Documentation/DocBook/deviceiobook.tmpl

   1 <?xml version="1.0" encoding="UTF-8"?>
   2 <!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
   3         "http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" []>
   4
   5 <book id="DoingIO">
   6  <bookinfo>
   7   <title>Bus-Independent Device Accesses</title>
   8
   9   <authorgroup>
  10    <author>
  11     <firstname>Matthew</firstname>
  12     <surname>Wilcox</surname>
  13     <affiliation>
  14      <address>
  15       <email>matthew@wil.cx</email>
  16      </address>
  17     </affiliation>
  18    </author>
  19   </authorgroup>
  20
  21   <authorgroup>
  22    <author>
  23     <firstname>Alan</firstname>
  24     <surname>Cox</surname>
  25     <affiliation>
  26      <address>
  27       <email>alan@lxorguk.ukuu.org.uk</email>
  28      </address>
  29     </affiliation>
  30    </author>
  31   </authorgroup>
  32
  33   <copyright>
  34    <year>2001</year>
  35    <holder>Matthew Wilcox</holder>
  36   </copyright>
  37
  38   <legalnotice>
  39    <para>
  40      This documentation is free software; you can redistribute
  41      it and/or modify it under the terms of the GNU General Public
  42      License as published by the Free Software Foundation; either
  43      version 2 of the License, or (at your option) any later
  44      version.
  45    </para>
  46
  47    <para>
  48      This program is distributed in the hope that it will be
  49      useful, but WITHOUT ANY WARRANTY; without even the implied
  50      warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
  51      See the GNU General Public License for more details.
  52    </para>
  53
  54    <para>
  55      You should have received a copy of the GNU General Public
  56      License along with this program; if not, write to the Free
  57      Software Foundation, Inc., 59 Temple Place, Suite 330, Boston,
  58      MA 02111-1307 USA
  59    </para>
  60
  61    <para>
  62      For more details see the file COPYING in the source
  63      distribution of Linux.
  64    </para>
  65   </legalnotice>
  66  </bookinfo>
  67
  68 <toc></toc>
  69
  70   <chapter id="intro">
  71       <title>Introduction</title>
  72   <para>
  73         Linux provides an API which abstracts performing IO across all busses
  74         and devices, allowing device drivers to be written independently of
  75         bus type.
  76   </para>
  77   </chapter>
  78
  79   <chapter id="bugs">
  80      <title>Known Bugs And Assumptions</title>
  81   <para>
  82         None.
  83   </para>
  84   </chapter>
  85
  86   <chapter id="mmio">
  87     <title>Memory Mapped IO</title>
  88     <sect1 id="getting_access_to_the_device">
  89       <title>Getting Access to the Device</title>
  90       <para>
  91         The most widely supported form of IO is memory mapped IO.
  92         That is, a part of the CPU's address space is interpreted
  93         not as accesses to memory, but as accesses to a device.  Some
  94         architectures define devices to be at a fixed address, but most
  95         have some method of discovering devices.  The PCI bus walk is a
  96         good example of such a scheme.  This document does not cover how
  97         to receive such an address, but assumes you are starting with one.
  98         Physical addresses are of type unsigned long.
  99       </para>
 100
 101       <para>
 102         This address should not be used directly.  Instead, to get an
 103         address suitable for passing to the accessor functions described
 104         below, you should call <function>ioremap</function>.
 105         An address suitable for accessing the device will be returned to you.
 106       </para>
 107
 108       <para>
 109         After you've finished using the device (say, in your module's
 110         exit routine), call <function>iounmap</function> in order to return
 111         the address space to the kernel.  Most architectures allocate new
 112         address space each time you call <function>ioremap</function>, and
 113         they can run out unless you call <function>iounmap</function>.
 114       </para>
 115     </sect1>
 116
 117     <sect1 id="accessing_the_device">
 118       <title>Accessing the device</title>
 119       <para>
 120         The part of the interface most used by drivers is reading and
 121         writing memory-mapped registers on the device.  Linux provides
 122         interfaces to read and write 8-bit, 16-bit, 32-bit and 64-bit
 123         quantities.  Due to a historical accident, these are named byte,
 124         word, long and quad accesses.  Both read and write accesses are
 125         supported; there is no prefetch support at this time.
 126       </para>
 127
 128       <para>
 129         The functions are named <function>readb</function>,
 130         <function>readw</function>, <function>readl</function>,
 131         <function>readq</function>, <function>readb_relaxed</function>,
 132         <function>readw_relaxed</function>, <function>readl_relaxed</function>,
 133         <function>readq_relaxed</function>, <function>writeb</function>,
 134         <function>writew</function>, <function>writel</function> and
 135         <function>writeq</function>.
 136       </para>
 137
 138       <para>
 139         Some devices (such as framebuffers) would like to use larger
 140         transfers than 8 bytes at a time.  For these devices, the
 141         <function>memcpy_toio</function>, <function>memcpy_fromio</function>
 142         and <function>memset_io</function> functions are provided.
 143         Do not use memset or memcpy on IO addresses; they
 144         are not guaranteed to copy data in order.
 145       </para>
 146
 147       <para>
 148         The read and write functions are defined to be ordered. That is the
 149         compiler is not permitted to reorder the I/O sequence. When the
 150         ordering can be compiler optimised, you can use <function>
 151         __readb</function> and friends to indicate the relaxed ordering. Use
 152         this with care.
 153       </para>
 154
 155       <para>
 156         While the basic functions are defined to be synchronous with respect
 157         to each other and ordered with respect to each other the busses the
 158         devices sit on may themselves have asynchronicity. In particular many
 159         authors are burned by the fact that PCI bus writes are posted
 160         asynchronously. A driver author must issue a read from the same
 161         device to ensure that writes have occurred in the specific cases the
 162         author cares. This kind of property cannot be hidden from driver
 163         writers in the API.  In some cases, the read used to flush the device
 164         may be expected to fail (if the card is resetting, for example).  In
 165         that case, the read should be done from config space, which is
 166         guaranteed to soft-fail if the card doesn't respond.
 167       </para>
 168
 169       <para>
 170         The following is an example of flushing a write to a device when
 171         the driver would like to ensure the write's effects are visible prior
 172         to continuing execution.
 173       </para>
 174
 175 <programlisting>
 176 static inline void
 177 qla1280_disable_intrs(struct scsi_qla_host *ha)
 178 {
 179         struct device_reg *reg;
 180
 181         reg = ha->iobase;
 182         /* disable risc and host interrupts */
 183         WRT_REG_WORD(&amp;reg->ictrl, 0);
 184         /*
 185          * The following read will ensure that the above write
 186          * has been received by the device before we return from this
 187          * function.
 188          */
 189         RD_REG_WORD(&amp;reg->ictrl);
 190         ha->flags.ints_enabled = 0;
 191 }
 192 </programlisting>
 193
 194       <para>
 195         In addition to write posting, on some large multiprocessing systems
 196         (e.g. SGI Challenge, Origin and Altix machines) posted writes won't
 197         be strongly ordered coming from different CPUs.  Thus it's important
 198         to properly protect parts of your driver that do memory-mapped writes
 199         with locks and use the <function>mmiowb</function> to make sure they
 200         arrive in the order intended.  Issuing a regular <function>readX
 201         </function> will also ensure write ordering, but should only be used
 202         when the driver has to be sure that the write has actually arrived
 203         at the device (not that it's simply ordered with respect to other
 204         writes), since a full <function>readX</function> is a relatively
 205         expensive operation.
 206       </para>
 207
 208       <para>
 209         Generally, one should use <function>mmiowb</function> prior to
 210         releasing a spinlock that protects regions using <function>writeb
 211         </function> or similar functions that aren't surrounded by <function>
 212         readb</function> calls, which will ensure ordering and flushing.  The
 213         following pseudocode illustrates what might occur if write ordering
 214         isn't guaranteed via <function>mmiowb</function> or one of the
 215         <function>readX</function> functions.
 216       </para>
 217
 218 <programlisting>
 219 CPU A:  spin_lock_irqsave(&amp;dev_lock, flags)
 220 CPU A:  ...
 221 CPU A:  writel(newval, ring_ptr);
 222 CPU A:  spin_unlock_irqrestore(&amp;dev_lock, flags)
 223         ...
 224 CPU B:  spin_lock_irqsave(&amp;dev_lock, flags)
 225 CPU B:  writel(newval2, ring_ptr);
 226 CPU B:  ...
 227 CPU B:  spin_unlock_irqrestore(&amp;dev_lock, flags)
 228 </programlisting>
 229
 230       <para>
 231         In the case above, newval2 could be written to ring_ptr before
 232         newval.  Fixing it is easy though:
 233       </para>
 234
 235 <programlisting>
 236 CPU A:  spin_lock_irqsave(&amp;dev_lock, flags)
 237 CPU A:  ...
 238 CPU A:  writel(newval, ring_ptr);
 239 CPU A:  mmiowb(); /* ensure no other writes beat us to the device */
 240 CPU A:  spin_unlock_irqrestore(&amp;dev_lock, flags)
 241         ...
 242 CPU B:  spin_lock_irqsave(&amp;dev_lock, flags)
 243 CPU B:  writel(newval2, ring_ptr);
 244 CPU B:  ...
 245 CPU B:  mmiowb();
 246 CPU B:  spin_unlock_irqrestore(&amp;dev_lock, flags)
 247 </programlisting>
 248
 249       <para>
 250         See tg3.c for a real world example of how to use <function>mmiowb
 251         </function>
 252       </para>
 253
 254       <para>
 255         PCI ordering rules also guarantee that PIO read responses arrive
 256         after any outstanding DMA writes from that bus, since for some devices
 257         the result of a <function>readb</function> call may signal to the
 258         driver that a DMA transaction is complete.  In many cases, however,
 259         the driver may want to indicate that the next
 260         <function>readb</function> call has no relation to any previous DMA
 261         writes performed by the device.  The driver can use
 262         <function>readb_relaxed</function> for these cases, although only
 263         some platforms will honor the relaxed semantics.  Using the relaxed
 264         read functions will provide significant performance benefits on
 265         platforms that support it.  The qla2xxx driver provides examples
 266         of how to use <function>readX_relaxed</function>.  In many cases,
 267         a majority of the driver's <function>readX</function> calls can
 268         safely be converted to <function>readX_relaxed</function> calls, since
 269         only a few will indicate or depend on DMA completion.
 270       </para>
 271     </sect1>
 272
 273   </chapter>
 274
 275   <chapter id="port_space_accesses">
 276     <title>Port Space Accesses</title>
 277     <sect1 id="port_space_explained">
 278       <title>Port Space Explained</title>
 279
 280       <para>
 281         Another form of IO commonly supported is Port Space.  This is a
 282         range of addresses separate to the normal memory address space.
 283         Access to these addresses is generally not as fast as accesses
 284         to the memory mapped addresses, and it also has a potentially
 285         smaller address space.
 286       </para>
 287
 288       <para>
 289         Unlike memory mapped IO, no preparation is required
 290         to access port space.
 291       </para>
 292
 293     </sect1>
 294     <sect1 id="accessing_port_space">
 295       <title>Accessing Port Space</title>
 296       <para>
 297         Accesses to this space are provided through a set of functions
 298         which allow 8-bit, 16-bit and 32-bit accesses; also
 299         known as byte, word and long.  These functions are
 300         <function>inb</function>, <function>inw</function>,
 301         <function>inl</function>, <function>outb</function>,
 302         <function>outw</function> and <function>outl</function>.
 303       </para>
 304
 305       <para>
 306         Some variants are provided for these functions.  Some devices
 307         require that accesses to their ports are slowed down.  This
 308         functionality is provided by appending a <function>_p</function>
 309         to the end of the function.  There are also equivalents to memcpy.
 310         The <function>ins</function> and <function>outs</function>
 311         functions copy bytes, words or longs to the given port.
 312       </para>
 313     </sect1>
 314
 315   </chapter>
 316
 317   <chapter id="pubfunctions">
 318      <title>Public Functions Provided</title>
 319 !Iarch/x86/include/asm/io_32.h
 320 !Elib/iomap.c
 321   </chapter>
 322
 323 </book>