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Subject[PATCH 2/3] UIO: Documentation
Date
From: Hans J. Koch <hjk@linutronix.de>

Documentation for the UIO interface

From: Hans J. Koch <hjk@linutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
---
Documentation/DocBook/kernel-api.tmpl | 4 +
Documentation/DocBook/uio-howto.tmpl | 498 +++++++++++++++++++++++++++++++++
2 files changed, 502 insertions(+), 0 deletions(-)
create mode 100644 Documentation/DocBook/uio-howto.tmpl
diff --git a/Documentation/DocBook/kernel-api.tmpl b/Documentation/DocBook/kernel-api.tmpl
index 0bb9023..40e968d 100644
--- a/Documentation/DocBook/kernel-api.tmpl
+++ b/Documentation/DocBook/kernel-api.tmpl
@@ -396,6 +396,10 @@ X!Edrivers/pnp/system.c
!Edrivers/pnp/manager.c
!Edrivers/pnp/support.c
</sect1>
+ <sect1><title>Userspace IO devices</title>
+!Edrivers/uio/uio.c
+!Iinclude/linux/uio_driver.h
+ </sect1>
</chapter>

<chapter id="blkdev">
diff --git a/Documentation/DocBook/uio-howto.tmpl b/Documentation/DocBook/uio-howto.tmpl
new file mode 100644
index 0000000..bd2209f
--- /dev/null
+++ b/Documentation/DocBook/uio-howto.tmpl
@@ -0,0 +1,498 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
+"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
+
+<article id="index">
+<articleinfo>
+<title>The Userspace I/O HOWTO</title>
+
+<author>
+ <firstname>Hans-Jürgen</firstname>
+ <surname>Koch</surname>
+ <authorblurb><para>Linux developer, Linutronix</para></authorblurb>
+ <affiliation>
+ <orgname>
+ <ulink url="http://www.linutronix.de">Linutronix</ulink>
+ </orgname>
+
+ <address>
+ <email>hjk@linutronix.de</email>
+ </address>
+ </affiliation>
+</author>
+
+<pubdate>2006-12-11</pubdate>
+
+<abstract>
+ <para>This HOWTO describes concept and usage of Linux kernel's
+ Userspace I/O system.</para>
+</abstract>
+
+<revhistory>
+ <revision>
+ <revnumber>0.2</revnumber>
+ <date>2007-02-13</date>
+ <authorinitials>hjk</authorinitials>
+ <revremark>Update after multiple mappings were added.</revremark>
+ </revision>
+ <revision>
+ <revnumber>0.1</revnumber>
+ <date>2006-12-11</date>
+ <authorinitials>hjk</authorinitials>
+ <revremark>First draft.</revremark>
+ </revision>
+</revhistory>
+</articleinfo>
+
+<sect1 id="aboutthisdoc">
+<?dbhtml filename="about.html"?>
+<title>About this document</title>
+
+<sect2 id="copyright">
+<?dbhtml filename="copyright.html"?>
+<title>Copyright and License</title>
+<para>
+ Copyright (c) 2006 by Hans-Jürgen Koch.</para>
+<para>
+This documentation is Free Software licensed under the terms of the
+GPL version 2.
+</para>
+</sect2>
+
+<sect2 id="translations">
+<?dbhtml filename="translations.html"?>
+<title>Translations</title>
+
+<para>If you know of any translations for this document, or you are
+interested in translating it, please email me
+<email>hjk@linutronix.de</email>.
+</para>
+</sect2>
+
+<sect2 id="preface">
+<title>Preface</title>
+ <para>
+ For many types of devices, creating a Linux kernel driver is
+ overkill. All that is really needed is some way to handle an
+ interrupt and provide access to the memory space of the
+ device. The logic of controlling the device does not
+ necessarily have to be within the kernel, as the device does
+ not need to take advantage of any of other resources that the
+ kernel provides. One such common class of devices that are
+ like this are for industrial I/O cards.
+ </para>
+ <para>
+ To address this situation, the userspace I/O system (UIO) was
+ designed. For typical industrial I/O cards, only a very small
+ kernel module is needed. The main part of the driver will run in
+ user space. This simplifies development and reduces the risk of
+ serious bugs within a kernel module.
+ </para>
+</sect2>
+
+<sect2 id="thanks">
+<title>Acknowledgments</title>
+ <para>I'd like to thank Thomas Gleixner and Benedikt Spranger of
+ Linutronix, who have not only written most of the UIO code, but also
+ helped greatly writing this HOWTO by giving me all kinds of background
+ information.</para>
+</sect2>
+
+<sect2 id="feedback">
+<title>Feedback</title>
+ <para>Find something wrong with this document? (Or perhaps something
+ right?) I would love to hear from you. Please email me at
+ <email>hjk@linutronix.de</email>.</para>
+</sect2>
+</sect1>
+
+<sect1 id="about">
+<?dbhtml filename="about.html"?>
+<title>About UIO</title>
+
+<para>If you use UIO for your card's driver, here's what you get:</para>
+
+<itemizedlist>
+<listitem>
+ <para>only one small kernel module to write and maintain.</para>
+</listitem>
+<listitem>
+ <para>develop the main part of your driver in user space,
+ with all the tools and libraries you're used to.</para>
+</listitem>
+<listitem>
+ <para>bugs in your driver won't crash the kernel.</para>
+</listitem>
+<listitem>
+ <para>updates of your driver can take place without recompiling
+ the kernel.</para>
+</listitem>
+<listitem>
+ <para>if you need to keep some parts of your driver closed source,
+ you can do so without violating the GPL license on the kernel.</para>
+</listitem>
+</itemizedlist>
+
+<sect2 id="how_uio_works">
+<title>How UIO works</title>
+ <para>
+ Each UIO device is accessed through a device file and several
+ sysfs attribute files. The device file will be called
+ <filename>/dev/uio0</filename> for the first device, and
+ <filename>/dev/uio1</filename>, <filename>/dev/uio2</filename>
+ and so on for subsequent devices.
+ </para>
+
+ <para><filename>/dev/uioX</filename> is used to access the
+ address space of the card. Just use
+ <function>mmap()</function> to access registers or RAM
+ locations of your card.
+ </para>
+
+ <para>
+ Interrupts are handled by reading from
+ <filename>/dev/uioX</filename>. A blocking
+ <function>read()</function> from
+ <filename>/dev/uioX</filename> will return as soon as an
+ interrupt occurs. You can also use
+ <function>select()</function> on
+ <filename>/dev/uioX</filename> to wait for an interrupt. The
+ integer value read from <filename>/dev/uioX</filename>
+ represents the total interrupt count. You can use this number
+ to figure out if you missed some interrupts.
+ </para>
+
+ <para>
+ To handle interrupts properly, your custom kernel module can
+ provide its own interrupt handler. It will automatically be
+ called by the built-in handler.
+ </para>
+
+ <para>
+ For cards that don't generate interrupts but need to be
+ polled, there is the possibility to set up a timer that
+ triggers the interrupt handler at configurable time intervals.
+ See <filename>drivers/uio/uio_dummy.c</filename> for an
+ example of this technique.
+ </para>
+
+ <para>
+ Each driver provides attributes that are used to read or write
+ variables. These attributes are accessible through sysfs
+ files. A custom kernel driver module can add its own
+ attributes to the device owned by the uio driver, but not added
+ to the UIO device itself at this time. This might change in the
+ future if it would be found to be useful
+ </para>
+
+ <para>
+ The following standard attributes are provided by the UIO
+ framework:
+ </para>
+<itemizedlist>
+<listitem>
+ <para>
+ <filename>name</filename>: The name of your device. It is
+ recommended to use the name of your kernel module for this.
+ </para>
+</listitem>
+<listitem>
+ <para>
+ <filename>version</filename>: A version string defined by your
+ driver. This allows the user space part of your driver to deal
+ with different versions of the kernel module.
+ </para>
+</listitem>
+<listitem>
+ <para>
+ <filename>event</filename>: The total number of interrupts
+ handled by the driver since the last time the device node was
+ read.
+ </para>
+</listitem>
+</itemizedlist>
+<para>
+ These attributes appear under the
+ <filename>/sys/class/uio/uioX</filename> directory. Please
+ note that this directory might be a symlink, and not a real
+ directory. Any userspace code that accesses it must be able
+ to handle this.
+</para>
+<para>
+ Each UIO device can make one or more memory regions available for
+ memory mapping. This is necessary because some industrial I/O cards
+ require access to more than one PCI memory region in a driver.
+</para>
+<para>
+ Each mapping has its own directory in sysfs, the first mapping
+ appears as <filename>/sys/class/uio/uioX/maps/map0/</filename>.
+ Subsequent mappings create directories <filename>map1/</filename>,
+ <filename>map2/</filename>, and so on. These directories will only
+ appear if the size of the mapping is not 0.
+</para>
+<para>
+ Each <filename>mapX/</filename> directory contains two read-only files
+ that show start address and size of the memory:
+</para>
+<itemizedlist>
+<listitem>
+ <para>
+ <filename>addr</filename>: The address of memory that can be mapped.
+ </para>
+</listitem>
+<listitem>
+ <para>
+ <filename>size</filename>: The size, in bytes, of the memory
+ pointed to by addr.
+ </para>
+</listitem>
+</itemizedlist>
+
+<para>
+ From userspace, the different mappings are distinguished by adjusting
+ the <varname>offset</varname> parameter of the
+ <function>mmap()</function> call. To map the memory of mapping N, you
+ have to use N times the page size as your offset:
+</para>
+<programlisting format="linespecific">
+offset = N * getpagesize();
+</programlisting>
+
+</sect2>
+</sect1>
+
+<sect1 id="using-uio_dummy" xreflabel="Using uio_dummy">
+<?dbhtml filename="using-uio_dummy.html"?>
+<title>Using uio_dummy</title>
+ <para>
+ Well, there is no real use for uio_dummy. Its only purpose is
+ to test most parts of the UIO system (everything except
+ hardware interrupts), and to serve as an example for the
+ kernel module that you will have to write yourself.
+ </para>
+
+<sect2 id="what_uio_dummy_does">
+<title>What uio_dummy does</title>
+ <para>
+ The kernel module <filename>uio_dummy.ko</filename> creates a
+ device that uses a timer to generate periodic interrupts. The
+ interrupt handler does nothing but increment a counter. The
+ driver adds two custom attributes, <varname>count</varname>
+ and <varname>freq</varname>, that appear under
+ <filename>/sys/devices/platform/uio_dummy/</filename>.
+ </para>
+
+ <para>
+ The attribute <varname>count</varname> can be read and
+ written. The associated file
+ <filename>/sys/devices/platform/uio_dummy/count</filename>
+ appears as a normal text file and contains the total number of
+ timer interrupts. If you look at it (e.g. using
+ <function>cat</function>), you'll notice it is slowly counting
+ up.
+ </para>
+
+ <para>
+ The attribute <varname>freq</varname> can be read and written.
+ The content of
+ <filename>/sys/devices/platform/uio_dummy/freq</filename>
+ represents the number of system timer ticks between two timer
+ interrupts. The default value of <varname>freq</varname> is
+ the value of the kernel variable <varname>HZ</varname>, which
+ gives you an interval of one second. Lower values will
+ increase the frequency. Try the following:
+ </para>
+<programlisting format="linespecific">
+cd /sys/devices/platform/uio_dummy/
+echo 100 > freq
+</programlisting>
+ <para>
+ Use <function>cat count</function> to see how the interrupt
+ frequency changes.
+ </para>
+</sect2>
+</sect1>
+
+<sect1 id="custom_kernel_module" xreflabel="Writing your own kernel module">
+<?dbhtml filename="custom_kernel_module.html"?>
+<title>Writing your own kernel module</title>
+ <para>
+ Please have a look at <filename>uio_dummy.c</filename> as an
+ example. The following paragraphs explain the different
+ sections of this file.
+ </para>
+
+<sect2 id="uio_info">
+<title>struct uio_info</title>
+ <para>
+ This structure tells the framework the details of your driver,
+ Some of the members are required, others are optional.
+ </para>
+
+<itemizedlist>
+<listitem>
+<varname>char *name</varname>: Required. The name of your driver as
+it will appear in sysfs. I recommend using the name of your module for this.
+</listitem>
+
+<listitem>
+<varname>char *version</varname>: Required. This string appears in
+<filename>/sys/class/uio/uioX/version</filename>.
+</listitem>
+
+<listitem>
+<varname>struct uio_mem mem[ MAX_UIO_MAPS ]</varname>: Required if you
+have memory that can be mapped with <function>mmap()</function>. For each
+mapping you need to fill one of the <varname>uio_mem</varname> structures.
+See the description below for details.
+</listitem>
+
+<listitem>
+<varname>long irq</varname>: Required. If your hardware generates an
+interrupt, it's your modules task to determine the irq number during
+initialization. If you don't have a hardware generated interrupt but
+want to trigger the interrupt handler in some other way, set
+<varname>irq</varname> to <varname>UIO_IRQ_CUSTOM</varname>. The
+uio_dummy module does this as it triggers the event mechanism in a timer
+routine. If you had no interrupt at all, you could set
+<varname>irq</varname> to <varname>UIO_IRQ_NONE</varname>, though this
+rarely makes sense.
+</listitem>
+
+<listitem>
+<varname>unsigned long irq_flags</varname>: Required if you've set
+<varname>irq</varname> to a hardware interrupt number. The flags given
+here will be used in the call to <function>request_irq()</function>.
+</listitem>
+
+<listitem>
+<varname>int (*mmap)(struct uio_info *info, struct vm_area_struct
+*vma)</varname>: Optional. If you need a special
+<function>mmap()</function> function, you can set it here. If this
+pointer is not NULL, your <function>mmap()</function> will be called
+instead of the built-in one.
+</listitem>
+
+<listitem>
+<varname>int (*open)(struct uio_info *info, struct inode *inode)
+</varname>: Optional. You might want to have your own
+<function>open()</function>, e.g. to enable interrupts only when your
+device is actually used.
+</listitem>
+
+<listitem>
+<varname>int (*release)(struct uio_info *info, struct inode *inode)
+</varname>: Optional. If you define your own
+<function>open()</function>, you will probably also want a custom
+<function>release()</function> function.
+</listitem>
+</itemizedlist>
+
+<para>
+Usually, your device will have one or more memory regions that can be mapped
+to user space. For each region, you have to set up a
+<varname>struct uio_mem</varname> in the <varname>mem[]</varname> array.
+Here's a description of the fields of <varname>struct uio_mem</varname>:
+</para>
+
+<itemizedlist>
+<listitem>
+<varname>int memtype</varname>: Required if the mapping is used. Set this to
+<varname>UIO_MEM_PHYS</varname> if you you have physical memory on your
+card to be mapped. Use <varname>UIO_MEM_LOGICAL</varname> for logical
+memory (e.g. allocated with <function>kmalloc()</function>). There's also
+<varname>UIO_MEM_VIRTUAL</varname> for virtual memory.
+</listitem>
+
+<listitem>
+<varname>unsigned long addr</varname>: Required if the mapping is used.
+Fill in the address of your memory block. This address is the one that
+appears in sysfs.
+</listitem>
+
+<listitem>
+<varname>unsigned long size</varname>: Fill in the size of the
+memory block that <varname>addr</varname> points to. If <varname>size</varname>
+is zero, the mapping is considered unused. Note that you
+<emphasis>must</emphasis> initialize <varname>size</varname> with zero for
+all unused mappings.
+</listitem>
+
+<listitem>
+<varname>void *internal_addr</varname>: If you have to access this memory
+region from within your kernel module, you will want to map it internally by
+using something like <function>ioremap_nocache()</function>. Addresses
+returned by this function can not be mapped to user space, so you must not
+store it in <varname>addr</varname>. Use <varname>internal_addr</varname>
+instead to remember such an address.
+</listitem>
+</itemizedlist>
+
+<para>
+Please do not touch the <varname>kobj</varname> element of
+<varname>struct uio_mem</varname>! It is used by the UIO framework
+to set up sysfs files for this mapping. Simply leave it alone.
+</para>
+</sect2>
+
+<sect2 id="adding_irq_handler">
+<title>Adding an interrupt handler</title>
+ <para>
+ What you need to do in your interrupt handler depends on your
+ hardware and on how you want to handle it. You should try to
+ keep the amount of code in your kernel interrupt handler low.
+ If your hardware requires no action that you
+ <emphasis>have</emphasis> to perform after each interrupt,
+ then your handler can be empty.</para> <para>If, on the other
+ hand, your hardware <emphasis>needs</emphasis> some action to
+ be performed after each interrupt, then you
+ <emphasis>must</emphasis> do it in your kernel module. Note
+ that you cannot rely on the userspace part of your driver.Your
+ userspace program can terminate at any time, possibly leaving
+ your hardware in a state where proper interrupt handling is
+ still required.
+ </para>
+
+ <para>
+ There might also be applications where you want to read data
+ from your hardware at each interrupt and buffer it in a piece
+ of kernel memory you've allocated for that purpose. With this
+ technique you could avoid loss of data if your userspace
+ program misses an interrupt.
+ </para>
+
+ <para>
+ A note on shared interrupts: Your driver should support
+ interrupt sharing whenever this is possible. It is possible if
+ and only if your driver can detect whether your hardware has
+ triggered the interrupt or not. This is usually done by looking
+ at an interrupt status register. If your driver sees that the
+ IRQ bit is actually set, it will perform its actions, and the
+ handler returns IRQ_HANDLED. If the driver detects that it was
+ not your hardware that caused the interrupt, it will do nothing
+ and return IRQ_NONE, allowing the kernel to call the next
+ possible interrupt handler.
+ </para>
+
+ <para>
+ If you decide not to support shared interrupts, your card
+ won't work in computers with no free interrupts. As this
+ frequently happens on the PC platform, you can save yourself a
+ lot of trouble by supporting interrupt sharing.
+ </para>
+</sect2>
+
+</sect1>
+
+<appendix id="app1">
+<title>Further information</title>
+<itemizedlist>
+ <listitem><para>
+ <ulink url="http://www.linutronix.de">
+ Linutronix homepage.</ulink>
+ </para></listitem>
+ <listitem><para>...</para></listitem>
+</itemizedlist>
+</appendix>
+
+</article>
--
1.5.1.2
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