<?xml version="1.0" encoding="UTF-8"?>
  <!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
  "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd" []>
  
  <book id="index">
  <bookinfo>
  <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@hansjkoch.de</email>

  	</address>
      </affiliation>
  </author>

  <copyright>
  	<year>2006-2008</year>
  	<holder>Hans-Jürgen Koch.</holder>
  </copyright>

  <copyright>
  	<year>2009</year>
  	<holder>Red Hat Inc, Michael S. Tsirkin (mst@redhat.com)</holder>
  </copyright>

  
  <legalnotice>
  <para>
  This documentation is Free Software licensed under the terms of the
  GPL version 2.
  </para>
  </legalnotice>

  <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>0.9</revnumber>
  	<date>2009-07-16</date>
  	<authorinitials>mst</authorinitials>
  	<revremark>Added generic pci driver
  		</revremark>
  	</revision>
  	<revision>

  	<revnumber>0.8</revnumber>
  	<date>2008-12-24</date>
  	<authorinitials>hjk</authorinitials>
  	<revremark>Added name attributes in mem and portio sysfs directories.
  		</revremark>
  	</revision>
  	<revision>

  	<revnumber>0.7</revnumber>
  	<date>2008-12-23</date>
  	<authorinitials>hjk</authorinitials>
  	<revremark>Added generic platform drivers and offset attribute.</revremark>
  	</revision>
  	<revision>

  	<revnumber>0.6</revnumber>
  	<date>2008-12-05</date>
  	<authorinitials>hjk</authorinitials>
  	<revremark>Added description of portio sysfs attributes.</revremark>
  	</revision>
  	<revision>

  	<revnumber>0.5</revnumber>
  	<date>2008-05-22</date>
  	<authorinitials>hjk</authorinitials>
  	<revremark>Added description of write() function.</revremark>
  	</revision>
  	<revision>

  	<revnumber>0.4</revnumber>
  	<date>2007-11-26</date>
  	<authorinitials>hjk</authorinitials>
  	<revremark>Removed section about uio_dummy.</revremark>
  	</revision>
  	<revision>

  	<revnumber>0.3</revnumber>
  	<date>2007-04-29</date>
  	<authorinitials>hjk</authorinitials>
  	<revremark>Added section about userspace drivers.</revremark>
  	</revision>
  	<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>
  </bookinfo>
  
  <chapter id="aboutthisdoc">

  <?dbhtml filename="aboutthis.html"?>

  <title>About this document</title>

  <sect1 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@hansjkoch.de</email>.

  </para>
  </sect1>
  
  <sect1 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>

  	<para>
  	Please note that UIO is not an universal driver interface. Devices
  	that are already handled well by other kernel subsystems (like
  	networking or serial or USB) are no candidates for an UIO driver.
  	Hardware that is ideally suited for an UIO driver fulfills all of
  	the following:
  	</para>
  <itemizedlist>
  <listitem>
  	<para>The device has memory that can be mapped. The device can be
  	controlled completely by writing to this memory.</para>
  </listitem>
  <listitem>
  	<para>The device usually generates interrupts.</para>
  </listitem>
  <listitem>
  	<para>The device does not fit into one of the standard kernel
  	subsystems.</para>
  </listitem>
  </itemizedlist>

  </sect1>
  
  <sect1 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>
  </sect1>
  
  <sect1 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@hansjkoch.de</email>.</para>

  </sect1>
  </chapter>
  
  <chapter 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>

  </itemizedlist>
  
  <sect1 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>
  	For some hardware that has more than one interrupt source internally,
  	but not separate IRQ mask and status registers, there might be
  	situations where userspace cannot determine what the interrupt source
  	was if the kernel handler disables them by writing to the chip's IRQ
  	register. In such a case, the kernel has to disable the IRQ completely
  	to leave the chip's register untouched. Now the userspace part can
  	determine the cause of the interrupt, but it cannot re-enable
  	interrupts. Another cornercase is chips where re-enabling interrupts
  	is a read-modify-write operation to a combined IRQ status/acknowledge
  	register. This would be racy if a new interrupt occurred
  	simultaneously.
  	</para>
  	<para>
  	To address these problems, UIO also implements a write() function. It
  	is normally not used and can be ignored for hardware that has only a
  	single interrupt source or has separate IRQ mask and status registers.
  	If you need it, however, a write to <filename>/dev/uioX</filename>
  	will call the <function>irqcontrol()</function> function implemented
  	by the driver. You have to write a 32-bit value that is usually either
  	0 or 1 to disable or enable interrupts. If a driver does not implement
  	<function>irqcontrol()</function>, <function>write()</function> will
  	return with <varname>-ENOSYS</varname>.
  	</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.

  	This interrupt simulation is done by calling
  	<function>uio_event_notify()</function>
  	from the timer's event handler.

  	</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 four read-only files
  	that show attributes of the memory:

  </para>
  <itemizedlist>
  <listitem>
  	<para>

  	<filename>name</filename>: A string identifier for this mapping. This
  	is optional, the string can be empty. Drivers can set this to make it
  	easier for userspace to find the correct mapping.
  	</para>
  </listitem>
  <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>

  <listitem>
  	<para>
  	<filename>offset</filename>: The offset, in bytes, that has to be
  	added to the pointer returned by <function>mmap()</function> to get
  	to the actual device memory. This is important if the device's memory
  	is not page aligned. Remember that pointers returned by
  	<function>mmap()</function> are always page aligned, so it is good
  	style to always add this offset.
  	</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>

  <para>
  	Sometimes there is hardware with memory-like regions that can not be
  	mapped with the technique described here, but there are still ways to
  	access them from userspace. The most common example are x86 ioports.
  	On x86 systems, userspace can access these ioports using
  	<function>ioperm()</function>, <function>iopl()</function>,
  	<function>inb()</function>, <function>outb()</function>, and similar
  	functions.
  </para>
  <para>
  	Since these ioport regions can not be mapped, they will not appear under
  	<filename>/sys/class/uio/uioX/maps/</filename> like the normal memory
  	described above. Without information about the port regions a hardware
  	has to offer, it becomes difficult for the userspace part of the
  	driver to find out which ports belong to which UIO device.
  </para>
  <para>
  	To address this situation, the new directory
  	<filename>/sys/class/uio/uioX/portio/</filename> was added. It only
  	exists if the driver wants to pass information about one or more port
  	regions to userspace. If that is the case, subdirectories named
  	<filename>port0</filename>, <filename>port1</filename>, and so on,
  	will appear underneath
  	<filename>/sys/class/uio/uioX/portio/</filename>.
  </para>
  <para>

  	Each <filename>portX/</filename> directory contains four read-only
  	files that show name, start, size, and type of the port region:

  </para>
  <itemizedlist>
  <listitem>
  	<para>

  	<filename>name</filename>: A string identifier for this port region.
  	The string is optional and can be empty. Drivers can set it to make it
  	easier for userspace to find a certain port region.
  	</para>
  </listitem>
  <listitem>
  	<para>

  	<filename>start</filename>: The first port of this region.
  	</para>
  </listitem>
  <listitem>
  	<para>
  	<filename>size</filename>: The number of ports in this region.
  	</para>
  </listitem>
  <listitem>
  	<para>
  	<filename>porttype</filename>: A string describing the type of port.
  	</para>
  </listitem>
  </itemizedlist>

  </sect1>
  </chapter>

  <chapter 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_cif.c</filename> as an

  	example. The following paragraphs explain the different
  	sections of this file.
  	</para>
  
  <sect1 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><para>

  <varname>const 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.
  </para></listitem>
  
  <listitem><para>

  <varname>const char *version</varname>: Required. This string appears in

  <filename>/sys/class/uio/uioX/version</filename>.
  </para></listitem>
  
  <listitem><para>
  <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.
  </para></listitem>
  
  <listitem><para>

  <varname>struct uio_port port[ MAX_UIO_PORTS_REGIONS ]</varname>: Required
  if you want to pass information about ioports to userspace. For each port
  region you need to fill one of the <varname>uio_port</varname> structures.
  See the description below for details.
  </para></listitem>
  
  <listitem><para>

  <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>.
  If you had no interrupt at all, you could set

  <varname>irq</varname> to <varname>UIO_IRQ_NONE</varname>, though this
  rarely makes sense.
  </para></listitem>
  
  <listitem><para>
  <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>.
  </para></listitem>
  
  <listitem><para>
  <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.
  </para></listitem>
  
  <listitem><para>
  <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.
  </para></listitem>
  
  <listitem><para>
  <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.
  </para></listitem>

  
  <listitem><para>
  <varname>int (*irqcontrol)(struct uio_info *info, s32 irq_on)
  </varname>: Optional. If you need to be able to enable or disable
  interrupts from userspace by writing to <filename>/dev/uioX</filename>,
  you can implement this function. The parameter <varname>irq_on</varname>
  will be 0 to disable interrupts and 1 to enable them.
  </para></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><para>

  <varname>const char *name</varname>: Optional. Set this to help identify
  the memory region, it will show up in the corresponding sysfs node.
  </para></listitem>
  
  <listitem><para>

  <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.
  </para></listitem>
  
  <listitem><para>

  <varname>phys_addr_t 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.
  </para></listitem>
  
  <listitem><para>
  <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.
  </para></listitem>
  
  <listitem><para>
  <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()</function>. Addresses
  returned by this function cannot 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.
  </para></listitem>
  </itemizedlist>
  
  <para>

  Please do not touch the <varname>map</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>

  
  <para>
  Sometimes, your device can have one or more port regions which can not be
  mapped to userspace. But if there are other possibilities for userspace to
  access these ports, it makes sense to make information about the ports
  available in sysfs. For each region, you have to set up a
  <varname>struct uio_port</varname> in the <varname>port[]</varname> array.
  Here's a description of the fields of <varname>struct uio_port</varname>:
  </para>
  
  <itemizedlist>
  <listitem><para>
  <varname>char *porttype</varname>: Required. Set this to one of the predefined
  constants. Use <varname>UIO_PORT_X86</varname> for the ioports found in x86
  architectures.
  </para></listitem>
  
  <listitem><para>
  <varname>unsigned long start</varname>: Required if the port region is used.
  Fill in the number of the first port of this region.
  </para></listitem>
  
  <listitem><para>
  <varname>unsigned long size</varname>: Fill in the number of ports in this
  region. If <varname>size</varname> is zero, the region is considered unused.
  Note that you <emphasis>must</emphasis> initialize <varname>size</varname>
  with zero for all unused regions.
  </para></listitem>
  </itemizedlist>
  
  <para>
  Please do not touch the <varname>portio</varname> element of
  <varname>struct uio_port</varname>! It is used internally by the UIO
  framework to set up sysfs files for this region. Simply leave it alone.
  </para>

  </sect1>
  
  <sect1 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>
  </sect1>

  <sect1 id="using_uio_pdrv">
  <title>Using uio_pdrv for platform devices</title>
  	<para>
  	In many cases, UIO drivers for platform devices can be handled in a
  	generic way. In the same place where you define your
  	<varname>struct platform_device</varname>, you simply also implement
  	your interrupt handler and fill your
  	<varname>struct uio_info</varname>. A pointer to this
  	<varname>struct uio_info</varname> is then used as
  	<varname>platform_data</varname> for your platform device.
  	</para>
  	<para>
  	You also need to set up an array of <varname>struct resource</varname>
  	containing addresses and sizes of your memory mappings. This
  	information is passed to the driver using the
  	<varname>.resource</varname> and <varname>.num_resources</varname>
  	elements of <varname>struct platform_device</varname>.
  	</para>
  	<para>
  	You now have to set the <varname>.name</varname> element of
  	<varname>struct platform_device</varname> to
  	<varname>"uio_pdrv"</varname> to use the generic UIO platform device
  	driver. This driver will fill the <varname>mem[]</varname> array
  	according to the resources given, and register the device.
  	</para>
  	<para>
  	The advantage of this approach is that you only have to edit a file
  	you need to edit anyway. You do not have to create an extra driver.
  	</para>
  </sect1>
  
  <sect1 id="using_uio_pdrv_genirq">
  <title>Using uio_pdrv_genirq for platform devices</title>
  	<para>
  	Especially in embedded devices, you frequently find chips where the
  	irq pin is tied to its own dedicated interrupt line. In such cases,
  	where you can be really sure the interrupt is not shared, we can take
  	the concept of <varname>uio_pdrv</varname> one step further and use a
  	generic interrupt handler. That's what
  	<varname>uio_pdrv_genirq</varname> does.
  	</para>
  	<para>
  	The setup for this driver is the same as described above for
  	<varname>uio_pdrv</varname>, except that you do not implement an
  	interrupt handler. The <varname>.handler</varname> element of
  	<varname>struct uio_info</varname> must remain
  	<varname>NULL</varname>. The  <varname>.irq_flags</varname> element
  	must not contain <varname>IRQF_SHARED</varname>.
  	</para>
  	<para>
  	You will set the <varname>.name</varname> element of
  	<varname>struct platform_device</varname> to
  	<varname>"uio_pdrv_genirq"</varname> to use this driver.
  	</para>
  	<para>
  	The generic interrupt handler of <varname>uio_pdrv_genirq</varname>
  	will simply disable the interrupt line using
  	<function>disable_irq_nosync()</function>. After doing its work,
  	userspace can reenable the interrupt by writing 0x00000001 to the UIO
  	device file. The driver already implements an
  	<function>irq_control()</function> to make this possible, you must not
  	implement your own.
  	</para>
  	<para>
  	Using <varname>uio_pdrv_genirq</varname> not only saves a few lines of
  	interrupt handler code. You also do not need to know anything about
  	the chip's internal registers to create the kernel part of the driver.
  	All you need to know is the irq number of the pin the chip is
  	connected to.
  	</para>
  </sect1>

  </chapter>
  
  <chapter id="userspace_driver" xreflabel="Writing a driver in user space">
  <?dbhtml filename="userspace_driver.html"?>
  <title>Writing a driver in userspace</title>
  	<para>
  	Once you have a working kernel module for your hardware, you can
  	write the userspace part of your driver. You don't need any special
  	libraries, your driver can be written in any reasonable language,
  	you can use floating point numbers and so on. In short, you can
  	use all the tools and libraries you'd normally use for writing a
  	userspace application.
  	</para>
  
  <sect1 id="getting_uio_information">
  <title>Getting information about your UIO device</title>
  	<para>
  	Information about all UIO devices is available in sysfs. The
  	first thing you should do in your driver is check
  	<varname>name</varname> and <varname>version</varname> to
  	make sure your talking to the right device and that its kernel
  	driver has the version you expect.
  	</para>
  	<para>
  	You should also make sure that the memory mapping you need
  	exists and has the size you expect.
  	</para>
  	<para>
  	There is a tool called <varname>lsuio</varname> that lists
  	UIO devices and their attributes. It is available here:
  	</para>
  	<para>
  	<ulink url="http://www.osadl.org/projects/downloads/UIO/user/">
  		http://www.osadl.org/projects/downloads/UIO/user/</ulink>
  	</para>
  	<para>
  	With <varname>lsuio</varname> you can quickly check if your
  	kernel module is loaded and which attributes it exports.
  	Have a look at the manpage for details.
  	</para>
  	<para>
  	The source code of <varname>lsuio</varname> can serve as an
  	example for getting information about an UIO device.
  	The file <filename>uio_helper.c</filename> contains a lot of
  	functions you could use in your userspace driver code.
  	</para>
  </sect1>
  
  <sect1 id="mmap_device_memory">
  <title>mmap() device memory</title>
  	<para>
  	After you made sure you've got the right device with the
  	memory mappings you need, all you have to do is to call
  	<function>mmap()</function> to map the device's memory
  	to userspace.
  	</para>
  	<para>
  	The parameter <varname>offset</varname> of the
  	<function>mmap()</function> call has a special meaning
  	for UIO devices: It is used to select which mapping of
  	your device you want to map. 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>
  	<para>
  	N starts from zero, so if you've got only one memory
  	range to map, set <varname>offset = 0</varname>.
  	A drawback of this technique is that memory is always
  	mapped beginning with its start address.
  	</para>
  </sect1>
  
  <sect1 id="wait_for_interrupts">
  <title>Waiting for interrupts</title>
  	<para>
  	After you successfully mapped your devices memory, you
  	can access it like an ordinary array. Usually, you will
  	perform some initialization. After that, your hardware
  	starts working and will generate an interrupt as soon
  	as it's finished, has some data available, or needs your

  	attention because an error occurred.

  	</para>
  	<para>
  	<filename>/dev/uioX</filename> is a read-only file. A
  	<function>read()</function> will always block until an
  	interrupt occurs. There is only one legal value for the
  	<varname>count</varname> parameter of
  	<function>read()</function>, and that is the size of a
  	signed 32 bit integer (4). Any other value for
  	<varname>count</varname> causes <function>read()</function>
  	to fail. The signed 32 bit integer read is the interrupt
  	count of your device. If the value is one more than the value
  	you read the last time, everything is OK. If the difference
  	is greater than one, you missed interrupts.
  	</para>
  	<para>
  	You can also use <function>select()</function> on
  	<filename>/dev/uioX</filename>.
  	</para>
  </sect1>
  
  </chapter>

  <chapter id="uio_pci_generic" xreflabel="Using Generic driver for PCI cards">
  <?dbhtml filename="uio_pci_generic.html"?>
  <title>Generic PCI UIO driver</title>
  	<para>
  	The generic driver is a kernel module named uio_pci_generic.
  	It can work with any device compliant to PCI 2.3 (circa 2002) and
  	any compliant PCI Express device. Using this, you only need to
          write the userspace driver, removing the need to write
          a hardware-specific kernel module.
  	</para>
  
  <sect1 id="uio_pci_generic_binding">
  <title>Making the driver recognize the device</title>
  	<para>
  Since the driver does not declare any device ids, it will not get loaded
  automatically and will not automatically bind to any devices, you must load it
  and allocate id to the driver yourself. For example:
  	<programlisting>
   modprobe uio_pci_generic
   echo &quot;8086 10f5&quot; &gt; /sys/bus/pci/drivers/uio_pci_generic/new_id
  	</programlisting>
  	</para>
  	<para>
  If there already is a hardware specific kernel driver for your device, the
  generic driver still won't bind to it, in this case if you want to use the
  generic driver (why would you?) you'll have to manually unbind the hardware
  specific driver and bind the generic driver, like this:
  	<programlisting>
      echo -n 0000:00:19.0 &gt; /sys/bus/pci/drivers/e1000e/unbind
      echo -n 0000:00:19.0 &gt; /sys/bus/pci/drivers/uio_pci_generic/bind
  	</programlisting>
  	</para>
  	<para>
  You can verify that the device has been bound to the driver
  by looking for it in sysfs, for example like the following:
  	<programlisting>
      ls -l /sys/bus/pci/devices/0000:00:19.0/driver
  	</programlisting>
  Which if successful should print
  	<programlisting>
    .../0000:00:19.0/driver -&gt; ../../../bus/pci/drivers/uio_pci_generic
  	</programlisting>
  Note that the generic driver will not bind to old PCI 2.2 devices.
  If binding the device failed, run the following command:
  	<programlisting>
    dmesg
  	</programlisting>
  and look in the output for failure reasons
  	</para>
  </sect1>
  
  <sect1 id="uio_pci_generic_internals">
  <title>Things to know about uio_pci_generic</title>
  	<para>
  Interrupts are handled using the Interrupt Disable bit in the PCI command
  register and Interrupt Status bit in the PCI status register.  All devices
  compliant to PCI 2.3 (circa 2002) and all compliant PCI Express devices should
  support these bits.  uio_pci_generic detects this support, and won't bind to
  devices which do not support the Interrupt Disable Bit in the command register.
  	</para>
  	<para>
  On each interrupt, uio_pci_generic sets the Interrupt Disable bit.
  This prevents the device from generating further interrupts
  until the bit is cleared. The userspace driver should clear this
  bit before blocking and waiting for more interrupts.
  	</para>
  </sect1>
  <sect1 id="uio_pci_generic_userspace">
  <title>Writing userspace driver using uio_pci_generic</title>
  	<para>
  Userspace driver can use pci sysfs interface, or the
  libpci libray that wraps it, to talk to the device and to
  re-enable interrupts by writing to the command register.
  	</para>
  </sect1>
  <sect1 id="uio_pci_generic_example">
  <title>Example code using uio_pci_generic</title>
  	<para>
  Here is some sample userspace driver code using uio_pci_generic:
  <programlisting>
  #include &lt;stdlib.h&gt;
  #include &lt;stdio.h&gt;
  #include &lt;unistd.h&gt;
  #include &lt;sys/types.h&gt;
  #include &lt;sys/stat.h&gt;
  #include &lt;fcntl.h&gt;
  #include &lt;errno.h&gt;
  
  int main()
  {
  	int uiofd;
  	int configfd;
  	int err;
  	int i;
  	unsigned icount;
  	unsigned char command_high;
  
  	uiofd = open(&quot;/dev/uio0&quot;, O_RDONLY);
  	if (uiofd &lt; 0) {
  		perror(&quot;uio open:&quot;);
  		return errno;
  	}
  	configfd = open(&quot;/sys/class/uio/uio0/device/config&quot;, O_RDWR);
  	if (uiofd &lt; 0) {
  		perror(&quot;config open:&quot;);
  		return errno;
  	}
  
  	/* Read and cache command value */
  	err = pread(configfd, &amp;command_high, 1, 5);
  	if (err != 1) {
  		perror(&quot;command config read:&quot;);
  		return errno;
  	}
  	command_high &amp;= ~0x4;
  
  	for(i = 0;; ++i) {
  		/* Print out a message, for debugging. */
  		if (i == 0)
  			fprintf(stderr, &quot;Started uio test driver.
  &quot;);
  		else
  			fprintf(stderr, &quot;Interrupts: %d
  &quot;, icount);
  
  		/****************************************/
  		/* Here we got an interrupt from the
  		   device. Do something to it. */
  		/****************************************/
  
  		/* Re-enable interrupts. */
  		err = pwrite(configfd, &amp;command_high, 1, 5);
  		if (err != 1) {
  			perror(&quot;config write:&quot;);
  			break;
  		}
  
  		/* Wait for next interrupt. */
  		err = read(uiofd, &amp;icount, 4);
  		if (err != 4) {
  			perror(&quot;uio read:&quot;);
  			break;
  		}
  
  	}
  	return errno;
  }
  
  </programlisting>
  	</para>
  </sect1>
  
  </chapter>

  <appendix id="app1">
  <title>Further information</title>
  <itemizedlist>
  	<listitem><para>
  			<ulink url="http://www.osadl.org">
  				OSADL homepage.</ulink>
  		</para></listitem>
  	<listitem><para>
  		<ulink url="http://www.linutronix.de">
  		 Linutronix homepage.</ulink>
  		</para></listitem>
  </itemizedlist>
  </appendix>
  
  </book>