'>"GPL"</span></span><span class='ectt-0800'>);</span></pre>
<!-- l. 1877 --><p class='indent'> Install the module:
<!-- l. 1878 --><p class='indent'> Install the module:
</p><!-- l. 1 --><p class='indent'>
</p>
<pre class='fancyvrb' id='fancyvrb82'><a id='x1-61132r1'></a><span class='ecrm-0500'>1</span><span class='ectt-1000'>sudo insmod cryptosha256.ko</span>
<a id='x1-61134r2'></a><span class='ecrm-0500'>2</span><span class='ectt-1000'>sudo dmesg</span></pre>
<!-- l. 1884 --><p class='indent'> And you should see that the hash was calculated for the test string.
</p><!-- l. 1886 --><p class='indent'> Finally, remove the test module:
<!-- l. 1885 --><p class='indent'> And you should see that the hash was calculated for the test string.
</p><!-- l. 1887 --><p class='indent'> Finally, remove the test module:
</p><!-- l. 1 --><p class='indent'>
</p>
<pre class='fancyvrb' id='fancyvrb83'><a id='x1-61137r1'></a><span class='ecrm-0500'>1</span><span class='ectt-1000'>sudo rmmod cryptosha256</span></pre>
<!-- l. 1892 --><p class='noindent'>
<!-- l. 1893 --><p class='noindent'>
</p>
<h4 class='subsectionHead' id='symmetric-key-encryption'><span class='titlemark'>16.2 </span> <a id='x1-6200016.2'></a>Symmetric key encryption</h4>
<!-- l. 1894 --><p class='noindent'>Here is an example of symmetrically encrypting a string using the AES algorithm
<!-- l. 1895 --><p class='noindent'>Here is an example of symmetrically encrypting a string using the AES algorithm
and a password.
</p><!-- l. 1 --><p class='indent'>
@ -5591,10 +5592,10 @@ and a password.
<a id='x1-62392r196'></a><span class='ecrm-0500'>196</span>
<a id='x1-62394r197'></a><span class='ecrm-0500'>197</span><span class='ectt-0800'>MODULE_DESCRIPTION(</span><span id='textcolor2919'><span class='ectt-0800'>"Symmetric key encryption example"</span></span><span class='ectt-0800'>);</span>
<a id='x1-62396r198'></a><span class='ecrm-0500'>198</span><span class='ectt-0800'>MODULE_LICENSE(</span><span id='textcolor2920'><span class='ectt-0800'>"GPL"</span></span><span class='ectt-0800'>);</span></pre>
<!-- l. 1898 --><p class='noindent'>
<!-- l. 1899 --><p class='noindent'>
</p>
<h3 class='sectionHead' id='virtual-input-device-driver'><span class='titlemark'>17 </span> <a id='x1-6300017'></a>Virtual Input Device Driver</h3>
<!-- l. 1900 --><p class='noindent'>The input device driver is a module that provides a way to communicate
<!-- l. 1901 --><p class='noindent'>The input device driver is a module that provides a way to communicate
with the interaction device via the event. For example, the keyboard
can send the press or release event to tell the kernel what we want to
do. The input device driver will allocate a new input structure with
@ -5602,7 +5603,7 @@ do. The input device driver will allocate a new input structure with
</code> and sets up input bitfields, device id, version, etc. After that, registers it by calling
<code> <span class='ectt-1000'>input_register_device()</span>
</code>.
</p><!-- l. 1905 --><p class='indent'> Here is an example, vinput, It is an API to allow easy
</p><!-- l. 1906 --><p class='indent'> Here is an example, vinput, It is an API to allow easy
development of virtual input drivers. The drivers needs to export a
<code> <span class='ectt-1000'>vinput_device()</span>
</code> that contains the virtual device name and
@ -5618,7 +5619,7 @@ development of virtual input drivers. The drivers needs to export a
</li>
<li class='itemize'>the readback function: <code> <span class='ectt-1000'>read()</span>
</code></li></ul>
<!-- l. 1915 --><p class='indent'> Then using <code> <span class='ectt-1000'>vinput_register_device()</span>
<!-- l. 1916 --><p class='indent'> Then using <code> <span class='ectt-1000'>vinput_register_device()</span>
</code> and <code> <span class='ectt-1000'>vinput_unregister_device()</span>
</code> will add a new device to the list of support virtual input devices.
</p><!-- l. 1 --><p class='indent'>
@ -5627,7 +5628,7 @@ development of virtual input drivers. The drivers needs to export a
</p>
<pre class='fancyvrb' id='fancyvrb85'><a id='x1-63012r1'></a><span class='ecrm-0500'>1</span><span id='textcolor2921'><span class='ectt-0800'>int</span></span><span class='ectt-0800'> init(</span><span id='textcolor2922'><span class='ectt-0800'>struct</span></span><span class='ectt-0800'> vinput *);</span></pre>
<!-- l. 1921 --><p class='indent'> This function is passed a <code> <span id='textcolor2923'><span class='ectt-1000'>struct</span></span><span class='ectt-1000'> vinput</span>
<!-- l. 1922 --><p class='indent'> This function is passed a <code> <span id='textcolor2923'><span class='ectt-1000'>struct</span></span><span class='ectt-1000'> vinput</span>
</code> already initialized with an allocated <code> <span id='textcolor2924'><span class='ectt-1000'>struct</span></span><span class='ectt-1000'> input_dev</span>
</code>. The <code> <span class='ectt-1000'>init()</span>
</code> function is responsible for initializing the capabilities of the input device and register
@ -5635,20 +5636,20 @@ it.
</p><!-- l. 1 --><p class='indent'>
</p>
<pre class='fancyvrb' id='fancyvrb86'><a id='x1-63018r1'></a><span class='ecrm-0500'>1</span><span id='textcolor2925'><span class='ectt-0800'>int</span></span><span class='ectt-0800'> send(</span><span id='textcolor2926'><span class='ectt-0800'>struct</span></span><span class='ectt-0800'> vinput *, </span><span id='textcolor2927'><span class='ectt-0800'>char</span></span><span class='ectt-0800'> *, </span><span id='textcolor2928'><span class='ectt-0800'>int</span></span><span class='ectt-0800'>);</span></pre>
<!-- l. 1928 --><p class='indent'> This function will receive a user string to interpret and inject the event using the
<!-- l. 1929 --><p class='indent'> This function will receive a user string to interpret and inject the event using the
<code> <span class='ectt-1000'>input_report_XXXX</span>
</code> or <code> <span class='ectt-1000'>input_event</span>
</code> call. The string is already copied from user.
</p><!-- l. 1 --><p class='indent'>
</p>
<pre class='fancyvrb' id='fancyvrb87'><a id='x1-63023r1'></a><span class='ecrm-0500'>1</span><span id='textcolor2929'><span class='ectt-0800'>int</span></span><span class='ectt-0800'> read(</span><span id='textcolor2930'><span class='ectt-0800'>struct</span></span><span class='ectt-0800'> vinput *, </span><span id='textcolor2931'><span class='ectt-0800'>char</span></span><span class='ectt-0800'> *, </span><span id='textcolor2932'><span class='ectt-0800'>int</span></span><span class='ectt-0800'>);</span></pre>
<!-- l. 1935 --><p class='indent'> This function is used for debugging and should fill the buffer parameter with the
<!-- l. 1936 --><p class='indent'> This function is used for debugging and should fill the buffer parameter with the
last event sent in the virtual input device format. The buffer will then be copied to
user.
</p><!-- l. 1938 --><p class='indent'> vinput devices are created and destroyed using sysfs. And, event injection is done
</p><!-- l. 1939 --><p class='indent'> vinput devices are created and destroyed using sysfs. And, event injection is done
through a <span class='obeylines-h'><span class='verb'><span class='ectt-1000'>/dev</span></span></span> node. The device name will be used by the userland to export a new
virtual input device.
</p><!-- l. 1942 --><p class='indent'> The <code> <span class='ectt-1000'>class_attribute</span>
</p><!-- l. 1943 --><p class='indent'> The <code> <span class='ectt-1000'>class_attribute</span>
</code> structure is similar to other attribute types we talked about in section <a href='#sysfs-interacting-with-your-module'>8<!-- tex4ht:ref: sec:sysfs --></a>:
</p><!-- l. 1 --><p class='indent'>
</p>
@ -5659,7 +5660,7 @@ virtual input device.
<a id='x1-63041r5'></a><span class='ecrm-0500'>5</span><span class='ectt-0800'>    </span><span id='textcolor2939'><span class='ectt-0800'>ssize_t</span></span><span class='ectt-0800'> (*store)(</span><span id='textcolor2940'><span class='ectt-0800'>struct</span></span><span class='ectt-0800'> class *class, </span><span id='textcolor2941'><span class='ectt-0800'>struct</span></span><span class='ectt-0800'> class_attribute *attr,</span>
<a id='x1-63043r6'></a><span class='ecrm-0500'>6</span><span class='ectt-0800'>                    </span><span id='textcolor2942'><span class='ectt-0800'>const</span></span><span class='ectt-0800'> </span><span id='textcolor2943'><span class='ectt-0800'>char</span></span><span class='ectt-0800'> *buf, </span><span id='textcolor2944'><span class='ectt-0800'>size_t</span></span><span class='ectt-0800'> count);</span>
<a id='x1-63045r7'></a><span class='ecrm-0500'>7</span><span class='ectt-0800'>};</span></pre>
<!-- l. 1954 --><p class='indent'> In <span class='obeylines-h'><span class='verb'><span class='ectt-1000'>vinput.c</span></span></span>, the macro <code> <span class='ectt-1000'>CLASS_ATTR_WO(export/unexport)</span>
<!-- l. 1955 --><p class='indent'> In <span class='obeylines-h'><span class='verb'><span class='ectt-1000'>vinput.c</span></span></span>, the macro <code> <span class='ectt-1000'>CLASS_ATTR_WO(export/unexport)</span>
</code> defined in <a href='https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/include/linux/device.h'>include/linux/device.h</a> (in this case, <span class='obeylines-h'><span class='verb'><span class='ectt-1000'>device.h</span></span></span> is included in <a href='https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/include/linux/input.h'>include/linux/input.h</a>)
will generate the <code> <span class='ectt-1000'>class_attribute</span>
</code> structures which are named <span class='obeylines-h'><span class='verb'><span class='ectt-1000'>class_attr_export/unexport</span></span></span>. Then, put them into
@ -5669,14 +5670,14 @@ will generate the <code> <span class='ectt-1000'>class_attribute</span>
</code> that should be assigned in <code> <span class='ectt-1000'>vinput_class</span>
</code>. Finally, call <code> <span class='ectt-1000'>class_register(&amp;vinput_class)</span>
</code> to create attributes in sysfs.
</p><!-- l. 1958 --><p class='indent'> To create a <span class='obeylines-h'><span class='verb'><span class='ectt-1000'>vinputX</span></span></span> sysfs entry and <span class='obeylines-h'><span class='verb'><span class='ectt-1000'>/dev</span></span></span> node.
</p><!-- l. 1959 --><p class='indent'> To create a <span class='obeylines-h'><span class='verb'><span class='ectt-1000'>vinputX</span></span></span> sysfs entry and <span class='obeylines-h'><span class='verb'><span class='ectt-1000'>/dev</span></span></span> node.
</p><!-- l. 1 --><p class='indent'>
</p>
<pre class='fancyvrb' id='fancyvrb89'><a id='x1-63055r1'></a><span class='ecrm-0500'>1</span><span class='ectt-1000'>echo </span><span id='textcolor2946'><span class='ectt-1000'>"vkbd"</span></span><span class='ectt-1000'> | sudo tee /sys/class/vinput/export</span></pre>
<!-- l. 1964 --><p class='indent'> To unexport the device, just echo its id in unexport:
<!-- l. 1965 --><p class='indent'> To unexport the device, just echo its id in unexport:
</p><!-- l. 1 --><p class='indent'>
</p>
<pre class='fancyvrb' id='fancyvrb90'><a id='x1-63058r1'></a><span class='ecrm-0500'>1</span><span class='ectt-1000'>echo </span><span id='textcolor2947'><span class='ectt-1000'>"0"</span></span><span class='ectt-1000'> | sudo tee /sys/class/vinput/unexport</span></pre>
@ -6137,7 +6138,7 @@ will generate the <code> <span class='ectt-1000'>class_attribute</span>
<a id='x1-63956r400'></a><span class='ecrm-0500'>400</span>
<a id='x1-63958r401'></a><span class='ecrm-0500'>401</span><span class='ectt-0800'>MODULE_LICENSE(</span><span id='textcolor3242'><span class='ectt-0800'>"GPL"</span></span><span class='ectt-0800'>);</span>
<a id='x1-63960r402'></a><span class='ecrm-0500'>402</span><span class='ectt-0800'>MODULE_DESCRIPTION(</span><span id='textcolor3243'><span class='ectt-0800'>"Emulate input events"</span></span><span class='ectt-0800'>);</span></pre>
<!-- l. 1973 --><p class='indent'> Here the virtual keyboard is one of example to use vinput. It supports all
<!-- l. 1974 --><p class='indent'> Here the virtual keyboard is one of example to use vinput. It supports all
<code> <span class='ectt-1000'>KEY_MAX</span>
</code> keycodes. The injection format is the <code> <span class='ectt-1000'>KEY_CODE</span>
</code> such as defined in <a href='https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/include/linux/input.h'>include/linux/input.h</a>. A positive value means
@ -6145,12 +6146,12 @@ will generate the <code> <span class='ectt-1000'>class_attribute</span>
</code> while a negative value is a <code> <span class='ectt-1000'>KEY_RELEASE</span>
</code>. The keyboard supports repetition when the key stays pressed for too long. The
following demonstrates how simulation work.
</p><!-- l. 1980 --><p class='indent'> Simulate a key press on "g" (<code> <span class='ectt-1000'>KEY_G</span>
</p><!-- l. 1981 --><p class='indent'> Simulate a key press on "g" (<code> <span class='ectt-1000'>KEY_G</span>
</code> = 34):
</p><!-- l. 1 --><p class='indent'>
</p>
<pre class='fancyvrb' id='fancyvrb93'><a id='x1-63968r1'></a><span class='ecrm-0500'>1</span><span class='ectt-1000'>echo </span><span id='textcolor3244'><span class='ectt-1000'>"+34"</span></span><span class='ectt-1000'> | sudo tee /dev/vinput0</span></pre>
<!-- l. 1986 --><p class='indent'> Simulate a key release on "g" (<code> <span class='ectt-1000'>KEY_G</span>
<!-- l. 1987 --><p class='indent'> Simulate a key release on "g" (<code> <span class='ectt-1000'>KEY_G</span>
</code> = 34):
</p><!-- l. 1 --><p class='indent'>
</p>
@ -6268,13 +6269,13 @@ following demonstrates how simulation work.
<a id='x1-64188r108'></a><span class='ecrm-0500'>108</span>
<a id='x1-64190r109'></a><span class='ecrm-0500'>109</span><span class='ectt-0800'>MODULE_LICENSE(</span><span id='textcolor3327'><span class='ectt-0800'>"GPL"</span></span><span class='ectt-0800'>);</span>
<a id='x1-64192r110'></a><span class='ecrm-0500'>110</span><span class='ectt-0800'>MODULE_DESCRIPTION(</span><span id='textcolor3328'><span class='ectt-0800'>"Emulate keyboard input events through /dev/vinput"</span></span><span class='ectt-0800'>);</span></pre>
<!-- l. 1996 --><p class='noindent'>
<!-- l. 1997 --><p class='noindent'>
</p>
<h3 class='sectionHead' id='standardizing-the-interfaces-the-device-model'><span class='titlemark'>18 </span> <a id='x1-6500018'></a>Standardizing the interfaces: The Device Model</h3>
<!-- l. 1998 --><p class='noindent'>Up to this point we have seen all kinds of modules doing all kinds of things, but there
<!-- l. 1999 --><p class='noindent'>Up to this point we have seen all kinds of modules doing all kinds of things, but there
was no consistency in their interfaces with the rest of the kernel. To impose some
consistency such that there is at minimum a standardized way to start, suspend and
resume a device a device model was added. An example is shown below, and you can
@ -6381,13 +6382,13 @@ functions.
<a id='x1-65194r97'></a><span class='ecrm-0500'>97</span>
<a id='x1-65196r98'></a><span class='ecrm-0500'>98</span><span class='ectt-0800'>MODULE_LICENSE(</span><span id='textcolor3403'><span class='ectt-0800'>"GPL"</span></span><span class='ectt-0800'>);</span>
<a id='x1-65198r99'></a><span class='ecrm-0500'>99</span><span class='ectt-0800'>MODULE_DESCRIPTION(</span><span id='textcolor3404'><span class='ectt-0800'>"Linux Device Model example"</span></span><span class='ectt-0800'>);</span></pre>
<!-- l. 2004 --><p class='noindent'>
<!-- l. 2005 --><p class='noindent'>
</p>
<h3 class='sectionHead' id='optimizations'><span class='titlemark'>19 </span> <a id='x1-6600019'></a>Optimizations</h3>
<!-- l. 2006 --><p class='noindent'>
<!-- l. 2007 --><p class='noindent'>
</p>
<h4 class='subsectionHead' id='likely-and-unlikely-conditions'><span class='titlemark'>19.1 </span> <a id='x1-6700019.1'></a>Likely and Unlikely conditions</h4>
<!-- l. 2008 --><p class='noindent'>Sometimes you might want your code to run as quickly as possible,
<!-- l. 2009 --><p class='noindent'>Sometimes you might want your code to run as quickly as possible,
especially if it is handling an interrupt or doing something which might
cause noticeable latency. If your code contains boolean conditions and if
you know that the conditions are almost always likely to evaluate as either
@ -6406,7 +6407,7 @@ to succeed.
<a id='x1-67018r4'></a><span class='ecrm-0500'>4</span><span class='ectt-0800'>    bio = NULL;</span>
<a id='x1-67020r5'></a><span class='ecrm-0500'>5</span><span class='ectt-0800'>    </span><span id='textcolor3406'><span class='ectt-0800'>goto</span></span><span class='ectt-0800'> out;</span>
<a id='x1-67022r6'></a><span class='ecrm-0500'>6</span><span class='ectt-0800'>}</span></pre>
<!-- l. 2022 --><p class='indent'> When the <code> <span class='ectt-1000'>unlikely</span>
<!-- l. 2023 --><p class='indent'> When the <code> <span class='ectt-1000'>unlikely</span>
</code> macro is used, the compiler alters its machine instruction output, so that it
continues along the false branch and only jumps if the condition is true. That
avoids flushing the processor pipeline. The opposite happens if you use the
@ -6415,34 +6416,34 @@ avoids flushing the processor pipeline. The opposite happens if you use the
</p><!-- l. 2026 --><p class='noindent'>
</p><!-- l. 2027 --><p class='noindent'>
</p>
<h3 class='sectionHead' id='common-pitfalls'><span class='titlemark'>20 </span> <a id='x1-6800020'></a>Common Pitfalls</h3>
<!-- l. 2029 --><p class='noindent'>
<!-- l. 2030 --><p class='noindent'>
</p>
<h4 class='subsectionHead' id='using-standard-libraries'><span class='titlemark'>20.1 </span> <a id='x1-6900020.1'></a>Using standard libraries</h4>
<!-- l. 2031 --><p class='noindent'>You can not do that. In a kernel module, you can only use kernel functions which are
<!-- l. 2032 --><p class='noindent'>You can not do that. In a kernel module, you can only use kernel functions which are
the functions you can see in <span class='obeylines-h'><span class='verb'><span class='ectt-1000'>/proc/kallsyms</span></span></span>.
</p><!-- l. 2034 --><p class='noindent'>
</p><!-- l. 2035 --><p class='noindent'>
</p>
<h4 class='subsectionHead' id='disabling-interrupts'><span class='titlemark'>20.2 </span> <a id='x1-7000020.2'></a>Disabling interrupts</h4>
<!-- l. 2036 --><p class='noindent'>You might need to do this for a short time and that is OK, but if you do not enable
<!-- l. 2037 --><p class='noindent'>You might need to do this for a short time and that is OK, but if you do not enable
them afterwards, your system will be stuck and you will have to power it
off.
</p><!-- l. 2038 --><p class='noindent'>
</p><!-- l. 2039 --><p class='noindent'>
</p>
<h3 class='sectionHead' id='where-to-go-from-here'><span class='titlemark'>21 </span> <a id='x1-7100021'></a>Where To Go From Here?</h3>
<!-- l. 2040 --><p class='noindent'>For people seriously interested in kernel programming, I recommend <a href='https://kernelnewbies.org'>kernelnewbies.org</a>
<!-- l. 2041 --><p class='noindent'>For people seriously interested in kernel programming, I recommend <a href='https://kernelnewbies.org'>kernelnewbies.org</a>
and the <a href='https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/Documentation'>Documentation</a> subdirectory within the kernel source code which is not
always easy to understand but can be a starting point for further investigation. Also,
as Linus Torvalds said, the best way to learn the kernel is to read the source code
yourself.
</p><!-- l. 2043 --><p class='indent'> If you would like to contribute to this guide or notice anything glaringly wrong,
</p><!-- l. 2044 --><p class='indent'> If you would like to contribute to this guide or notice anything glaringly wrong,
please create an issue at <a class='url' href='https://github.com/sysprog21/lkmpg'><span class='ectt-1000'>https://github.com/sysprog21/lkmpg</span></a>. Your pull requests
will be appreciated.
</p><!-- l. 2046 --><p class='indent'> Happy hacking!
</p><!-- l. 2047 --><p class='indent'> Happy hacking!
</p>
<div class='footnotes'><!-- l. 1783 --><p class='indent'> <span class='footnote-mark'><a href='#fn1x0-bk' id='fn1x0'><sup class='textsuperscript'>1</sup></a></span><span class='ecrm-0800'>The goal of threaded interrupts is to push more of the work to separate threads, so that the
<div class='footnotes'><!-- l. 1784 --><p class='indent'> <span class='footnote-mark'><a href='#fn1x0-bk' id='fn1x0'><sup class='textsuperscript'>1</sup></a></span><span class='ecrm-0800'>The goal of threaded interrupts is to push more of the work to separate threads, so that the
</span><span class='ecrm-0800'>minimum needed for acknowledging an interrupt is reduced, and therefore the time spent handling
</span><span class='ecrm-0800'>the interrupt (where it can’t handle any other interrupts at the same time) is reduced. See</span>
<a class='url' href='https://lwn.net/Articles/302043/'><span class='ectt-0800'>https://lwn.net/Articles/302043/</span></a><span class='ecrm-0800'>.</span></p> </div>

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@ -4769,9 +4769,10 @@ better suited to running multiple things in a sequence.
<h4 class='subsectionHead' id='tasklets'><span class='titlemark'>14.1 </span> <a id='x1-5400014.1'></a>Tasklets</h4>
<!-- l. 1765 --><p class='noindent'>Here is an example tasklet module. The
<code> <span class='ectt-1000'>tasklet_fn</span>
</code> function runs for a few seconds and in the mean time execution of the
</code> function runs for a few seconds. In the meantime, execution of the
<code> <span class='ectt-1000'>example_tasklet_init</span>
</code> function continues to the exit point.
</code> function may continue to the exit point, depending on whether it is interrupted by
<span class='ecbx-1000'>softirq</span>.
</p><!-- l. 1 --><p class='indent'>
</p>
<pre class='fancyvrb' id='fancyvrb77'><a id='x1-54004r1'></a><span class='ecrm-0500'>1</span><span id='textcolor2403'><span class='ectt-0800'>/*</span></span>
@ -4818,7 +4819,7 @@ better suited to running multiple things in a sequence.
<a id='x1-54086r42'></a><span class='ecrm-0500'>42</span>
<a id='x1-54088r43'></a><span class='ecrm-0500'>43</span><span class='ectt-0800'>MODULE_DESCRIPTION(</span><span id='textcolor2447'><span class='ectt-0800'>"Tasklet example"</span></span><span class='ectt-0800'>);</span>
<a id='x1-54090r44'></a><span class='ecrm-0500'>44</span><span class='ectt-0800'>MODULE_LICENSE(</span><span id='textcolor2448'><span class='ectt-0800'>"GPL"</span></span><span class='ectt-0800'>);</span></pre>
<!-- l. 1770 --><p class='indent'> So with this example loaded <code> <span class='ectt-1000'>dmesg</span>
<!-- l. 1771 --><p class='indent'> So with this example loaded <code> <span class='ectt-1000'>dmesg</span>
</code> should show:
@ -4830,23 +4831,23 @@ Example tasklet starts
Example tasklet init continues...
Example tasklet ends
</pre>
<!-- l. 1777 --><p class='nopar'>Although tasklet is easy to use, it comes with several defators, and developers are
<!-- l. 1778 --><p class='nopar'>Although tasklet is easy to use, it comes with several defators, and developers are
discussing about getting rid of tasklet in linux kernel. The tasklet callback
runs in atomic context, inside a software interrupt, meaning that it cannot
sleep or access user-space data, so not all work can be done in a tasklet
handler. Also, the kernel only allows one instance of any given tasklet to be
running at any given time; multiple different tasklet callbacks can run in
parallel.
</p><!-- l. 1782 --><p class='indent'> In recent kernels, tasklets can be replaced by workqueues, timers, or threaded
</p><!-- l. 1783 --><p class='indent'> In recent kernels, tasklets can be replaced by workqueues, timers, or threaded
interrupts.<span class='footnote-mark'><a href='#fn1x0' id='fn1x0-bk'><sup class='textsuperscript'>1</sup></a></span><a id='x1-54092f1'></a>
While the removal of tasklets remains a longer-term goal, the current kernel contains more
than a hundred uses of tasklets. Now developers are proceeding with the API changes and
the macro <code> <span class='ectt-1000'>DECLARE_TASKLET_OLD</span>
</code> exists for compatibility. For further information, see <a class='url' href='https://lwn.net/Articles/830964/'><span class='ectt-1000'>https://lwn.net/Articles/830964/</span></a>.
</p><!-- l. 1788 --><p class='noindent'>
</p><!-- l. 1789 --><p class='noindent'>
</p>
<h4 class='subsectionHead' id='work-queues'><span class='titlemark'>14.2 </span> <a id='x1-5500014.2'></a>Work queues</h4>
<!-- l. 1790 --><p class='noindent'>To add a task to the scheduler we can use a workqueue. The kernel then uses the
<!-- l. 1791 --><p class='noindent'>To add a task to the scheduler we can use a workqueue. The kernel then uses the
Completely Fair Scheduler (CFS) to execute work within the queue.
</p><!-- l. 1 --><p class='indent'>
</p>
@ -4883,36 +4884,36 @@ Completely Fair Scheduler (CFS) to execute work within the queue.
<a id='x1-55062r31'></a><span class='ecrm-0500'>31</span>
<a id='x1-55064r32'></a><span class='ecrm-0500'>32</span><span class='ectt-0800'>MODULE_LICENSE(</span><span id='textcolor2476'><span class='ectt-0800'>"GPL"</span></span><span class='ectt-0800'>);</span>
<a id='x1-55066r33'></a><span class='ecrm-0500'>33</span><span class='ectt-0800'>MODULE_DESCRIPTION(</span><span id='textcolor2477'><span class='ectt-0800'>"Workqueue example"</span></span><span class='ectt-0800'>);</span></pre>
<!-- l. 1795 --><p class='noindent'>
<!-- l. 1796 --><p class='noindent'>
</p>
<h3 class='sectionHead' id='interrupt-handlers'><span class='titlemark'>15 </span> <a id='x1-5600015'></a>Interrupt Handlers</h3>
<!-- l. 1797 --><p class='noindent'>
<!-- l. 1798 --><p class='noindent'>
</p>
<h4 class='subsectionHead' id='interrupt-handlers1'><span class='titlemark'>15.1 </span> <a id='x1-5700015.1'></a>Interrupt Handlers</h4>
<!-- l. 1799 --><p class='noindent'>Except for the last chapter, everything we did in the kernel so far we have done as a
<!-- l. 1800 --><p class='noindent'>Except for the last chapter, everything we did in the kernel so far we have done as a
response to a process asking for it, either by dealing with a special file, sending an
<code> <span class='ectt-1000'>ioctl()</span>
</code>, or issuing a system call. But the job of the kernel is not just to respond to process
requests. Another job, which is every bit as important, is to speak to the hardware
connected to the machine.
</p><!-- l. 1803 --><p class='indent'> There are two types of interaction between the CPU and the rest of the
</p><!-- l. 1804 --><p class='indent'> There are two types of interaction between the CPU and the rest of the
computer’s hardware. The first type is when the CPU gives orders to the hardware,
the order is when the hardware needs to tell the CPU something. The second, called
interrupts, is much harder to implement because it has to be dealt with when
convenient for the hardware, not the CPU. Hardware devices typically have a very
small amount of RAM, and if you do not read their information when available, it is
lost.
</p><!-- l. 1808 --><p class='indent'> Under Linux, hardware interrupts are called IRQ’s (Interrupt ReQuests). There
</p><!-- l. 1809 --><p class='indent'> Under Linux, hardware interrupts are called IRQ’s (Interrupt ReQuests). There
are two types of IRQ’s, short and long. A short IRQ is one which is expected to take
a very short period of time, during which the rest of the machine will be blocked and
no other interrupts will be handled. A long IRQ is one which can take longer, and
during which other interrupts may occur (but not interrupts from the same
device). If at all possible, it is better to declare an interrupt handler to be
long.
</p><!-- l. 1814 --><p class='indent'> When the CPU receives an interrupt, it stops whatever it is doing (unless it is
</p><!-- l. 1815 --><p class='indent'> When the CPU receives an interrupt, it stops whatever it is doing (unless it is
processing a more important interrupt, in which case it will deal with this one only
when the more important one is done), saves certain parameters on the stack and
calls the interrupt handler. This means that certain things are not allowed in the
@ -4924,10 +4925,10 @@ heavy work deferred from an interrupt handler. Historically, BH (Linux
naming for <span class='ecti-1000'>Bottom Halves</span>) statistically book-keeps the deferred functions.
<span class='ecbx-1000'>Softirq </span>and its higher level abstraction, <span class='ecbx-1000'>Tasklet</span>, replace BH since Linux
2.3.
</p><!-- l. 1824 --><p class='indent'> The way to implement this is to call
</p><!-- l. 1825 --><p class='indent'> The way to implement this is to call
<code> <span class='ectt-1000'>request_irq()</span>
</code> to get your interrupt handler called when the relevant IRQ is received.
</p><!-- l. 1826 --><p class='indent'> In practice IRQ handling can be a bit more complex. Hardware is often
</p><!-- l. 1827 --><p class='indent'> In practice IRQ handling can be a bit more complex. Hardware is often
designed in a way that chains two interrupt controllers, so that all the IRQs
from interrupt controller B are cascaded to a certain IRQ from interrupt
controller A. Of course, that requires that the kernel finds out which IRQ it
@ -4944,7 +4945,7 @@ need to solve another truckload of problems. It is not enough to know if a
certain IRQs has happened, it’s also important to know what CPU(s) it was
for. People still interested in more details, might want to refer to "APIC"
now.
</p><!-- l. 1835 --><p class='indent'> This function receives the IRQ number, the name of the function,
</p><!-- l. 1836 --><p class='indent'> This function receives the IRQ number, the name of the function,
flags, a name for <span class='obeylines-h'><span class='verb'><span class='ectt-1000'>/proc/interrupts</span></span></span> and a parameter to be passed to the
interrupt handler. Usually there is a certain number of IRQs available.
How many IRQs there are is hardware-dependent. The flags can include
@ -4954,16 +4955,16 @@ How many IRQs there are is hardware-dependent. The flags can include
<code> <span class='ectt-1000'>SA_INTERRUPT</span>
</code> to indicate this is a fast interrupt. This function will only succeed if there is not
already a handler on this IRQ, or if you are both willing to share.
</p><!-- l. 1841 --><p class='noindent'>
</p><!-- l. 1842 --><p class='noindent'>
</p>
<h4 class='subsectionHead' id='detecting-button-presses'><span class='titlemark'>15.2 </span> <a id='x1-5800015.2'></a>Detecting button presses</h4>
<!-- l. 1843 --><p class='noindent'>Many popular single board computers, such as Raspberry Pi or Beagleboards, have a
<!-- l. 1844 --><p class='noindent'>Many popular single board computers, such as Raspberry Pi or Beagleboards, have a
bunch of GPIO pins. Attaching buttons to those and then having a button press do
something is a classic case in which you might need to use interrupts, so that instead
of having the CPU waste time and battery power polling for a change in input state,
it is better for the input to trigger the CPU to then run a particular handling
function.
</p><!-- l. 1847 --><p class='indent'> Here is an example where buttons are connected to GPIO numbers 17 and 18 and
</p><!-- l. 1848 --><p class='indent'> Here is an example where buttons are connected to GPIO numbers 17 and 18 and
an LED is connected to GPIO 4. You can change those numbers to whatever is
appropriate for your board.
</p><!-- l. 1 --><p class='indent'>
@ -5112,14 +5113,14 @@ appropriate for your board.
<a id='x1-58284r142'></a><span class='ecrm-0500'>142</span>
<a id='x1-58286r143'></a><span class='ecrm-0500'>143</span><span class='ectt-0800'>MODULE_LICENSE(</span><span id='textcolor2584'><span class='ectt-0800'>"GPL"</span></span><span class='ectt-0800'>);</span>
<a id='x1-58288r144'></a><span class='ecrm-0500'>144</span><span class='ectt-0800'>MODULE_DESCRIPTION(</span><span id='textcolor2585'><span class='ectt-0800'>"Handle some GPIO interrupts"</span></span><span class='ectt-0800'>);</span></pre>
<!-- l. 1852 --><p class='noindent'>
<!-- l. 1853 --><p class='noindent'>
</p>
<h4 class='subsectionHead' id='bottom-half'><span class='titlemark'>15.3 </span> <a id='x1-5900015.3'></a>Bottom Half</h4>
<!-- l. 1854 --><p class='noindent'>Suppose you want to do a bunch of stuff inside of an interrupt routine. A common
<!-- l. 1855 --><p class='noindent'>Suppose you want to do a bunch of stuff inside of an interrupt routine. A common
way to do that without rendering the interrupt unavailable for a significant duration
is to combine it with a tasklet. This pushes the bulk of the work off into the
scheduler.
</p><!-- l. 1858 --><p class='indent'> The example below modifies the previous example to also run an additional task
</p><!-- l. 1859 --><p class='indent'> The example below modifies the previous example to also run an additional task
when an interrupt is triggered.
</p><!-- l. 1 --><p class='indent'>
@ -5293,19 +5294,19 @@ when an interrupt is triggered.
<a id='x1-59330r165'></a><span class='ecrm-0500'>165</span>
<a id='x1-59332r166'></a><span class='ecrm-0500'>166</span><span class='ectt-0800'>MODULE_LICENSE(</span><span id='textcolor2713'><span class='ectt-0800'>"GPL"</span></span><span class='ectt-0800'>);</span>
<a id='x1-59334r167'></a><span class='ecrm-0500'>167</span><span class='ectt-0800'>MODULE_DESCRIPTION(</span><span id='textcolor2714'><span class='ectt-0800'>"Interrupt with top and bottom half"</span></span><span class='ectt-0800'>);</span></pre>
<!-- l. 1862 --><p class='noindent'>
<!-- l. 1863 --><p class='noindent'>
</p>
<h3 class='sectionHead' id='crypto'><span class='titlemark'>16 </span> <a id='x1-6000016'></a>Crypto</h3>
<!-- l. 1864 --><p class='noindent'>At the dawn of the internet, everybody trusted everybody completely…but that did
<!-- l. 1865 --><p class='noindent'>At the dawn of the internet, everybody trusted everybody completely…but that did
not work out so well. When this guide was originally written, it was a more innocent
era in which almost nobody actually gave a damn about crypto - least of all kernel
developers. That is certainly no longer the case now. To handle crypto stuff, the
kernel has its own API enabling common methods of encryption, decryption and your
favourite hash functions.
</p><!-- l. 1869 --><p class='noindent'>
</p><!-- l. 1870 --><p class='noindent'>
</p>
<h4 class='subsectionHead' id='hash-functions'><span class='titlemark'>16.1 </span> <a id='x1-6100016.1'></a>Hash functions</h4>
<!-- l. 1872 --><p class='noindent'>Calculating and checking the hashes of things is a common operation. Here is a
<!-- l. 1873 --><p class='noindent'>Calculating and checking the hashes of things is a common operation. Here is a
demonstration of how to calculate a sha256 hash within a kernel module.
</p><!-- l. 1 --><p class='indent'>
</p>
@ -5373,20 +5374,20 @@ demonstration of how to calculate a sha256 hash within a kernel module.
<a id='x1-61124r62'></a><span class='ecrm-0500'>62</span>
<a id='x1-61126r63'></a><span class='ecrm-0500'>63</span><span class='ectt-0800'>MODULE_DESCRIPTION(</span><span id='textcolor2766'><span class='ectt-0800'>"sha256 hash test"</span></span><span class='ectt-0800'>);</span>
<a id='x1-61128r64'></a><span class='ecrm-0500'>64</span><span class='ectt-0800'>MODULE_LICENSE(</span><span id='textcolor2767'><span class='ectt-0800'>"GPL"</span></span><span class='ectt-0800'>);</span></pre>
<!-- l. 1877 --><p class='indent'> Install the module:
<!-- l. 1878 --><p class='indent'> Install the module:
</p><!-- l. 1 --><p class='indent'>
</p>
<pre class='fancyvrb' id='fancyvrb82'><a id='x1-61132r1'></a><span class='ecrm-0500'>1</span><span class='ectt-1000'>sudo insmod cryptosha256.ko</span>
<a id='x1-61134r2'></a><span class='ecrm-0500'>2</span><span class='ectt-1000'>sudo dmesg</span></pre>
<!-- l. 1884 --><p class='indent'> And you should see that the hash was calculated for the test string.
</p><!-- l. 1886 --><p class='indent'> Finally, remove the test module:
<!-- l. 1885 --><p class='indent'> And you should see that the hash was calculated for the test string.
</p><!-- l. 1887 --><p class='indent'> Finally, remove the test module:
</p><!-- l. 1 --><p class='indent'>
</p>
<pre class='fancyvrb' id='fancyvrb83'><a id='x1-61137r1'></a><span class='ecrm-0500'>1</span><span class='ectt-1000'>sudo rmmod cryptosha256</span></pre>
<!-- l. 1892 --><p class='noindent'>
<!-- l. 1893 --><p class='noindent'>
</p>
<h4 class='subsectionHead' id='symmetric-key-encryption'><span class='titlemark'>16.2 </span> <a id='x1-6200016.2'></a>Symmetric key encryption</h4>
<!-- l. 1894 --><p class='noindent'>Here is an example of symmetrically encrypting a string using the AES algorithm
<!-- l. 1895 --><p class='noindent'>Here is an example of symmetrically encrypting a string using the AES algorithm
and a password.
</p><!-- l. 1 --><p class='indent'>
@ -5591,10 +5592,10 @@ and a password.
<a id='x1-62392r196'></a><span class='ecrm-0500'>196</span>
<a id='x1-62394r197'></a><span class='ecrm-0500'>197</span><span class='ectt-0800'>MODULE_DESCRIPTION(</span><span id='textcolor2919'><span class='ectt-0800'>"Symmetric key encryption example"</span></span><span class='ectt-0800'>);</span>
<a id='x1-62396r198'></a><span class='ecrm-0500'>198</span><span class='ectt-0800'>MODULE_LICENSE(</span><span id='textcolor2920'><span class='ectt-0800'>"GPL"</span></span><span class='ectt-0800'>);</span></pre>
<!-- l. 1898 --><p class='noindent'>
<!-- l. 1899 --><p class='noindent'>
</p>
<h3 class='sectionHead' id='virtual-input-device-driver'><span class='titlemark'>17 </span> <a id='x1-6300017'></a>Virtual Input Device Driver</h3>
<!-- l. 1900 --><p class='noindent'>The input device driver is a module that provides a way to communicate
<!-- l. 1901 --><p class='noindent'>The input device driver is a module that provides a way to communicate
with the interaction device via the event. For example, the keyboard
can send the press or release event to tell the kernel what we want to
do. The input device driver will allocate a new input structure with
@ -5602,7 +5603,7 @@ do. The input device driver will allocate a new input structure with
</code> and sets up input bitfields, device id, version, etc. After that, registers it by calling
<code> <span class='ectt-1000'>input_register_device()</span>
</code>.
</p><!-- l. 1905 --><p class='indent'> Here is an example, vinput, It is an API to allow easy
</p><!-- l. 1906 --><p class='indent'> Here is an example, vinput, It is an API to allow easy
development of virtual input drivers. The drivers needs to export a
<code> <span class='ectt-1000'>vinput_device()</span>
</code> that contains the virtual device name and
@ -5618,7 +5619,7 @@ development of virtual input drivers. The drivers needs to export a
</li>
<li class='itemize'>the readback function: <code> <span class='ectt-1000'>read()</span>
</code></li></ul>
<!-- l. 1915 --><p class='indent'> Then using <code> <span class='ectt-1000'>vinput_register_device()</span>
<!-- l. 1916 --><p class='indent'> Then using <code> <span class='ectt-1000'>vinput_register_device()</span>
</code> and <code> <span class='ectt-1000'>vinput_unregister_device()</span>
</code> will add a new device to the list of support virtual input devices.
</p><!-- l. 1 --><p class='indent'>
@ -5627,7 +5628,7 @@ development of virtual input drivers. The drivers needs to export a
</p>
<pre class='fancyvrb' id='fancyvrb85'><a id='x1-63012r1'></a><span class='ecrm-0500'>1</span><span id='textcolor2921'><span class='ectt-0800'>int</span></span><span class='ectt-0800'> init(</span><span id='textcolor2922'><span class='ectt-0800'>struct</span></span><span class='ectt-0800'> vinput *);</span></pre>
<!-- l. 1921 --><p class='indent'> This function is passed a <code> <span id='textcolor2923'><span class='ectt-1000'>struct</span></span><span class='ectt-1000'> vinput</span>
<!-- l. 1922 --><p class='indent'> This function is passed a <code> <span id='textcolor2923'><span class='ectt-1000'>struct</span></span><span class='ectt-1000'> vinput</span>
</code> already initialized with an allocated <code> <span id='textcolor2924'><span class='ectt-1000'>struct</span></span><span class='ectt-1000'> input_dev</span>
</code>. The <code> <span class='ectt-1000'>init()</span>
</code> function is responsible for initializing the capabilities of the input device and register
@ -5635,20 +5636,20 @@ it.
</p><!-- l. 1 --><p class='indent'>
</p>
<pre class='fancyvrb' id='fancyvrb86'><a id='x1-63018r1'></a><span class='ecrm-0500'>1</span><span id='textcolor2925'><span class='ectt-0800'>int</span></span><span class='ectt-0800'> send(</span><span id='textcolor2926'><span class='ectt-0800'>struct</span></span><span class='ectt-0800'> vinput *, </span><span id='textcolor2927'><span class='ectt-0800'>char</span></span><span class='ectt-0800'> *, </span><span id='textcolor2928'><span class='ectt-0800'>int</span></span><span class='ectt-0800'>);</span></pre>
<!-- l. 1928 --><p class='indent'> This function will receive a user string to interpret and inject the event using the
<!-- l. 1929 --><p class='indent'> This function will receive a user string to interpret and inject the event using the
<code> <span class='ectt-1000'>input_report_XXXX</span>
</code> or <code> <span class='ectt-1000'>input_event</span>
</code> call. The string is already copied from user.
</p><!-- l. 1 --><p class='indent'>
</p>
<pre class='fancyvrb' id='fancyvrb87'><a id='x1-63023r1'></a><span class='ecrm-0500'>1</span><span id='textcolor2929'><span class='ectt-0800'>int</span></span><span class='ectt-0800'> read(</span><span id='textcolor2930'><span class='ectt-0800'>struct</span></span><span class='ectt-0800'> vinput *, </span><span id='textcolor2931'><span class='ectt-0800'>char</span></span><span class='ectt-0800'> *, </span><span id='textcolor2932'><span class='ectt-0800'>int</span></span><span class='ectt-0800'>);</span></pre>
<!-- l. 1935 --><p class='indent'> This function is used for debugging and should fill the buffer parameter with the
<!-- l. 1936 --><p class='indent'> This function is used for debugging and should fill the buffer parameter with the
last event sent in the virtual input device format. The buffer will then be copied to
user.
</p><!-- l. 1938 --><p class='indent'> vinput devices are created and destroyed using sysfs. And, event injection is done
</p><!-- l. 1939 --><p class='indent'> vinput devices are created and destroyed using sysfs. And, event injection is done
through a <span class='obeylines-h'><span class='verb'><span class='ectt-1000'>/dev</span></span></span> node. The device name will be used by the userland to export a new
virtual input device.
</p><!-- l. 1942 --><p class='indent'> The <code> <span class='ectt-1000'>class_attribute</span>
</p><!-- l. 1943 --><p class='indent'> The <code> <span class='ectt-1000'>class_attribute</span>
</code> structure is similar to other attribute types we talked about in section <a href='#sysfs-interacting-with-your-module'>8<!-- tex4ht:ref: sec:sysfs --></a>:
</p><!-- l. 1 --><p class='indent'>
</p>
@ -5659,7 +5660,7 @@ virtual input device.
<a id='x1-63041r5'></a><span class='ecrm-0500'>5</span><span class='ectt-0800'>    </span><span id='textcolor2939'><span class='ectt-0800'>ssize_t</span></span><span class='ectt-0800'> (*store)(</span><span id='textcolor2940'><span class='ectt-0800'>struct</span></span><span class='ectt-0800'> class *class, </span><span id='textcolor2941'><span class='ectt-0800'>struct</span></span><span class='ectt-0800'> class_attribute *attr,</span>
<a id='x1-63043r6'></a><span class='ecrm-0500'>6</span><span class='ectt-0800'>                    </span><span id='textcolor2942'><span class='ectt-0800'>const</span></span><span class='ectt-0800'> </span><span id='textcolor2943'><span class='ectt-0800'>char</span></span><span class='ectt-0800'> *buf, </span><span id='textcolor2944'><span class='ectt-0800'>size_t</span></span><span class='ectt-0800'> count);</span>
<a id='x1-63045r7'></a><span class='ecrm-0500'>7</span><span class='ectt-0800'>};</span></pre>
<!-- l. 1954 --><p class='indent'> In <span class='obeylines-h'><span class='verb'><span class='ectt-1000'>vinput.c</span></span></span>, the macro <code> <span class='ectt-1000'>CLASS_ATTR_WO(export/unexport)</span>
<!-- l. 1955 --><p class='indent'> In <span class='obeylines-h'><span class='verb'><span class='ectt-1000'>vinput.c</span></span></span>, the macro <code> <span class='ectt-1000'>CLASS_ATTR_WO(export/unexport)</span>
</code> defined in <a href='https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/include/linux/device.h'>include/linux/device.h</a> (in this case, <span class='obeylines-h'><span class='verb'><span class='ectt-1000'>device.h</span></span></span> is included in <a href='https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/include/linux/input.h'>include/linux/input.h</a>)
will generate the <code> <span class='ectt-1000'>class_attribute</span>
</code> structures which are named <span class='obeylines-h'><span class='verb'><span class='ectt-1000'>class_attr_export/unexport</span></span></span>. Then, put them into
@ -5669,14 +5670,14 @@ will generate the <code> <span class='ectt-1000'>class_attribute</span>
</code> that should be assigned in <code> <span class='ectt-1000'>vinput_class</span>
</code>. Finally, call <code> <span class='ectt-1000'>class_register(&amp;vinput_class)</span>
</code> to create attributes in sysfs.
</p><!-- l. 1958 --><p class='indent'> To create a <span class='obeylines-h'><span class='verb'><span class='ectt-1000'>vinputX</span></span></span> sysfs entry and <span class='obeylines-h'><span class='verb'><span class='ectt-1000'>/dev</span></span></span> node.
</p><!-- l. 1959 --><p class='indent'> To create a <span class='obeylines-h'><span class='verb'><span class='ectt-1000'>vinputX</span></span></span> sysfs entry and <span class='obeylines-h'><span class='verb'><span class='ectt-1000'>/dev</span></span></span> node.
</p><!-- l. 1 --><p class='indent'>
</p>
<pre class='fancyvrb' id='fancyvrb89'><a id='x1-63055r1'></a><span class='ecrm-0500'>1</span><span class='ectt-1000'>echo </span><span id='textcolor2946'><span class='ectt-1000'>"vkbd"</span></span><span class='ectt-1000'> | sudo tee /sys/class/vinput/export</span></pre>
<!-- l. 1964 --><p class='indent'> To unexport the device, just echo its id in unexport:
<!-- l. 1965 --><p class='indent'> To unexport the device, just echo its id in unexport:
</p><!-- l. 1 --><p class='indent'>
</p>
<pre class='fancyvrb' id='fancyvrb90'><a id='x1-63058r1'></a><span class='ecrm-0500'>1</span><span class='ectt-1000'>echo </span><span id='textcolor2947'><span class='ectt-1000'>"0"</span></span><span class='ectt-1000'> | sudo tee /sys/class/vinput/unexport</span></pre>
@ -6137,7 +6138,7 @@ will generate the <code> <span class='ectt-1000'>class_attribute</span>
<a id='x1-63956r400'></a><span class='ecrm-0500'>400</span>
<a id='x1-63958r401'></a><span class='ecrm-0500'>401</span><span class='ectt-0800'>MODULE_LICENSE(</span><span id='textcolor3242'><span class='ectt-0800'>"GPL"</span></span><span class='ectt-0800'>);</span>
<a id='x1-63960r402'></a><span class='ecrm-0500'>402</span><span class='ectt-0800'>MODULE_DESCRIPTION(</span><span id='textcolor3243'><span class='ectt-0800'>"Emulate input events"</span></span><span class='ectt-0800'>);</span></pre>
<!-- l. 1973 --><p class='indent'> Here the virtual keyboard is one of example to use vinput. It supports all
<!-- l. 1974 --><p class='indent'> Here the virtual keyboard is one of example to use vinput. It supports all
<code> <span class='ectt-1000'>KEY_MAX</span>
</code> keycodes. The injection format is the <code> <span class='ectt-1000'>KEY_CODE</span>
</code> such as defined in <a href='https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/include/linux/input.h'>include/linux/input.h</a>. A positive value means
@ -6145,12 +6146,12 @@ will generate the <code> <span class='ectt-1000'>class_attribute</span>
</code> while a negative value is a <code> <span class='ectt-1000'>KEY_RELEASE</span>
</code>. The keyboard supports repetition when the key stays pressed for too long. The
following demonstrates how simulation work.
</p><!-- l. 1980 --><p class='indent'> Simulate a key press on "g" (<code> <span class='ectt-1000'>KEY_G</span>
</p><!-- l. 1981 --><p class='indent'> Simulate a key press on "g" (<code> <span class='ectt-1000'>KEY_G</span>
</code> = 34):
</p><!-- l. 1 --><p class='indent'>
</p>
<pre class='fancyvrb' id='fancyvrb93'><a id='x1-63968r1'></a><span class='ecrm-0500'>1</span><span class='ectt-1000'>echo </span><span id='textcolor3244'><span class='ectt-1000'>"+34"</span></span><span class='ectt-1000'> | sudo tee /dev/vinput0</span></pre>
<!-- l. 1986 --><p class='indent'> Simulate a key release on "g" (<code> <span class='ectt-1000'>KEY_G</span>
<!-- l. 1987 --><p class='indent'> Simulate a key release on "g" (<code> <span class='ectt-1000'>KEY_G</span>
</code> = 34):
</p><!-- l. 1 --><p class='indent'>
</p>
@ -6268,13 +6269,13 @@ following demonstrates how simulation work.
<a id='x1-64188r108'></a><span class='ecrm-0500'>108</span>
<a id='x1-64190r109'></a><span class='ecrm-0500'>109</span><span class='ectt-0800'>MODULE_LICENSE(</span><span id='textcolor3327'><span class='ectt-0800'>"GPL"</span></span><span class='ectt-0800'>);</span>
<a id='x1-64192r110'></a><span class='ecrm-0500'>110</span><span class='ectt-0800'>MODULE_DESCRIPTION(</span><span id='textcolor3328'><span class='ectt-0800'>"Emulate keyboard input events through /dev/vinput"</span></span><span class='ectt-0800'>);</span></pre>
<!-- l. 1996 --><p class='noindent'>
<!-- l. 1997 --><p class='noindent'>
</p>
<h3 class='sectionHead' id='standardizing-the-interfaces-the-device-model'><span class='titlemark'>18 </span> <a id='x1-6500018'></a>Standardizing the interfaces: The Device Model</h3>
<!-- l. 1998 --><p class='noindent'>Up to this point we have seen all kinds of modules doing all kinds of things, but there
<!-- l. 1999 --><p class='noindent'>Up to this point we have seen all kinds of modules doing all kinds of things, but there
was no consistency in their interfaces with the rest of the kernel. To impose some
consistency such that there is at minimum a standardized way to start, suspend and
resume a device a device model was added. An example is shown below, and you can
@ -6381,13 +6382,13 @@ functions.
<a id='x1-65194r97'></a><span class='ecrm-0500'>97</span>
<a id='x1-65196r98'></a><span class='ecrm-0500'>98</span><span class='ectt-0800'>MODULE_LICENSE(</span><span id='textcolor3403'><span class='ectt-0800'>"GPL"</span></span><span class='ectt-0800'>);</span>
<a id='x1-65198r99'></a><span class='ecrm-0500'>99</span><span class='ectt-0800'>MODULE_DESCRIPTION(</span><span id='textcolor3404'><span class='ectt-0800'>"Linux Device Model example"</span></span><span class='ectt-0800'>);</span></pre>
<!-- l. 2004 --><p class='noindent'>
<!-- l. 2005 --><p class='noindent'>
</p>
<h3 class='sectionHead' id='optimizations'><span class='titlemark'>19 </span> <a id='x1-6600019'></a>Optimizations</h3>
<!-- l. 2006 --><p class='noindent'>
<!-- l. 2007 --><p class='noindent'>
</p>
<h4 class='subsectionHead' id='likely-and-unlikely-conditions'><span class='titlemark'>19.1 </span> <a id='x1-6700019.1'></a>Likely and Unlikely conditions</h4>
<!-- l. 2008 --><p class='noindent'>Sometimes you might want your code to run as quickly as possible,
<!-- l. 2009 --><p class='noindent'>Sometimes you might want your code to run as quickly as possible,
especially if it is handling an interrupt or doing something which might
cause noticeable latency. If your code contains boolean conditions and if
you know that the conditions are almost always likely to evaluate as either
@ -6406,7 +6407,7 @@ to succeed.
<a id='x1-67018r4'></a><span class='ecrm-0500'>4</span><span class='ectt-0800'>    bio = NULL;</span>
<a id='x1-67020r5'></a><span class='ecrm-0500'>5</span><span class='ectt-0800'>    </span><span id='textcolor3406'><span class='ectt-0800'>goto</span></span><span class='ectt-0800'> out;</span>
<a id='x1-67022r6'></a><span class='ecrm-0500'>6</span><span class='ectt-0800'>}</span></pre>
<!-- l. 2022 --><p class='indent'> When the <code> <span class='ectt-1000'>unlikely</span>
<!-- l. 2023 --><p class='indent'> When the <code> <span class='ectt-1000'>unlikely</span>
</code> macro is used, the compiler alters its machine instruction output, so that it
continues along the false branch and only jumps if the condition is true. That
avoids flushing the processor pipeline. The opposite happens if you use the
@ -6415,34 +6416,34 @@ avoids flushing the processor pipeline. The opposite happens if you use the
</p><!-- l. 2026 --><p class='noindent'>
</p><!-- l. 2027 --><p class='noindent'>
</p>
<h3 class='sectionHead' id='common-pitfalls'><span class='titlemark'>20 </span> <a id='x1-6800020'></a>Common Pitfalls</h3>
<!-- l. 2029 --><p class='noindent'>
<!-- l. 2030 --><p class='noindent'>
</p>
<h4 class='subsectionHead' id='using-standard-libraries'><span class='titlemark'>20.1 </span> <a id='x1-6900020.1'></a>Using standard libraries</h4>
<!-- l. 2031 --><p class='noindent'>You can not do that. In a kernel module, you can only use kernel functions which are
<!-- l. 2032 --><p class='noindent'>You can not do that. In a kernel module, you can only use kernel functions which are
the functions you can see in <span class='obeylines-h'><span class='verb'><span class='ectt-1000'>/proc/kallsyms</span></span></span>.
</p><!-- l. 2034 --><p class='noindent'>
</p><!-- l. 2035 --><p class='noindent'>
</p>
<h4 class='subsectionHead' id='disabling-interrupts'><span class='titlemark'>20.2 </span> <a id='x1-7000020.2'></a>Disabling interrupts</h4>
<!-- l. 2036 --><p class='noindent'>You might need to do this for a short time and that is OK, but if you do not enable
<!-- l. 2037 --><p class='noindent'>You might need to do this for a short time and that is OK, but if you do not enable
them afterwards, your system will be stuck and you will have to power it
off.
</p><!-- l. 2038 --><p class='noindent'>
</p><!-- l. 2039 --><p class='noindent'>
</p>
<h3 class='sectionHead' id='where-to-go-from-here'><span class='titlemark'>21 </span> <a id='x1-7100021'></a>Where To Go From Here?</h3>
<!-- l. 2040 --><p class='noindent'>For people seriously interested in kernel programming, I recommend <a href='https://kernelnewbies.org'>kernelnewbies.org</a>
<!-- l. 2041 --><p class='noindent'>For people seriously interested in kernel programming, I recommend <a href='https://kernelnewbies.org'>kernelnewbies.org</a>
and the <a href='https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/Documentation'>Documentation</a> subdirectory within the kernel source code which is not
always easy to understand but can be a starting point for further investigation. Also,
as Linus Torvalds said, the best way to learn the kernel is to read the source code
yourself.
</p><!-- l. 2043 --><p class='indent'> If you would like to contribute to this guide or notice anything glaringly wrong,
</p><!-- l. 2044 --><p class='indent'> If you would like to contribute to this guide or notice anything glaringly wrong,
please create an issue at <a class='url' href='https://github.com/sysprog21/lkmpg'><span class='ectt-1000'>https://github.com/sysprog21/lkmpg</span></a>. Your pull requests
will be appreciated.
</p><!-- l. 2046 --><p class='indent'> Happy hacking!
</p><!-- l. 2047 --><p class='indent'> Happy hacking!
</p>
<div class='footnotes'><!-- l. 1783 --><p class='indent'> <span class='footnote-mark'><a href='#fn1x0-bk' id='fn1x0'><sup class='textsuperscript'>1</sup></a></span><span class='ecrm-0800'>The goal of threaded interrupts is to push more of the work to separate threads, so that the
<div class='footnotes'><!-- l. 1784 --><p class='indent'> <span class='footnote-mark'><a href='#fn1x0-bk' id='fn1x0'><sup class='textsuperscript'>1</sup></a></span><span class='ecrm-0800'>The goal of threaded interrupts is to push more of the work to separate threads, so that the
</span><span class='ecrm-0800'>minimum needed for acknowledging an interrupt is reduced, and therefore the time spent handling
</span><span class='ecrm-0800'>the interrupt (where it can’t handle any other interrupts at the same time) is reduced. See</span>
<a class='url' href='https://lwn.net/Articles/302043/'><span class='ectt-0800'>https://lwn.net/Articles/302043/</span></a><span class='ecrm-0800'>.</span></p> </div>