1MTRR (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 <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
154static 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
165int 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===============================================================================
198Creating 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
251static 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
262int 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===============================================================================
306