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Linus Torvalds1da177e2005-04-16 15:20:36 -07001MTRR (Memory Type Range Register) control
23 Jun 1999
3Richard 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
34There are two interfaces to /proc/mtrr: one is an ASCII interface
35which allows you to read and write. The other is an ioctl()
36interface. The ASCII interface is meant for administration. The
37ioctl() interface is meant for C programs (i.e. the X server). The
38interfaces are described below, with sample commands and C code.
39
40===============================================================================
41Reading MTRRs from the shell:
42
43% cat /proc/mtrr
44reg00: base=0x00000000 ( 0MB), size= 128MB: write-back, count=1
45reg01: base=0x08000000 ( 128MB), size= 64MB: write-back, count=1
46===============================================================================
47Creating MTRRs from the C-shell:
48# echo "base=0xf8000000 size=0x400000 type=write-combining" >! /proc/mtrr
49or if you use bash:
50# echo "base=0xf8000000 size=0x400000 type=write-combining" >| /proc/mtrr
51
52And the result thereof:
53% cat /proc/mtrr
54reg00: base=0x00000000 ( 0MB), size= 128MB: write-back, count=1
55reg01: base=0x08000000 ( 128MB), size= 64MB: write-back, count=1
56reg02: base=0xf8000000 (3968MB), size= 4MB: write-combining, count=1
57
58This is for video RAM at base address 0xf8000000 and size 4 megabytes. To
59find out your base address, you need to look at the output of your X
60server, which tells you where the linear framebuffer address is. A
61typical line that you may get is:
62
63(--) S3: PCI: 968 rev 0, Linear FB @ 0xf8000000
64
65Note that you should only use the value from the X server, as it may
66move the framebuffer base address, so the only value you can trust is
67that reported by the X server.
68
69To find out the size of your framebuffer (what, you don't actually
70know?), the following line will tell you:
71
72(--) S3: videoram: 4096k
73
74That's 4 megabytes, which is 0x400000 bytes (in hexadecimal).
75A patch is being written for XFree86 which will make this automatic:
76in other words the X server will manipulate /proc/mtrr using the
77ioctl() interface, so users won't have to do anything. If you use a
78commercial X server, lobby your vendor to add support for MTRRs.
79===============================================================================
80Creating 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
85And the results: cat /proc/mtrr
86reg00: base=0x00000000 ( 0MB), size= 64MB: write-back, count=1
87reg01: base=0xfb000000 (4016MB), size= 16MB: write-combining, count=1
88reg02: base=0xfb000000 (4016MB), size= 4kB: uncachable, count=1
89
90Some cards (especially Voodoo Graphics boards) need this 4 kB area
91excluded from the beginning of the region because it is used for
92registers.
93
94NOTE: You can only create type=uncachable region, if the first
95region that you created is type=write-combining.
96===============================================================================
97Removing MTRRs from the C-shell:
98% echo "disable=2" >! /proc/mtrr
99or using bash:
100% echo "disable=2" >| /proc/mtrr
101===============================================================================
102Reading 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 <string.h>
142#include <sys/types.h>
143#include <sys/stat.h>
144#include <fcntl.h>
145#include <sys/ioctl.h>
146#include <errno.h>
147#define MTRR_NEED_STRINGS
148#include <asm/mtrr.h>
149
150#define TRUE 1
151#define FALSE 0
152#define ERRSTRING strerror (errno)
153
154
155int main ()
156{
157 int fd;
158 struct mtrr_gentry gentry;
159
160 if ( ( fd = open ("/proc/mtrr", O_RDONLY, 0) ) == -1 )
161 {
162 if (errno == ENOENT)
163 {
164 fputs ("/proc/mtrr not found: not supported or you don't have a PPro?\n",
165 stderr);
166 exit (1);
167 }
168 fprintf (stderr, "Error opening /proc/mtrr\t%s\n", ERRSTRING);
169 exit (2);
170 }
171 for (gentry.regnum = 0; ioctl (fd, MTRRIOC_GET_ENTRY, &gentry) == 0;
172 ++gentry.regnum)
173 {
174 if (gentry.size < 1)
175 {
176 fprintf (stderr, "Register: %u disabled\n", gentry.regnum);
177 continue;
178 }
179 fprintf (stderr, "Register: %u base: 0x%lx size: 0x%lx type: %s\n",
180 gentry.regnum, gentry.base, gentry.size,
181 mtrr_strings[gentry.type]);
182 }
183 if (errno == EINVAL) exit (0);
184 fprintf (stderr, "Error doing ioctl(2) on /dev/mtrr\t%s\n", ERRSTRING);
185 exit (3);
186} /* End Function main */
187===============================================================================
188Creating MTRRs from a C programme using ioctl()'s:
189
190/* mtrr-add.c
191
192 Source file for mtrr-add (example programme to add an MTRRs using ioctl())
193
194 Copyright (C) 1997-1998 Richard Gooch
195
196 This program is free software; you can redistribute it and/or modify
197 it under the terms of the GNU General Public License as published by
198 the Free Software Foundation; either version 2 of the License, or
199 (at your option) any later version.
200
201 This program is distributed in the hope that it will be useful,
202 but WITHOUT ANY WARRANTY; without even the implied warranty of
203 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
204 GNU General Public License for more details.
205
206 You should have received a copy of the GNU General Public License
207 along with this program; if not, write to the Free Software
208 Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
209
210 Richard Gooch may be reached by email at rgooch@atnf.csiro.au
211 The postal address is:
212 Richard Gooch, c/o ATNF, P. O. Box 76, Epping, N.S.W., 2121, Australia.
213*/
214
215/*
216 This programme will use an ioctl() on /proc/mtrr to add an entry. The first
217 available mtrr is used. This is an alternative to writing /proc/mtrr.
218
219
220 Written by Richard Gooch 17-DEC-1997
221
222 Last updated by Richard Gooch 2-MAY-1998
223
224
225*/
226#include <stdio.h>
227#include <string.h>
228#include <stdlib.h>
229#include <unistd.h>
230#include <sys/types.h>
231#include <sys/stat.h>
232#include <fcntl.h>
233#include <sys/ioctl.h>
234#include <errno.h>
235#define MTRR_NEED_STRINGS
236#include <asm/mtrr.h>
237
238#define TRUE 1
239#define FALSE 0
240#define ERRSTRING strerror (errno)
241
242
243int main (int argc, char **argv)
244{
245 int fd;
246 struct mtrr_sentry sentry;
247
248 if (argc != 4)
249 {
250 fprintf (stderr, "Usage:\tmtrr-add base size type\n");
251 exit (1);
252 }
253 sentry.base = strtoul (argv[1], NULL, 0);
254 sentry.size = strtoul (argv[2], NULL, 0);
255 for (sentry.type = 0; sentry.type < MTRR_NUM_TYPES; ++sentry.type)
256 {
257 if (strcmp (argv[3], mtrr_strings[sentry.type]) == 0) break;
258 }
259 if (sentry.type >= MTRR_NUM_TYPES)
260 {
261 fprintf (stderr, "Illegal type: \"%s\"\n", argv[3]);
262 exit (2);
263 }
264 if ( ( fd = open ("/proc/mtrr", O_WRONLY, 0) ) == -1 )
265 {
266 if (errno == ENOENT)
267 {
268 fputs ("/proc/mtrr not found: not supported or you don't have a PPro?\n",
269 stderr);
270 exit (3);
271 }
272 fprintf (stderr, "Error opening /proc/mtrr\t%s\n", ERRSTRING);
273 exit (4);
274 }
275 if (ioctl (fd, MTRRIOC_ADD_ENTRY, &sentry) == -1)
276 {
277 fprintf (stderr, "Error doing ioctl(2) on /dev/mtrr\t%s\n", ERRSTRING);
278 exit (5);
279 }
280 fprintf (stderr, "Sleeping for 5 seconds so you can see the new entry\n");
281 sleep (5);
282 close (fd);
283 fputs ("I've just closed /proc/mtrr so now the new entry should be gone\n",
284 stderr);
285} /* End Function main */
286===============================================================================