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<book id="DoingIO">
 <bookinfo>
  <title>Bus-Independent Device Accesses</title>
  
  <authorgroup>
   <author>
    <firstname>Matthew</firstname>
    <surname>Wilcox</surname>
    <affiliation>
     <address>
      <email>matthew@wil.cx</email>
     </address>
    </affiliation>
   </author>
  </authorgroup>

  <authorgroup>
   <author>
    <firstname>Alan</firstname>
    <surname>Cox</surname>
    <affiliation>
     <address>
      <email>alan@redhat.com</email>
     </address>
    </affiliation>
   </author>
  </authorgroup>

  <copyright>
   <year>2001</year>
   <holder>Matthew Wilcox</holder>
  </copyright>

  <legalnotice>
   <para>
     This documentation is free software; you can redistribute
     it and/or modify it under the terms of the GNU General Public
     License as published by the Free Software Foundation; either
     version 2 of the License, or (at your option) any later
     version.
   </para>
      
   <para>
     This program is distributed in the hope that it will be
     useful, but WITHOUT ANY WARRANTY; without even the implied
     warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
     See the GNU General Public License for more details.
   </para>
      
   <para>
     You should have received a copy of the GNU General Public
     License along with this program; if not, write to the Free
     Software Foundation, Inc., 59 Temple Place, Suite 330, Boston,
     MA 02111-1307 USA
   </para>
      
   <para>
     For more details see the file COPYING in the source
     distribution of Linux.
   </para>
  </legalnotice>
 </bookinfo>

<toc></toc>

  <chapter id="intro">
      <title>Introduction</title>
  <para>
	Linux provides an API which abstracts performing IO across all busses
	and devices, allowing device drivers to be written independently of
	bus type.
  </para>
  </chapter>

  <chapter id="bugs">
     <title>Known Bugs And Assumptions</title>
  <para>
	None.	
  </para>
  </chapter>

  <chapter id="mmio">
    <title>Memory Mapped IO</title>
    <sect1>
      <title>Getting Access to the Device</title>
      <para>
	The most widely supported form of IO is memory mapped IO.
	That is, a part of the CPU's address space is interpreted
	not as accesses to memory, but as accesses to a device.  Some
	architectures define devices to be at a fixed address, but most
	have some method of discovering devices.  The PCI bus walk is a
	good example of such a scheme.	This document does not cover how
	to receive such an address, but assumes you are starting with one.
	Physical addresses are of type unsigned long. 
      </para>

      <para>
	This address should not be used directly.  Instead, to get an
	address suitable for passing to the accessor functions described
	below, you should call <function>ioremap</function>.
	An address suitable for accessing the device will be returned to you.
      </para>

      <para>
	After you've finished using the device (say, in your module's
	exit routine), call <function>iounmap</function> in order to return
	the address space to the kernel.  Most architectures allocate new
	address space each time you call <function>ioremap</function>, and
	they can run out unless you call <function>iounmap</function>.
      </para>
    </sect1>

    <sect1>
      <title>Accessing the device</title>
      <para>
	The part of the interface most used by drivers is reading and
	writing memory-mapped registers on the device.	Linux provides
	interfaces to read and write 8-bit, 16-bit, 32-bit and 64-bit
	quantities.  Due to a historical accident, these are named byte,
	word, long and quad accesses.  Both read and write accesses are
	supported; there is no prefetch support at this time.
      </para>

      <para>
	The functions are named <function>readb</function>,
	<function>readw</function>, <function>readl</function>,
	<function>readq</function>, <function>readb_relaxed</function>,
	<function>readw_relaxed</function>, <function>readl_relaxed</function>,
	<function>readq_relaxed</function>, <function>writeb</function>,
	<function>writew</function>, <function>writel</function> and
	<function>writeq</function>.
      </para>

      <para>
	Some devices (such as framebuffers) would like to use larger
	transfers than 8 bytes at a time.  For these devices, the
	<function>memcpy_toio</function>, <function>memcpy_fromio</function>
	and <function>memset_io</function> functions are provided.
	Do not use memset or memcpy on IO addresses; they
	are not guaranteed to copy data in order.
      </para>

      <para>
	The read and write functions are defined to be ordered. That is the
	compiler is not permitted to reorder the I/O sequence. When the 
	ordering can be compiler optimised, you can use <function>
	__readb</function> and friends to indicate the relaxed ordering. Use 
	this with care.
      </para>

      <para>
	While the basic functions are defined to be synchronous with respect
	to each other and ordered with respect to each other the busses the
	devices sit on may themselves have asynchronicity. In particular many
	authors are burned by the fact that PCI bus writes are posted
	asynchronously. A driver author must issue a read from the same
	device to ensure that writes have occurred in the specific cases the
	author cares. This kind of property cannot be hidden from driver
	writers in the API.  In some cases, the read used to flush the device
	may be expected to fail (if the card is resetting, for example).  In
	that case, the read should be done from config space, which is
	guaranteed to soft-fail if the card doesn't respond.
      </para>

      <para>
	The following is an example of flushing a write to a device when
	the driver would like to ensure the write's effects are visible prior
	to continuing execution.
      </para>

<programlisting>
static inline void
qla1280_disable_intrs(struct scsi_qla_host *ha)
{
	struct device_reg *reg;

	reg = ha->iobase;
	/* disable risc and host interrupts */
	WRT_REG_WORD(&amp;reg->ictrl, 0);
	/*
	 * The following read will ensure that the above write
	 * has been received by the device before we return from this
	 * function.
	 */
	RD_REG_WORD(&amp;reg->ictrl);
	ha->flags.ints_enabled = 0;
}
</programlisting>

      <para>
	In addition to write posting, on some large multiprocessing systems
	(e.g. SGI Challenge, Origin and Altix machines) posted writes won't
	be strongly ordered coming from different CPUs.  Thus it's important
	to properly protect parts of your driver that do memory-mapped writes
	with locks and use the <function>mmiowb</function> to make sure they
	arrive in the order intended.  Issuing a regular <function>readX
	</function> will also ensure write ordering, but should only be used
	when the driver has to be sure that the write has actually arrived
	at the device (not that it's simply ordered with respect to other
	writes), since a full <function>readX</function> is a relatively
	expensive operation.
      </para>

      <para>
	Generally, one should use <function>mmiowb</function> prior to
	releasing a spinlock that protects regions using <function>writeb
	</function> or similar functions that aren't surrounded by <function>
	readb</function> calls, which will ensure ordering and flushing.  The
	following pseudocode illustrates what might occur if write ordering
	isn't guaranteed via <function>mmiowb</function> or one of the
	<function>readX</function> functions.
      </para>

<programlisting>
CPU A:  spin_lock_irqsave(&amp;dev_lock, flags)
CPU A:  ...
CPU A:  writel(newval, ring_ptr);
CPU A:  spin_unlock_irqrestore(&amp;dev_lock, flags)
        ...
CPU B:  spin_lock_irqsave(&amp;dev_lock, flags)
CPU B:  writel(newval2, ring_ptr);
CPU B:  ...
CPU B:  spin_unlock_irqrestore(&amp;dev_lock, flags)
</programlisting>

      <para>
	In the case above, newval2 could be written to ring_ptr before
	newval.  Fixing it is easy though:
      </para>

<programlisting>
CPU A:  spin_lock_irqsave(&amp;dev_lock, flags)
CPU A:  ...
CPU A:  writel(newval, ring_ptr);
CPU A:  mmiowb(); /* ensure no other writes beat us to the device */
CPU A:  spin_unlock_irqrestore(&amp;dev_lock, flags)
        ...
CPU B:  spin_lock_irqsave(&amp;dev_lock, flags)
CPU B:  writel(newval2, ring_ptr);
CPU B:  ...
CPU B:  mmiowb();
CPU B:  spin_unlock_irqrestore(&amp;dev_lock, flags)
</programlisting>

      <para>
	See tg3.c for a real world example of how to use <function>mmiowb
	</function>
      </para>

      <para>
	PCI ordering rules also guarantee that PIO read responses arrive
	after any outstanding DMA writes from that bus, since for some devices
	the result of a <function>readb</function> call may signal to the
	driver that a DMA transaction is complete.  In many cases, however,
	the driver may want to indicate that the next
	<function>readb</function> call has no relation to any previous DMA
	writes performed by the device.  The driver can use
	<function>readb_relaxed</function> for these cases, although only
	some platforms will honor the relaxed semantics.  Using the relaxed
	read functions will provide significant performance benefits on
	platforms that support it.  The qla2xxx driver provides examples
	of how to use <function>readX_relaxed</function>.  In many cases,
	a majority of the driver's <function>readX</function> calls can
	safely be converted to <function>readX_relaxed</function> calls, since
	only a few will indicate or depend on DMA completion.
      </para>
    </sect1>

    <sect1>
      <title>ISA legacy functions</title>
      <para>
	On older kernels (2.2 and earlier) the ISA bus could be read or
	written with these functions and without ioremap being used. This is
	no longer true in Linux 2.4. A set of equivalent functions exist for
	easy legacy driver porting. The functions available are prefixed
	with 'isa_' and are <function>isa_readb</function>,
	<function>isa_writeb</function>, <function>isa_readw</function>, 
	<function>isa_writew</function>, <function>isa_readl</function>,
	<function>isa_writel</function>, <function>isa_memcpy_fromio</function>
	and <function>isa_memcpy_toio</function>
      </para>
      <para>
	These functions should not be used in new drivers, and will
	eventually be going away.
      </para>
    </sect1>

  </chapter>

  <chapter>
    <title>Port Space Accesses</title>
    <sect1>
      <title>Port Space Explained</title>

      <para>
	Another form of IO commonly supported is Port Space.  This is a
	range of addresses separate to the normal memory address space.
	Access to these addresses is generally not as fast as accesses
	to the memory mapped addresses, and it also has a potentially
	smaller address space.
      </para>

      <para>
	Unlike memory mapped IO, no preparation is required
	to access port space.
      </para>

    </sect1>
    <sect1>
      <title>Accessing Port Space</title>
      <para>
	Accesses to this space are provided through a set of functions
	which allow 8-bit, 16-bit and 32-bit accesses; also
	known as byte, word and long.  These functions are
	<function>inb</function>, <function>inw</function>,
	<function>inl</function>, <function>outb</function>,
	<function>outw</function> and <function>outl</function>.
      </para>

      <para>
	Some variants are provided for these functions.  Some devices
	require that accesses to their ports are slowed down.  This
	functionality is provided by appending a <function>_p</function>
	to the end of the function.  There are also equivalents to memcpy.
	The <function>ins</function> and <function>outs</function>
	functions copy bytes, words or longs to the given port.
      </para>
    </sect1>

  </chapter>

  <chapter id="pubfunctions">
     <title>Public Functions Provided</title>
!Einclude/asm-i386/io.h
  </chapter>

</book>