1 <!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook V3.1//EN"[]>
5 <title>Bus-Independent Device Accesses</title>
9 <firstname>Matthew</firstname>
10 <surname>Wilcox</surname>
13 <email>matthew@wil.cx</email>
21 <firstname>Alan</firstname>
22 <surname>Cox</surname>
25 <email>alan@redhat.com</email>
33 <holder>Matthew Wilcox</holder>
38 This documentation is free software; you can redistribute
39 it and/or modify it under the terms of the GNU General Public
40 License as published by the Free Software Foundation; either
41 version 2 of the License, or (at your option) any later
46 This program is distributed in the hope that it will be
47 useful, but WITHOUT ANY WARRANTY; without even the implied
48 warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
49 See the GNU General Public License for more details.
53 You should have received a copy of the GNU General Public
54 License along with this program; if not, write to the Free
55 Software Foundation, Inc., 59 Temple Place, Suite 330, Boston,
60 For more details see the file COPYING in the source
61 distribution of Linux.
69 <title>Introduction</title>
71 Linux provides an API which abstracts performing IO across all busses
72 and devices, allowing device drivers to be written independently of
78 <title>Known Bugs And Assumptions</title>
85 <title>Memory Mapped IO</title>
87 <title>Getting Access to the Device</title>
89 The most widely supported form of IO is memory mapped IO.
90 That is, a part of the CPU's address space is interpreted
91 not as accesses to memory, but as accesses to a device. Some
92 architectures define devices to be at a fixed address, but most
93 have some method of discovering devices. The PCI bus walk is a
94 good example of such a scheme. This document does not cover how
95 to receive such an address, but assumes you are starting with one.
96 Physical addresses are of type unsigned long.
100 This address should not be used directly. Instead, to get an
101 address suitable for passing to the accessor functions described
102 below, you should call <function>ioremap</function>.
103 An address suitable for accessing the device will be returned to you.
107 After you've finished using the device (say, in your module's
108 exit routine), call <function>iounmap</function> in order to return
109 the address space to the kernel. Most architectures allocate new
110 address space each time you call <function>ioremap</function>, and
111 they can run out unless you call <function>iounmap</function>.
116 <title>Accessing the device</title>
118 The part of the interface most used by drivers is reading and
119 writing memory-mapped registers on the device. Linux provides
120 interfaces to read and write 8-bit, 16-bit, 32-bit and 64-bit
121 quantities. Due to a historical accident, these are named byte,
122 word, long and quad accesses. Both read and write accesses are
123 supported; there is no prefetch support at this time.
127 The functions are named <function>readb</function>,
128 <function>readw</function>, <function>readl</function>,
129 <function>readq</function>, <function>readb_relaxed</function>,
130 <function>readw_relaxed</function>, <function>readl_relaxed</function>,
131 <function>readq_relaxed</function>, <function>writeb</function>,
132 <function>writew</function>, <function>writel</function> and
133 <function>writeq</function>.
137 Some devices (such as framebuffers) would like to use larger
138 transfers than 8 bytes at a time. For these devices, the
139 <function>memcpy_toio</function>, <function>memcpy_fromio</function>
140 and <function>memset_io</function> functions are provided.
141 Do not use memset or memcpy on IO addresses; they
142 are not guaranteed to copy data in order.
146 The read and write functions are defined to be ordered. That is the
147 compiler is not permitted to reorder the I/O sequence. When the
148 ordering can be compiler optimised, you can use <function>
149 __readb</function> and friends to indicate the relaxed ordering. Use
150 this with care. The <function>rmb</function> provides a read memory
151 barrier. The <function>wmb</function> provides a write memory barrier.
155 While the basic functions are defined to be synchronous with respect
156 to each other and ordered with respect to each other the busses the
157 devices sit on may themselves have asynchronicity. In particular many
158 authors are burned by the fact that PCI bus writes are posted
159 asynchronously. A driver author must issue a read from the same
160 device to ensure that writes have occurred in the specific cases the
161 author cares. This kind of property cannot be hidden from driver
166 PCI ordering rules also guarantee that PIO read responses arrive
167 after any outstanding DMA writes on that bus, since for some devices
168 the result of a <function>readb</function> call may signal to the
169 driver that a DMA transaction is complete. In many cases, however,
170 the driver may want to indicate that the next
171 <function>readb</function> call has no relation to any previous DMA
172 writes performed by the device. The driver can use
173 <function>readb_relaxed</function> for these cases, although only
174 some platforms will honor the relaxed semantics.
179 <title>ISA legacy functions</title>
181 On older kernels (2.2 and earlier) the ISA bus could be read or
182 written with these functions and without ioremap being used. This is
183 no longer true in Linux 2.4. A set of equivalent functions exist for
184 easy legacy driver porting. The functions available are prefixed
185 with 'isa_' and are <function>isa_readb</function>,
186 <function>isa_writeb</function>, <function>isa_readw</function>,
187 <function>isa_writew</function>, <function>isa_readl</function>,
188 <function>isa_writel</function>, <function>isa_memcpy_fromio</function>
189 and <function>isa_memcpy_toio</function>
192 These functions should not be used in new drivers, and will
193 eventually be going away.
200 <title>Port Space Accesses</title>
202 <title>Port Space Explained</title>
205 Another form of IO commonly supported is Port Space. This is a
206 range of addresses separate to the normal memory address space.
207 Access to these addresses is generally not as fast as accesses
208 to the memory mapped addresses, and it also has a potentially
209 smaller address space.
213 Unlike memory mapped IO, no preparation is required
214 to access port space.
219 <title>Accessing Port Space</title>
221 Accesses to this space are provided through a set of functions
222 which allow 8-bit, 16-bit and 32-bit accesses; also
223 known as byte, word and long. These functions are
224 <function>inb</function>, <function>inw</function>,
225 <function>inl</function>, <function>outb</function>,
226 <function>outw</function> and <function>outl</function>.
230 Some variants are provided for these functions. Some devices
231 require that accesses to their ports are slowed down. This
232 functionality is provided by appending a <function>_p</function>
233 to the end of the function. There are also equivalents to memcpy.
234 The <function>ins</function> and <function>outs</function>
235 functions copy bytes, words or longs to the given port.
241 <chapter id="pubfunctions">
242 <title>Public Functions Provided</title>
243 !Einclude/asm-i386/io.h