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Based on kernel version 3.15.4. Page generated on 2014-07-07 09:05 EST.

1	MTRR (Memory Type Range Register) control
2	3 Jun 1999
3	Richard Gooch
4	<rgooch@atnf.csiro.au>
5	
6	  On Intel P6 family processors (Pentium Pro, Pentium II and later)
7	  the Memory Type Range Registers (MTRRs) may be used to control
8	  processor access to memory ranges. This is most useful when you have
9	  a video (VGA) card on a PCI or AGP bus. Enabling write-combining
10	  allows bus write transfers to be combined into a larger transfer
11	  before bursting over the PCI/AGP bus. This can increase performance
12	  of image write operations 2.5 times or more.
13	
14	  The Cyrix 6x86, 6x86MX and M II processors have Address Range
15	  Registers (ARRs) which provide a similar functionality to MTRRs. For
16	  these, the ARRs are used to emulate the MTRRs.
17	
18	  The AMD K6-2 (stepping 8 and above) and K6-3 processors have two
19	  MTRRs. These are supported.  The AMD Athlon family provide 8 Intel
20	  style MTRRs.
21	
22	  The Centaur C6 (WinChip) has 8 MCRs, allowing write-combining. These
23	  are supported.
24	
25	  The VIA Cyrix III and VIA C3 CPUs offer 8 Intel style MTRRs.
26	
27	  The CONFIG_MTRR option creates a /proc/mtrr file which may be used
28	  to manipulate your MTRRs. Typically the X server should use
29	  this. This should have a reasonably generic interface so that
30	  similar control registers on other processors can be easily
31	  supported.
32	
33	
34	There are two interfaces to /proc/mtrr: one is an ASCII interface
35	which allows you to read and write. The other is an ioctl()
36	interface. The ASCII interface is meant for administration. The
37	ioctl() interface is meant for C programs (i.e. the X server). The
38	interfaces are described below, with sample commands and C code.
39	
40	===============================================================================
41	Reading MTRRs from the shell:
42	
43	% cat /proc/mtrr
44	reg00: base=0x00000000 (   0MB), size= 128MB: write-back, count=1
45	reg01: base=0x08000000 ( 128MB), size=  64MB: write-back, count=1
46	===============================================================================
47	Creating MTRRs from the C-shell:
48	# echo "base=0xf8000000 size=0x400000 type=write-combining" >! /proc/mtrr
49	or if you use bash:
50	# echo "base=0xf8000000 size=0x400000 type=write-combining" >| /proc/mtrr
51	
52	And the result thereof:
53	% cat /proc/mtrr
54	reg00: base=0x00000000 (   0MB), size= 128MB: write-back, count=1
55	reg01: base=0x08000000 ( 128MB), size=  64MB: write-back, count=1
56	reg02: base=0xf8000000 (3968MB), size=   4MB: write-combining, count=1
57	
58	This is for video RAM at base address 0xf8000000 and size 4 megabytes. To
59	find out your base address, you need to look at the output of your X
60	server, which tells you where the linear framebuffer address is. A
61	typical line that you may get is:
62	
63	(--) S3: PCI: 968 rev 0, Linear FB @ 0xf8000000
64	
65	Note that you should only use the value from the X server, as it may
66	move the framebuffer base address, so the only value you can trust is
67	that reported by the X server.
68	
69	To find out the size of your framebuffer (what, you don't actually
70	know?), the following line will tell you:
71	
72	(--) S3: videoram:  4096k
73	
74	That's 4 megabytes, which is 0x400000 bytes (in hexadecimal).
75	A patch is being written for XFree86 which will make this automatic:
76	in other words the X server will manipulate /proc/mtrr using the
77	ioctl() interface, so users won't have to do anything. If you use a
78	commercial X server, lobby your vendor to add support for MTRRs.
79	===============================================================================
80	Creating overlapping MTRRs:
81	
82	%echo "base=0xfb000000 size=0x1000000 type=write-combining" >/proc/mtrr
83	%echo "base=0xfb000000 size=0x1000 type=uncachable" >/proc/mtrr
84	
85	And the results: cat /proc/mtrr
86	reg00: base=0x00000000 (   0MB), size=  64MB: write-back, count=1
87	reg01: base=0xfb000000 (4016MB), size=  16MB: write-combining, count=1
88	reg02: base=0xfb000000 (4016MB), size=   4kB: uncachable, count=1
89	
90	Some cards (especially Voodoo Graphics boards) need this 4 kB area
91	excluded from the beginning of the region because it is used for
92	registers.
93	
94	NOTE: You can only create type=uncachable region, if the first
95	region that you created is type=write-combining.
96	===============================================================================
97	Removing MTRRs from the C-shell:
98	% echo "disable=2" >! /proc/mtrr
99	or using bash:
100	% echo "disable=2" >| /proc/mtrr
101	===============================================================================
102	Reading MTRRs from a C program using ioctl()'s:
103	
104	/*  mtrr-show.c
105	
106	    Source file for mtrr-show (example program to show MTRRs using ioctl()'s)
107	
108	    Copyright (C) 1997-1998  Richard Gooch
109	
110	    This program is free software; you can redistribute it and/or modify
111	    it under the terms of the GNU General Public License as published by
112	    the Free Software Foundation; either version 2 of the License, or
113	    (at your option) any later version.
114	
115	    This program is distributed in the hope that it will be useful,
116	    but WITHOUT ANY WARRANTY; without even the implied warranty of
117	    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
118	    GNU General Public License for more details.
119	
120	    You should have received a copy of the GNU General Public License
121	    along with this program; if not, write to the Free Software
122	    Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
123	
124	    Richard Gooch may be reached by email at  rgooch@atnf.csiro.au
125	    The postal address is:
126	      Richard Gooch, c/o ATNF, P. O. Box 76, Epping, N.S.W., 2121, Australia.
127	*/
128	
129	/*
130	    This program will use an ioctl() on /proc/mtrr to show the current MTRR
131	    settings. This is an alternative to reading /proc/mtrr.
132	
133	
134	    Written by      Richard Gooch   17-DEC-1997
135	
136	    Last updated by Richard Gooch   2-MAY-1998
137	
138	
139	*/
140	#include <stdio.h>
141	#include <stdlib.h>
142	#include <string.h>
143	#include <sys/types.h>
144	#include <sys/stat.h>
145	#include <fcntl.h>
146	#include <sys/ioctl.h>
147	#include <errno.h>
148	#include <asm/mtrr.h>
149	
150	#define TRUE 1
151	#define FALSE 0
152	#define ERRSTRING strerror (errno)
153	
154	static char *mtrr_strings[MTRR_NUM_TYPES] =
155	{
156	    "uncachable",               /* 0 */
157	    "write-combining",          /* 1 */
158	    "?",                        /* 2 */
159	    "?",                        /* 3 */
160	    "write-through",            /* 4 */
161	    "write-protect",            /* 5 */
162	    "write-back",               /* 6 */
163	};
164	
165	int main ()
166	{
167	    int fd;
168	    struct mtrr_gentry gentry;
169	
170	    if ( ( fd = open ("/proc/mtrr", O_RDONLY, 0) ) == -1 )
171	    {
172		if (errno == ENOENT)
173		{
174		    fputs ("/proc/mtrr not found: not supported or you don't have a PPro?\n",
175			   stderr);
176		    exit (1);
177		}
178		fprintf (stderr, "Error opening /proc/mtrr\t%s\n", ERRSTRING);
179		exit (2);
180	    }
181	    for (gentry.regnum = 0; ioctl (fd, MTRRIOC_GET_ENTRY, &gentry) == 0;
182		 ++gentry.regnum)
183	    {
184		if (gentry.size < 1)
185		{
186		    fprintf (stderr, "Register: %u disabled\n", gentry.regnum);
187		    continue;
188		}
189		fprintf (stderr, "Register: %u base: 0x%lx size: 0x%lx type: %s\n",
190			 gentry.regnum, gentry.base, gentry.size,
191			 mtrr_strings[gentry.type]);
192	    }
193	    if (errno == EINVAL) exit (0);
194	    fprintf (stderr, "Error doing ioctl(2) on /dev/mtrr\t%s\n", ERRSTRING);
195	    exit (3);
196	}   /*  End Function main  */
197	===============================================================================
198	Creating MTRRs from a C programme using ioctl()'s:
199	
200	/*  mtrr-add.c
201	
202	    Source file for mtrr-add (example programme to add an MTRRs using ioctl())
203	
204	    Copyright (C) 1997-1998  Richard Gooch
205	
206	    This program is free software; you can redistribute it and/or modify
207	    it under the terms of the GNU General Public License as published by
208	    the Free Software Foundation; either version 2 of the License, or
209	    (at your option) any later version.
210	
211	    This program is distributed in the hope that it will be useful,
212	    but WITHOUT ANY WARRANTY; without even the implied warranty of
213	    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
214	    GNU General Public License for more details.
215	
216	    You should have received a copy of the GNU General Public License
217	    along with this program; if not, write to the Free Software
218	    Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
219	
220	    Richard Gooch may be reached by email at  rgooch@atnf.csiro.au
221	    The postal address is:
222	      Richard Gooch, c/o ATNF, P. O. Box 76, Epping, N.S.W., 2121, Australia.
223	*/
224	
225	/*
226	    This programme will use an ioctl() on /proc/mtrr to add an entry. The first
227	    available mtrr is used. This is an alternative to writing /proc/mtrr.
228	
229	
230	    Written by      Richard Gooch   17-DEC-1997
231	
232	    Last updated by Richard Gooch   2-MAY-1998
233	
234	
235	*/
236	#include <stdio.h>
237	#include <string.h>
238	#include <stdlib.h>
239	#include <unistd.h>
240	#include <sys/types.h>
241	#include <sys/stat.h>
242	#include <fcntl.h>
243	#include <sys/ioctl.h>
244	#include <errno.h>
245	#include <asm/mtrr.h>
246	
247	#define TRUE 1
248	#define FALSE 0
249	#define ERRSTRING strerror (errno)
250	
251	static char *mtrr_strings[MTRR_NUM_TYPES] =
252	{
253	    "uncachable",               /* 0 */
254	    "write-combining",          /* 1 */
255	    "?",                        /* 2 */
256	    "?",                        /* 3 */
257	    "write-through",            /* 4 */
258	    "write-protect",            /* 5 */
259	    "write-back",               /* 6 */
260	};
261	
262	int main (int argc, char **argv)
263	{
264	    int fd;
265	    struct mtrr_sentry sentry;
266	
267	    if (argc != 4)
268	    {
269		fprintf (stderr, "Usage:\tmtrr-add base size type\n");
270		exit (1);
271	    }
272	    sentry.base = strtoul (argv[1], NULL, 0);
273	    sentry.size = strtoul (argv[2], NULL, 0);
274	    for (sentry.type = 0; sentry.type < MTRR_NUM_TYPES; ++sentry.type)
275	    {
276		if (strcmp (argv[3], mtrr_strings[sentry.type]) == 0) break;
277	    }
278	    if (sentry.type >= MTRR_NUM_TYPES)
279	    {
280		fprintf (stderr, "Illegal type: \"%s\"\n", argv[3]);
281		exit (2);
282	    }
283	    if ( ( fd = open ("/proc/mtrr", O_WRONLY, 0) ) == -1 )
284	    {
285		if (errno == ENOENT)
286		{
287		    fputs ("/proc/mtrr not found: not supported or you don't have a PPro?\n",
288			   stderr);
289		    exit (3);
290		}
291		fprintf (stderr, "Error opening /proc/mtrr\t%s\n", ERRSTRING);
292		exit (4);
293	    }
294	    if (ioctl (fd, MTRRIOC_ADD_ENTRY, &sentry) == -1)
295	    {
296		fprintf (stderr, "Error doing ioctl(2) on /dev/mtrr\t%s\n", ERRSTRING);
297		exit (5);
298	    }
299	    fprintf (stderr, "Sleeping for 5 seconds so you can see the new entry\n");
300	    sleep (5);
301	    close (fd);
302	    fputs ("I've just closed /proc/mtrr so now the new entry should be gone\n",
303		   stderr);
304	}   /*  End Function main  */
305	===============================================================================
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