Documentation/x86/mtrr.txt

   1 MTRR (Memory Type Range Register) control
   2
   3 Richard Gooch <rgooch@atnf.csiro.au> - 3 Jun 1999
   4 Luis R. Rodriguez <mcgrof@do-not-panic.com> - April 9, 2015
   5
   6 ===============================================================================
   7 Phasing out MTRR use
   8
   9 MTRR use is replaced on modern x86 hardware with PAT. Over time the only type
  10 of effective MTRR that is expected to be supported will be for write-combining.
  11 As MTRR use is phased out device drivers should use arch_phys_wc_add() to make
  12 MTRR effective on non-PAT systems while a no-op on PAT enabled systems.
  13
  14 For details refer to Documentation/x86/pat.txt.
  15
  16 ===============================================================================
  17
  18   On Intel P6 family processors (Pentium Pro, Pentium II and later)
  19   the Memory Type Range Registers (MTRRs) may be used to control
  20   processor access to memory ranges. This is most useful when you have
  21   a video (VGA) card on a PCI or AGP bus. Enabling write-combining
  22   allows bus write transfers to be combined into a larger transfer
  23   before bursting over the PCI/AGP bus. This can increase performance
  24   of image write operations 2.5 times or more.
  25
  26   The Cyrix 6x86, 6x86MX and M II processors have Address Range
  27   Registers (ARRs) which provide a similar functionality to MTRRs. For
  28   these, the ARRs are used to emulate the MTRRs.
  29
  30   The AMD K6-2 (stepping 8 and above) and K6-3 processors have two
  31   MTRRs. These are supported.  The AMD Athlon family provide 8 Intel
  32   style MTRRs.
  33
  34   The Centaur C6 (WinChip) has 8 MCRs, allowing write-combining. These
  35   are supported.
  36
  37   The VIA Cyrix III and VIA C3 CPUs offer 8 Intel style MTRRs.
  38
  39   The CONFIG_MTRR option creates a /proc/mtrr file which may be used
  40   to manipulate your MTRRs. Typically the X server should use
  41   this. This should have a reasonably generic interface so that
  42   similar control registers on other processors can be easily
  43   supported.
  44
  45
  46 There are two interfaces to /proc/mtrr: one is an ASCII interface
  47 which allows you to read and write. The other is an ioctl()
  48 interface. The ASCII interface is meant for administration. The
  49 ioctl() interface is meant for C programs (i.e. the X server). The
  50 interfaces are described below, with sample commands and C code.
  51
  52 ===============================================================================
  53 Reading MTRRs from the shell:
  54
  55 % cat /proc/mtrr
  56 reg00: base=0x00000000 (   0MB), size= 128MB: write-back, count=1
  57 reg01: base=0x08000000 ( 128MB), size=  64MB: write-back, count=1
  58 ===============================================================================
  59 Creating MTRRs from the C-shell:
  60 # echo "base=0xf8000000 size=0x400000 type=write-combining" >! /proc/mtrr
  61 or if you use bash:
  62 # echo "base=0xf8000000 size=0x400000 type=write-combining" >| /proc/mtrr
  63
  64 And the result thereof:
  65 % cat /proc/mtrr
  66 reg00: base=0x00000000 (   0MB), size= 128MB: write-back, count=1
  67 reg01: base=0x08000000 ( 128MB), size=  64MB: write-back, count=1
  68 reg02: base=0xf8000000 (3968MB), size=   4MB: write-combining, count=1
  69
  70 This is for video RAM at base address 0xf8000000 and size 4 megabytes. To
  71 find out your base address, you need to look at the output of your X
  72 server, which tells you where the linear framebuffer address is. A
  73 typical line that you may get is:
  74
  75 (--) S3: PCI: 968 rev 0, Linear FB @ 0xf8000000
  76
  77 Note that you should only use the value from the X server, as it may
  78 move the framebuffer base address, so the only value you can trust is
  79 that reported by the X server.
  80
  81 To find out the size of your framebuffer (what, you don't actually
  82 know?), the following line will tell you:
  83
  84 (--) S3: videoram:  4096k
  85
  86 That's 4 megabytes, which is 0x400000 bytes (in hexadecimal).
  87 A patch is being written for XFree86 which will make this automatic:
  88 in other words the X server will manipulate /proc/mtrr using the
  89 ioctl() interface, so users won't have to do anything. If you use a
  90 commercial X server, lobby your vendor to add support for MTRRs.
  91 ===============================================================================
  92 Creating overlapping MTRRs:
  93
  94 %echo "base=0xfb000000 size=0x1000000 type=write-combining" >/proc/mtrr
  95 %echo "base=0xfb000000 size=0x1000 type=uncachable" >/proc/mtrr
  96
  97 And the results: cat /proc/mtrr
  98 reg00: base=0x00000000 (   0MB), size=  64MB: write-back, count=1
  99 reg01: base=0xfb000000 (4016MB), size=  16MB: write-combining, count=1
 100 reg02: base=0xfb000000 (4016MB), size=   4kB: uncachable, count=1
 101
 102 Some cards (especially Voodoo Graphics boards) need this 4 kB area
 103 excluded from the beginning of the region because it is used for
 104 registers.
 105
 106 NOTE: You can only create type=uncachable region, if the first
 107 region that you created is type=write-combining.
 108 ===============================================================================
 109 Removing MTRRs from the C-shell:
 110 % echo "disable=2" >! /proc/mtrr
 111 or using bash:
 112 % echo "disable=2" >| /proc/mtrr
 113 ===============================================================================
 114 Reading MTRRs from a C program using ioctl()'s:
 115
 116 /*  mtrr-show.c
 117
 118     Source file for mtrr-show (example program to show MTRRs using ioctl()'s)
 119
 120     Copyright (C) 1997-1998  Richard Gooch
 121
 122     This program is free software; you can redistribute it and/or modify
 123     it under the terms of the GNU General Public License as published by
 124     the Free Software Foundation; either version 2 of the License, or
 125     (at your option) any later version.
 126
 127     This program is distributed in the hope that it will be useful,
 128     but WITHOUT ANY WARRANTY; without even the implied warranty of
 129     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 130     GNU General Public License for more details.
 131
 132     You should have received a copy of the GNU General Public License
 133     along with this program; if not, write to the Free Software
 134     Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
 135
 136     Richard Gooch may be reached by email at  rgooch@atnf.csiro.au
 137     The postal address is:
 138       Richard Gooch, c/o ATNF, P. O. Box 76, Epping, N.S.W., 2121, Australia.
 139 */
 140
 141 /*
 142     This program will use an ioctl() on /proc/mtrr to show the current MTRR
 143     settings. This is an alternative to reading /proc/mtrr.
 144
 145
 146     Written by      Richard Gooch   17-DEC-1997
 147
 148     Last updated by Richard Gooch   2-MAY-1998
 149
 150
 151 */
 152 #include <stdio.h>
 153 #include <stdlib.h>
 154 #include <string.h>
 155 #include <sys/types.h>
 156 #include <sys/stat.h>
 157 #include <fcntl.h>
 158 #include <sys/ioctl.h>
 159 #include <errno.h>
 160 #include <asm/mtrr.h>
 161
 162 #define TRUE 1
 163 #define FALSE 0
 164 #define ERRSTRING strerror (errno)
 165
 166 static char *mtrr_strings[MTRR_NUM_TYPES] =
 167 {
 168     "uncachable",               /* 0 */
 169     "write-combining",          /* 1 */
 170     "?",                        /* 2 */
 171     "?",                        /* 3 */
 172     "write-through",            /* 4 */
 173     "write-protect",            /* 5 */
 174     "write-back",               /* 6 */
 175 };
 176
 177 int main ()
 178 {
 179     int fd;
 180     struct mtrr_gentry gentry;
 181
 182     if ( ( fd = open ("/proc/mtrr", O_RDONLY, 0) ) == -1 )
 183     {
 184         if (errno == ENOENT)
 185         {
 186             fputs ("/proc/mtrr not found: not supported or you don't have a PPro?\n",
 187                    stderr);
 188             exit (1);
 189         }
 190         fprintf (stderr, "Error opening /proc/mtrr\t%s\n", ERRSTRING);
 191         exit (2);
 192     }
 193     for (gentry.regnum = 0; ioctl (fd, MTRRIOC_GET_ENTRY, &gentry) == 0;
 194          ++gentry.regnum)
 195     {
 196         if (gentry.size < 1)
 197         {
 198             fprintf (stderr, "Register: %u disabled\n", gentry.regnum);
 199             continue;
 200         }
 201         fprintf (stderr, "Register: %u base: 0x%lx size: 0x%lx type: %s\n",
 202                  gentry.regnum, gentry.base, gentry.size,
 203                  mtrr_strings[gentry.type]);
 204     }
 205     if (errno == EINVAL) exit (0);
 206     fprintf (stderr, "Error doing ioctl(2) on /dev/mtrr\t%s\n", ERRSTRING);
 207     exit (3);
 208 }   /*  End Function main  */
 209 ===============================================================================
 210 Creating MTRRs from a C programme using ioctl()'s:
 211
 212 /*  mtrr-add.c
 213
 214     Source file for mtrr-add (example programme to add an MTRRs using ioctl())
 215
 216     Copyright (C) 1997-1998  Richard Gooch
 217
 218     This program is free software; you can redistribute it and/or modify
 219     it under the terms of the GNU General Public License as published by
 220     the Free Software Foundation; either version 2 of the License, or
 221     (at your option) any later version.
 222
 223     This program is distributed in the hope that it will be useful,
 224     but WITHOUT ANY WARRANTY; without even the implied warranty of
 225     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 226     GNU General Public License for more details.
 227
 228     You should have received a copy of the GNU General Public License
 229     along with this program; if not, write to the Free Software
 230     Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
 231
 232     Richard Gooch may be reached by email at  rgooch@atnf.csiro.au
 233     The postal address is:
 234       Richard Gooch, c/o ATNF, P. O. Box 76, Epping, N.S.W., 2121, Australia.
 235 */
 236
 237 /*
 238     This programme will use an ioctl() on /proc/mtrr to add an entry. The first
 239     available mtrr is used. This is an alternative to writing /proc/mtrr.
 240
 241
 242     Written by      Richard Gooch   17-DEC-1997
 243
 244     Last updated by Richard Gooch   2-MAY-1998
 245
 246
 247 */
 248 #include <stdio.h>
 249 #include <string.h>
 250 #include <stdlib.h>
 251 #include <unistd.h>
 252 #include <sys/types.h>
 253 #include <sys/stat.h>
 254 #include <fcntl.h>
 255 #include <sys/ioctl.h>
 256 #include <errno.h>
 257 #include <asm/mtrr.h>
 258
 259 #define TRUE 1
 260 #define FALSE 0
 261 #define ERRSTRING strerror (errno)
 262
 263 static char *mtrr_strings[MTRR_NUM_TYPES] =
 264 {
 265     "uncachable",               /* 0 */
 266     "write-combining",          /* 1 */
 267     "?",                        /* 2 */
 268     "?",                        /* 3 */
 269     "write-through",            /* 4 */
 270     "write-protect",            /* 5 */
 271     "write-back",               /* 6 */
 272 };
 273
 274 int main (int argc, char **argv)
 275 {
 276     int fd;
 277     struct mtrr_sentry sentry;
 278
 279     if (argc != 4)
 280     {
 281         fprintf (stderr, "Usage:\tmtrr-add base size type\n");
 282         exit (1);
 283     }
 284     sentry.base = strtoul (argv[1], NULL, 0);
 285     sentry.size = strtoul (argv[2], NULL, 0);
 286     for (sentry.type = 0; sentry.type < MTRR_NUM_TYPES; ++sentry.type)
 287     {
 288         if (strcmp (argv[3], mtrr_strings[sentry.type]) == 0) break;
 289     }
 290     if (sentry.type >= MTRR_NUM_TYPES)
 291     {
 292         fprintf (stderr, "Illegal type: \"%s\"\n", argv[3]);
 293         exit (2);
 294     }
 295     if ( ( fd = open ("/proc/mtrr", O_WRONLY, 0) ) == -1 )
 296     {
 297         if (errno == ENOENT)
 298         {
 299             fputs ("/proc/mtrr not found: not supported or you don't have a PPro?\n",
 300                    stderr);
 301             exit (3);
 302         }
 303         fprintf (stderr, "Error opening /proc/mtrr\t%s\n", ERRSTRING);
 304         exit (4);
 305     }
 306     if (ioctl (fd, MTRRIOC_ADD_ENTRY, &sentry) == -1)
 307     {
 308         fprintf (stderr, "Error doing ioctl(2) on /dev/mtrr\t%s\n", ERRSTRING);
 309         exit (5);
 310     }
 311     fprintf (stderr, "Sleeping for 5 seconds so you can see the new entry\n");
 312     sleep (5);
 313     close (fd);
 314     fputs ("I've just closed /proc/mtrr so now the new entry should be gone\n",
 315            stderr);
 316 }   /*  End Function main  */
 317 ===============================================================================