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/Documentation/mtrr.txt

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  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 <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===============================================================================