Documentation/mtrr.txt

   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 ===============================================================================