Documentation/DocBook/uio-howto.tmpl

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   2 <!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
   3 "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd" []>
   4
   5 <book id="index">
   6 <bookinfo>
   7 <title>The Userspace I/O HOWTO</title>
   8
   9 <author>
  10       <firstname>Hans-Jürgen</firstname>
  11       <surname>Koch</surname>
  12       <authorblurb><para>Linux developer, Linutronix</para></authorblurb>
  13         <affiliation>
  14         <orgname>
  15                 <ulink url="http://www.linutronix.de">Linutronix</ulink>
  16         </orgname>
  17
  18         <address>
  19            <email>hjk@linutronix.de</email>
  20         </address>
  21     </affiliation>
  22 </author>
  23
  24 <pubdate>2006-12-11</pubdate>
  25
  26 <abstract>
  27         <para>This HOWTO describes concept and usage of Linux kernel's
  28                 Userspace I/O system.</para>
  29 </abstract>
  30
  31 <revhistory>
  32         <revision>
  33         <revnumber>0.3</revnumber>
  34         <date>2007-04-29</date>
  35         <authorinitials>hjk</authorinitials>
  36         <revremark>Added section about userspace drivers.</revremark>
  37         </revision>
  38         <revision>
  39         <revnumber>0.2</revnumber>
  40         <date>2007-02-13</date>
  41         <authorinitials>hjk</authorinitials>
  42         <revremark>Update after multiple mappings were added.</revremark>
  43         </revision>
  44         <revision>
  45         <revnumber>0.1</revnumber>
  46         <date>2006-12-11</date>
  47         <authorinitials>hjk</authorinitials>
  48         <revremark>First draft.</revremark>
  49         </revision>
  50 </revhistory>
  51 </bookinfo>
  52
  53 <chapter id="aboutthisdoc">
  54 <?dbhtml filename="about.html"?>
  55 <title>About this document</title>
  56
  57 <sect1 id="copyright">
  58 <?dbhtml filename="copyright.html"?>
  59 <title>Copyright and License</title>
  60 <para>
  61       Copyright (c) 2006 by Hans-Jürgen Koch.</para>
  62 <para>
  63 This documentation is Free Software licensed under the terms of the
  64 GPL version 2.
  65 </para>
  66 </sect1>
  67
  68 <sect1 id="translations">
  69 <?dbhtml filename="translations.html"?>
  70 <title>Translations</title>
  71
  72 <para>If you know of any translations for this document, or you are
  73 interested in translating it, please email me
  74 <email>hjk@linutronix.de</email>.
  75 </para>
  76 </sect1>
  77
  78 <sect1 id="preface">
  79 <title>Preface</title>
  80         <para>
  81         For many types of devices, creating a Linux kernel driver is
  82         overkill.  All that is really needed is some way to handle an
  83         interrupt and provide access to the memory space of the
  84         device.  The logic of controlling the device does not
  85         necessarily have to be within the kernel, as the device does
  86         not need to take advantage of any of other resources that the
  87         kernel provides.  One such common class of devices that are
  88         like this are for industrial I/O cards.
  89         </para>
  90         <para>
  91         To address this situation, the userspace I/O system (UIO) was
  92         designed.  For typical industrial I/O cards, only a very small
  93         kernel module is needed. The main part of the driver will run in
  94         user space. This simplifies development and reduces the risk of
  95         serious bugs within a kernel module.
  96         </para>
  97 </sect1>
  98
  99 <sect1 id="thanks">
 100 <title>Acknowledgments</title>
 101         <para>I'd like to thank Thomas Gleixner and Benedikt Spranger of
 102         Linutronix, who have not only written most of the UIO code, but also
 103         helped greatly writing this HOWTO by giving me all kinds of background
 104         information.</para>
 105 </sect1>
 106
 107 <sect1 id="feedback">
 108 <title>Feedback</title>
 109         <para>Find something wrong with this document? (Or perhaps something
 110         right?) I would love to hear from you. Please email me at
 111         <email>hjk@linutronix.de</email>.</para>
 112 </sect1>
 113 </chapter>
 114
 115 <chapter id="about">
 116 <?dbhtml filename="about.html"?>
 117 <title>About UIO</title>
 118
 119 <para>If you use UIO for your card's driver, here's what you get:</para>
 120
 121 <itemizedlist>
 122 <listitem>
 123         <para>only one small kernel module to write and maintain.</para>
 124 </listitem>
 125 <listitem>
 126         <para>develop the main part of your driver in user space,
 127         with all the tools and libraries you're used to.</para>
 128 </listitem>
 129 <listitem>
 130         <para>bugs in your driver won't crash the kernel.</para>
 131 </listitem>
 132 <listitem>
 133         <para>updates of your driver can take place without recompiling
 134         the kernel.</para>
 135 </listitem>
 136 </itemizedlist>
 137
 138 <sect1 id="how_uio_works">
 139 <title>How UIO works</title>
 140         <para>
 141         Each UIO device is accessed through a device file and several
 142         sysfs attribute files. The device file will be called
 143         <filename>/dev/uio0</filename> for the first device, and
 144         <filename>/dev/uio1</filename>, <filename>/dev/uio2</filename>
 145         and so on for subsequent devices.
 146         </para>
 147
 148         <para><filename>/dev/uioX</filename> is used to access the
 149         address space of the card. Just use
 150         <function>mmap()</function> to access registers or RAM
 151         locations of your card.
 152         </para>
 153
 154         <para>
 155         Interrupts are handled by reading from
 156         <filename>/dev/uioX</filename>. A blocking
 157         <function>read()</function> from
 158         <filename>/dev/uioX</filename> will return as soon as an
 159         interrupt occurs. You can also use
 160         <function>select()</function> on
 161         <filename>/dev/uioX</filename> to wait for an interrupt. The
 162         integer value read from <filename>/dev/uioX</filename>
 163         represents the total interrupt count. You can use this number
 164         to figure out if you missed some interrupts.
 165         </para>
 166
 167         <para>
 168         To handle interrupts properly, your custom kernel module can
 169         provide its own interrupt handler. It will automatically be
 170         called by the built-in handler.
 171         </para>
 172
 173         <para>
 174         For cards that don't generate interrupts but need to be
 175         polled, there is the possibility to set up a timer that
 176         triggers the interrupt handler at configurable time intervals.
 177         See <filename>drivers/uio/uio_dummy.c</filename> for an
 178         example of this technique.
 179         </para>
 180
 181         <para>
 182         Each driver provides attributes that are used to read or write
 183         variables. These attributes are accessible through sysfs
 184         files.  A custom kernel driver module can add its own
 185         attributes to the device owned by the uio driver, but not added
 186         to the UIO device itself at this time.  This might change in the
 187         future if it would be found to be useful.
 188         </para>
 189
 190         <para>
 191         The following standard attributes are provided by the UIO
 192         framework:
 193         </para>
 194 <itemizedlist>
 195 <listitem>
 196         <para>
 197         <filename>name</filename>: The name of your device. It is
 198         recommended to use the name of your kernel module for this.
 199         </para>
 200 </listitem>
 201 <listitem>
 202         <para>
 203         <filename>version</filename>: A version string defined by your
 204         driver. This allows the user space part of your driver to deal
 205         with different versions of the kernel module.
 206         </para>
 207 </listitem>
 208 <listitem>
 209         <para>
 210         <filename>event</filename>: The total number of interrupts
 211         handled by the driver since the last time the device node was
 212         read.
 213         </para>
 214 </listitem>
 215 </itemizedlist>
 216 <para>
 217         These attributes appear under the
 218         <filename>/sys/class/uio/uioX</filename> directory.  Please
 219         note that this directory might be a symlink, and not a real
 220         directory.  Any userspace code that accesses it must be able
 221         to handle this.
 222 </para>
 223 <para>
 224         Each UIO device can make one or more memory regions available for
 225         memory mapping. This is necessary because some industrial I/O cards
 226         require access to more than one PCI memory region in a driver.
 227 </para>
 228 <para>
 229         Each mapping has its own directory in sysfs, the first mapping
 230         appears as <filename>/sys/class/uio/uioX/maps/map0/</filename>.
 231         Subsequent mappings create directories <filename>map1/</filename>,
 232         <filename>map2/</filename>, and so on. These directories will only
 233         appear if the size of the mapping is not 0.
 234 </para>
 235 <para>
 236         Each <filename>mapX/</filename> directory contains two read-only files
 237         that show start address and size of the memory:
 238 </para>
 239 <itemizedlist>
 240 <listitem>
 241         <para>
 242         <filename>addr</filename>: The address of memory that can be mapped.
 243         </para>
 244 </listitem>
 245 <listitem>
 246         <para>
 247         <filename>size</filename>: The size, in bytes, of the memory
 248         pointed to by addr.
 249         </para>
 250 </listitem>
 251 </itemizedlist>
 252
 253 <para>
 254         From userspace, the different mappings are distinguished by adjusting
 255         the <varname>offset</varname> parameter of the
 256         <function>mmap()</function> call. To map the memory of mapping N, you
 257         have to use N times the page size as your offset:
 258 </para>
 259 <programlisting format="linespecific">
 260 offset = N * getpagesize();
 261 </programlisting>
 262
 263 </sect1>
 264 </chapter>
 265
 266 <chapter id="using-uio_dummy" xreflabel="Using uio_dummy">
 267 <?dbhtml filename="using-uio_dummy.html"?>
 268 <title>Using uio_dummy</title>
 269         <para>
 270         Well, there is no real use for uio_dummy. Its only purpose is
 271         to test most parts of the UIO system (everything except
 272         hardware interrupts), and to serve as an example for the
 273         kernel module that you will have to write yourself.
 274         </para>
 275
 276 <sect1 id="what_uio_dummy_does">
 277 <title>What uio_dummy does</title>
 278         <para>
 279         The kernel module <filename>uio_dummy.ko</filename> creates a
 280         device that uses a timer to generate periodic interrupts. The
 281         interrupt handler does nothing but increment a counter. The
 282         driver adds two custom attributes, <varname>count</varname>
 283         and <varname>freq</varname>, that appear under
 284         <filename>/sys/devices/platform/uio_dummy/</filename>.
 285         </para>
 286
 287         <para>
 288         The attribute <varname>count</varname> can be read and
 289         written.  The associated file
 290         <filename>/sys/devices/platform/uio_dummy/count</filename>
 291         appears as a normal text file and contains the total number of
 292         timer interrupts. If you look at it (e.g. using
 293         <function>cat</function>), you'll notice it is slowly counting
 294         up.
 295         </para>
 296
 297         <para>
 298         The attribute <varname>freq</varname> can be read and written.
 299         The content of
 300         <filename>/sys/devices/platform/uio_dummy/freq</filename>
 301         represents the number of system timer ticks between two timer
 302         interrupts. The default value of <varname>freq</varname> is
 303         the value of the kernel variable <varname>HZ</varname>, which
 304         gives you an interval of one second. Lower values will
 305         increase the frequency. Try the following:
 306         </para>
 307 <programlisting format="linespecific">
 308 cd /sys/devices/platform/uio_dummy/
 309 echo 100 > freq
 310 </programlisting>
 311         <para>
 312         Use <function>cat count</function> to see how the interrupt
 313         frequency changes.
 314         </para>
 315 </sect1>
 316 </chapter>
 317
 318 <chapter id="custom_kernel_module" xreflabel="Writing your own kernel module">
 319 <?dbhtml filename="custom_kernel_module.html"?>
 320 <title>Writing your own kernel module</title>
 321         <para>
 322         Please have a look at <filename>uio_dummy.c</filename> as an
 323         example. The following paragraphs explain the different
 324         sections of this file.
 325         </para>
 326
 327 <sect1 id="uio_info">
 328 <title>struct uio_info</title>
 329         <para>
 330         This structure tells the framework the details of your driver,
 331         Some of the members are required, others are optional.
 332         </para>
 333
 334 <itemizedlist>
 335 <listitem><para>
 336 <varname>char *name</varname>: Required. The name of your driver as
 337 it will appear in sysfs. I recommend using the name of your module for this.
 338 </para></listitem>
 339
 340 <listitem><para>
 341 <varname>char *version</varname>: Required. This string appears in
 342 <filename>/sys/class/uio/uioX/version</filename>.
 343 </para></listitem>
 344
 345 <listitem><para>
 346 <varname>struct uio_mem mem[ MAX_UIO_MAPS ]</varname>: Required if you
 347 have memory that can be mapped with <function>mmap()</function>. For each
 348 mapping you need to fill one of the <varname>uio_mem</varname> structures.
 349 See the description below for details.
 350 </para></listitem>
 351
 352 <listitem><para>
 353 <varname>long irq</varname>: Required. If your hardware generates an
 354 interrupt, it's your modules task to determine the irq number during
 355 initialization. If you don't have a hardware generated interrupt but
 356 want to trigger the interrupt handler in some other way, set
 357 <varname>irq</varname> to <varname>UIO_IRQ_CUSTOM</varname>. The
 358 uio_dummy module does this as it triggers the event mechanism in a timer
 359 routine. If you had no interrupt at all, you could set
 360 <varname>irq</varname> to <varname>UIO_IRQ_NONE</varname>, though this
 361 rarely makes sense.
 362 </para></listitem>
 363
 364 <listitem><para>
 365 <varname>unsigned long irq_flags</varname>: Required if you've set
 366 <varname>irq</varname> to a hardware interrupt number. The flags given
 367 here will be used in the call to <function>request_irq()</function>.
 368 </para></listitem>
 369
 370 <listitem><para>
 371 <varname>int (*mmap)(struct uio_info *info, struct vm_area_struct
 372 *vma)</varname>: Optional. If you need a special
 373 <function>mmap()</function> function, you can set it here. If this
 374 pointer is not NULL, your <function>mmap()</function> will be called
 375 instead of the built-in one.
 376 </para></listitem>
 377
 378 <listitem><para>
 379 <varname>int (*open)(struct uio_info *info, struct inode *inode)
 380 </varname>: Optional. You might want to have your own
 381 <function>open()</function>, e.g. to enable interrupts only when your
 382 device is actually used.
 383 </para></listitem>
 384
 385 <listitem><para>
 386 <varname>int (*release)(struct uio_info *info, struct inode *inode)
 387 </varname>: Optional. If you define your own
 388 <function>open()</function>, you will probably also want a custom
 389 <function>release()</function> function.
 390 </para></listitem>
 391 </itemizedlist>
 392
 393 <para>
 394 Usually, your device will have one or more memory regions that can be mapped
 395 to user space. For each region, you have to set up a
 396 <varname>struct uio_mem</varname> in the <varname>mem[]</varname> array.
 397 Here's a description of the fields of <varname>struct uio_mem</varname>:
 398 </para>
 399
 400 <itemizedlist>
 401 <listitem><para>
 402 <varname>int memtype</varname>: Required if the mapping is used. Set this to
 403 <varname>UIO_MEM_PHYS</varname> if you you have physical memory on your
 404 card to be mapped. Use <varname>UIO_MEM_LOGICAL</varname> for logical
 405 memory (e.g. allocated with <function>kmalloc()</function>). There's also
 406 <varname>UIO_MEM_VIRTUAL</varname> for virtual memory.
 407 </para></listitem>
 408
 409 <listitem><para>
 410 <varname>unsigned long addr</varname>: Required if the mapping is used.
 411 Fill in the address of your memory block. This address is the one that
 412 appears in sysfs.
 413 </para></listitem>
 414
 415 <listitem><para>
 416 <varname>unsigned long size</varname>: Fill in the size of the
 417 memory block that <varname>addr</varname> points to. If <varname>size</varname>
 418 is zero, the mapping is considered unused. Note that you
 419 <emphasis>must</emphasis> initialize <varname>size</varname> with zero for
 420 all unused mappings.
 421 </para></listitem>
 422
 423 <listitem><para>
 424 <varname>void *internal_addr</varname>: If you have to access this memory
 425 region from within your kernel module, you will want to map it internally by
 426 using something like <function>ioremap()</function>. Addresses
 427 returned by this function cannot be mapped to user space, so you must not
 428 store it in <varname>addr</varname>. Use <varname>internal_addr</varname>
 429 instead to remember such an address.
 430 </para></listitem>
 431 </itemizedlist>
 432
 433 <para>
 434 Please do not touch the <varname>kobj</varname> element of
 435 <varname>struct uio_mem</varname>! It is used by the UIO framework
 436 to set up sysfs files for this mapping. Simply leave it alone.
 437 </para>
 438 </sect1>
 439
 440 <sect1 id="adding_irq_handler">
 441 <title>Adding an interrupt handler</title>
 442         <para>
 443         What you need to do in your interrupt handler depends on your
 444         hardware and on how you want to handle it. You should try to
 445         keep the amount of code in your kernel interrupt handler low.
 446         If your hardware requires no action that you
 447         <emphasis>have</emphasis> to perform after each interrupt,
 448         then your handler can be empty.</para> <para>If, on the other
 449         hand, your hardware <emphasis>needs</emphasis> some action to
 450         be performed after each interrupt, then you
 451         <emphasis>must</emphasis> do it in your kernel module. Note
 452         that you cannot rely on the userspace part of your driver. Your
 453         userspace program can terminate at any time, possibly leaving
 454         your hardware in a state where proper interrupt handling is
 455         still required.
 456         </para>
 457
 458         <para>
 459         There might also be applications where you want to read data
 460         from your hardware at each interrupt and buffer it in a piece
 461         of kernel memory you've allocated for that purpose.  With this
 462         technique you could avoid loss of data if your userspace
 463         program misses an interrupt.
 464         </para>
 465
 466         <para>
 467         A note on shared interrupts: Your driver should support
 468         interrupt sharing whenever this is possible. It is possible if
 469         and only if your driver can detect whether your hardware has
 470         triggered the interrupt or not. This is usually done by looking
 471         at an interrupt status register. If your driver sees that the
 472         IRQ bit is actually set, it will perform its actions, and the
 473         handler returns IRQ_HANDLED. If the driver detects that it was
 474         not your hardware that caused the interrupt, it will do nothing
 475         and return IRQ_NONE, allowing the kernel to call the next
 476         possible interrupt handler.
 477         </para>
 478
 479         <para>
 480         If you decide not to support shared interrupts, your card
 481         won't work in computers with no free interrupts. As this
 482         frequently happens on the PC platform, you can save yourself a
 483         lot of trouble by supporting interrupt sharing.
 484         </para>
 485 </sect1>
 486
 487 </chapter>
 488
 489 <chapter id="userspace_driver" xreflabel="Writing a driver in user space">
 490 <?dbhtml filename="userspace_driver.html"?>
 491 <title>Writing a driver in userspace</title>
 492         <para>
 493         Once you have a working kernel module for your hardware, you can
 494         write the userspace part of your driver. You don't need any special
 495         libraries, your driver can be written in any reasonable language,
 496         you can use floating point numbers and so on. In short, you can
 497         use all the tools and libraries you'd normally use for writing a
 498         userspace application.
 499         </para>
 500
 501 <sect1 id="getting_uio_information">
 502 <title>Getting information about your UIO device</title>
 503         <para>
 504         Information about all UIO devices is available in sysfs. The
 505         first thing you should do in your driver is check
 506         <varname>name</varname> and <varname>version</varname> to
 507         make sure your talking to the right device and that its kernel
 508         driver has the version you expect.
 509         </para>
 510         <para>
 511         You should also make sure that the memory mapping you need
 512         exists and has the size you expect.
 513         </para>
 514         <para>
 515         There is a tool called <varname>lsuio</varname> that lists
 516         UIO devices and their attributes. It is available here:
 517         </para>
 518         <para>
 519         <ulink url="http://www.osadl.org/projects/downloads/UIO/user/">
 520                 http://www.osadl.org/projects/downloads/UIO/user/</ulink>
 521         </para>
 522         <para>
 523         With <varname>lsuio</varname> you can quickly check if your
 524         kernel module is loaded and which attributes it exports.
 525         Have a look at the manpage for details.
 526         </para>
 527         <para>
 528         The source code of <varname>lsuio</varname> can serve as an
 529         example for getting information about an UIO device.
 530         The file <filename>uio_helper.c</filename> contains a lot of
 531         functions you could use in your userspace driver code.
 532         </para>
 533 </sect1>
 534
 535 <sect1 id="mmap_device_memory">
 536 <title>mmap() device memory</title>
 537         <para>
 538         After you made sure you've got the right device with the
 539         memory mappings you need, all you have to do is to call
 540         <function>mmap()</function> to map the device's memory
 541         to userspace.
 542         </para>
 543         <para>
 544         The parameter <varname>offset</varname> of the
 545         <function>mmap()</function> call has a special meaning
 546         for UIO devices: It is used to select which mapping of
 547         your device you want to map. To map the memory of
 548         mapping N, you have to use N times the page size as
 549         your offset:
 550         </para>
 551 <programlisting format="linespecific">
 552         offset = N * getpagesize();
 553 </programlisting>
 554         <para>
 555         N starts from zero, so if you've got only one memory
 556         range to map, set <varname>offset = 0</varname>.
 557         A drawback of this technique is that memory is always
 558         mapped beginning with its start address.
 559         </para>
 560 </sect1>
 561
 562 <sect1 id="wait_for_interrupts">
 563 <title>Waiting for interrupts</title>
 564         <para>
 565         After you successfully mapped your devices memory, you
 566         can access it like an ordinary array. Usually, you will
 567         perform some initialization. After that, your hardware
 568         starts working and will generate an interrupt as soon
 569         as it's finished, has some data available, or needs your
 570         attention because an error occured.
 571         </para>
 572         <para>
 573         <filename>/dev/uioX</filename> is a read-only file. A
 574         <function>read()</function> will always block until an
 575         interrupt occurs. There is only one legal value for the
 576         <varname>count</varname> parameter of
 577         <function>read()</function>, and that is the size of a
 578         signed 32 bit integer (4). Any other value for
 579         <varname>count</varname> causes <function>read()</function>
 580         to fail. The signed 32 bit integer read is the interrupt
 581         count of your device. If the value is one more than the value
 582         you read the last time, everything is OK. If the difference
 583         is greater than one, you missed interrupts.
 584         </para>
 585         <para>
 586         You can also use <function>select()</function> on
 587         <filename>/dev/uioX</filename>.
 588         </para>
 589 </sect1>
 590
 591 </chapter>
 592
 593 <appendix id="app1">
 594 <title>Further information</title>
 595 <itemizedlist>
 596         <listitem><para>
 597                         <ulink url="http://www.osadl.org">
 598                                 OSADL homepage.</ulink>
 599                 </para></listitem>
 600         <listitem><para>
 601                 <ulink url="http://www.linutronix.de">
 602                  Linutronix homepage.</ulink>
 603                 </para></listitem>
 604 </itemizedlist>
 605 </appendix>
 606
 607 </book>