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Based on kernel version 3.16. Page generated on 2014-08-06 21:38 EST.

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.h
320	!Elib/pci_iomap.c
321	  </chapter>
322	
323	</book>
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