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. Direct MTRR use by
  10 drivers on Linux is now completely phased out, device drivers should use
  11 arch_phys_wc_add() in combination with ioremap_wc() to make MTRR effective on
  12 non-PAT systems while a no-op but equally effective on PAT enabled systems.
  13
  14 Even if Linux does not use MTRRs directly, some x86 platform firmware may still
  15 set up MTRRs early before booting the OS. They do this as some platform
  16 firmware may still have implemented access to MTRRs which would be controlled
  17 and handled by the platform firmware directly. An example of platform use of
  18 MTRRs is through the use of SMI handlers, one case could be for fan control,
  19 the platform code would need uncachable access to some of its fan control
  20 registers. Such platform access does not need any Operating System MTRR code in
  21 place other than mtrr_type_lookup() to ensure any OS specific mapping requests
  22 are aligned with platform MTRR setup. If MTRRs are only set up by the platform
  23 firmware code though and the OS does not make any specific MTRR mapping
  24 requests mtrr_type_lookup() should always return MTRR_TYPE_INVALID.
  25
  26 For details refer to Documentation/x86/pat.txt.
  27
  28 ===============================================================================
  29
  30   On Intel P6 family processors (Pentium Pro, Pentium II and later)
  31   the Memory Type Range Registers (MTRRs) may be used to control
  32   processor access to memory ranges. This is most useful when you have
  33   a video (VGA) card on a PCI or AGP bus. Enabling write-combining
  34   allows bus write transfers to be combined into a larger transfer
  35   before bursting over the PCI/AGP bus. This can increase performance
  36   of image write operations 2.5 times or more.
  37
  38   The Cyrix 6x86, 6x86MX and M II processors have Address Range
  39   Registers (ARRs) which provide a similar functionality to MTRRs. For
  40   these, the ARRs are used to emulate the MTRRs.
  41
  42   The AMD K6-2 (stepping 8 and above) and K6-3 processors have two
  43   MTRRs. These are supported.  The AMD Athlon family provide 8 Intel
  44   style MTRRs.
  45
  46   The Centaur C6 (WinChip) has 8 MCRs, allowing write-combining. These
  47   are supported.
  48
  49   The VIA Cyrix III and VIA C3 CPUs offer 8 Intel style MTRRs.
  50
  51   The CONFIG_MTRR option creates a /proc/mtrr file which may be used
  52   to manipulate your MTRRs. Typically the X server should use
  53   this. This should have a reasonably generic interface so that
  54   similar control registers on other processors can be easily
  55   supported.
  56
  57
  58 There are two interfaces to /proc/mtrr: one is an ASCII interface
  59 which allows you to read and write. The other is an ioctl()
  60 interface. The ASCII interface is meant for administration. The
  61 ioctl() interface is meant for C programs (i.e. the X server). The
  62 interfaces are described below, with sample commands and C code.
  63
  64 ===============================================================================
  65 Reading MTRRs from the shell:
  66
  67 % cat /proc/mtrr
  68 reg00: base=0x00000000 (   0MB), size= 128MB: write-back, count=1
  69 reg01: base=0x08000000 ( 128MB), size=  64MB: write-back, count=1
  70 ===============================================================================
  71 Creating MTRRs from the C-shell:
  72 # echo "base=0xf8000000 size=0x400000 type=write-combining" >! /proc/mtrr
  73 or if you use bash:
  74 # echo "base=0xf8000000 size=0x400000 type=write-combining" >| /proc/mtrr
  75
  76 And the result thereof:
  77 % cat /proc/mtrr
  78 reg00: base=0x00000000 (   0MB), size= 128MB: write-back, count=1
  79 reg01: base=0x08000000 ( 128MB), size=  64MB: write-back, count=1
  80 reg02: base=0xf8000000 (3968MB), size=   4MB: write-combining, count=1
  81
  82 This is for video RAM at base address 0xf8000000 and size 4 megabytes. To
  83 find out your base address, you need to look at the output of your X
  84 server, which tells you where the linear framebuffer address is. A
  85 typical line that you may get is:
  86
  87 (--) S3: PCI: 968 rev 0, Linear FB @ 0xf8000000
  88
  89 Note that you should only use the value from the X server, as it may
  90 move the framebuffer base address, so the only value you can trust is
  91 that reported by the X server.
  92
  93 To find out the size of your framebuffer (what, you don't actually
  94 know?), the following line will tell you:
  95
  96 (--) S3: videoram:  4096k
  97
  98 That's 4 megabytes, which is 0x400000 bytes (in hexadecimal).
  99 A patch is being written for XFree86 which will make this automatic:
 100 in other words the X server will manipulate /proc/mtrr using the
 101 ioctl() interface, so users won't have to do anything. If you use a
 102 commercial X server, lobby your vendor to add support for MTRRs.
 103 ===============================================================================
 104 Creating overlapping MTRRs:
 105
 106 %echo "base=0xfb000000 size=0x1000000 type=write-combining" >/proc/mtrr
 107 %echo "base=0xfb000000 size=0x1000 type=uncachable" >/proc/mtrr
 108
 109 And the results: cat /proc/mtrr
 110 reg00: base=0x00000000 (   0MB), size=  64MB: write-back, count=1
 111 reg01: base=0xfb000000 (4016MB), size=  16MB: write-combining, count=1
 112 reg02: base=0xfb000000 (4016MB), size=   4kB: uncachable, count=1
 113
 114 Some cards (especially Voodoo Graphics boards) need this 4 kB area
 115 excluded from the beginning of the region because it is used for
 116 registers.
 117
 118 NOTE: You can only create type=uncachable region, if the first
 119 region that you created is type=write-combining.
 120 ===============================================================================
 121 Removing MTRRs from the C-shell:
 122 % echo "disable=2" >! /proc/mtrr
 123 or using bash:
 124 % echo "disable=2" >| /proc/mtrr
 125 ===============================================================================
 126 Reading MTRRs from a C program using ioctl()'s:
 127
 128 /*  mtrr-show.c
 129
 130     Source file for mtrr-show (example program to show MTRRs using ioctl()'s)
 131
 132     Copyright (C) 1997-1998  Richard Gooch
 133
 134     This program is free software; you can redistribute it and/or modify
 135     it under the terms of the GNU General Public License as published by
 136     the Free Software Foundation; either version 2 of the License, or
 137     (at your option) any later version.
 138
 139     This program is distributed in the hope that it will be useful,
 140     but WITHOUT ANY WARRANTY; without even the implied warranty of
 141     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 142     GNU General Public License for more details.
 143
 144     You should have received a copy of the GNU General Public License
 145     along with this program; if not, write to the Free Software
 146     Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
 147
 148     Richard Gooch may be reached by email at  rgooch@atnf.csiro.au
 149     The postal address is:
 150       Richard Gooch, c/o ATNF, P. O. Box 76, Epping, N.S.W., 2121, Australia.
 151 */
 152
 153 /*
 154     This program will use an ioctl() on /proc/mtrr to show the current MTRR
 155     settings. This is an alternative to reading /proc/mtrr.
 156
 157
 158     Written by      Richard Gooch   17-DEC-1997
 159
 160     Last updated by Richard Gooch   2-MAY-1998
 161
 162
 163 */
 164 #include <stdio.h>
 165 #include <stdlib.h>
 166 #include <string.h>
 167 #include <sys/types.h>
 168 #include <sys/stat.h>
 169 #include <fcntl.h>
 170 #include <sys/ioctl.h>
 171 #include <errno.h>
 172 #include <asm/mtrr.h>
 173
 174 #define TRUE 1
 175 #define FALSE 0
 176 #define ERRSTRING strerror (errno)
 177
 178 static char *mtrr_strings[MTRR_NUM_TYPES] =
 179 {
 180     "uncachable",               /* 0 */
 181     "write-combining",          /* 1 */
 182     "?",                        /* 2 */
 183     "?",                        /* 3 */
 184     "write-through",            /* 4 */
 185     "write-protect",            /* 5 */
 186     "write-back",               /* 6 */
 187 };
 188
 189 int main ()
 190 {
 191     int fd;
 192     struct mtrr_gentry gentry;
 193
 194     if ( ( fd = open ("/proc/mtrr", O_RDONLY, 0) ) == -1 )
 195     {
 196         if (errno == ENOENT)
 197         {
 198             fputs ("/proc/mtrr not found: not supported or you don't have a PPro?\n",
 199                    stderr);
 200             exit (1);
 201         }
 202         fprintf (stderr, "Error opening /proc/mtrr\t%s\n", ERRSTRING);
 203         exit (2);
 204     }
 205     for (gentry.regnum = 0; ioctl (fd, MTRRIOC_GET_ENTRY, &gentry) == 0;
 206          ++gentry.regnum)
 207     {
 208         if (gentry.size < 1)
 209         {
 210             fprintf (stderr, "Register: %u disabled\n", gentry.regnum);
 211             continue;
 212         }
 213         fprintf (stderr, "Register: %u base: 0x%lx size: 0x%lx type: %s\n",
 214                  gentry.regnum, gentry.base, gentry.size,
 215                  mtrr_strings[gentry.type]);
 216     }
 217     if (errno == EINVAL) exit (0);
 218     fprintf (stderr, "Error doing ioctl(2) on /dev/mtrr\t%s\n", ERRSTRING);
 219     exit (3);
 220 }   /*  End Function main  */
 221 ===============================================================================
 222 Creating MTRRs from a C programme using ioctl()'s:
 223
 224 /*  mtrr-add.c
 225
 226     Source file for mtrr-add (example programme to add an MTRRs using ioctl())
 227
 228     Copyright (C) 1997-1998  Richard Gooch
 229
 230     This program is free software; you can redistribute it and/or modify
 231     it under the terms of the GNU General Public License as published by
 232     the Free Software Foundation; either version 2 of the License, or
 233     (at your option) any later version.
 234
 235     This program is distributed in the hope that it will be useful,
 236     but WITHOUT ANY WARRANTY; without even the implied warranty of
 237     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 238     GNU General Public License for more details.
 239
 240     You should have received a copy of the GNU General Public License
 241     along with this program; if not, write to the Free Software
 242     Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
 243
 244     Richard Gooch may be reached by email at  rgooch@atnf.csiro.au
 245     The postal address is:
 246       Richard Gooch, c/o ATNF, P. O. Box 76, Epping, N.S.W., 2121, Australia.
 247 */
 248
 249 /*
 250     This programme will use an ioctl() on /proc/mtrr to add an entry. The first
 251     available mtrr is used. This is an alternative to writing /proc/mtrr.
 252
 253
 254     Written by      Richard Gooch   17-DEC-1997
 255
 256     Last updated by Richard Gooch   2-MAY-1998
 257
 258
 259 */
 260 #include <stdio.h>
 261 #include <string.h>
 262 #include <stdlib.h>
 263 #include <unistd.h>
 264 #include <sys/types.h>
 265 #include <sys/stat.h>
 266 #include <fcntl.h>
 267 #include <sys/ioctl.h>
 268 #include <errno.h>
 269 #include <asm/mtrr.h>
 270
 271 #define TRUE 1
 272 #define FALSE 0
 273 #define ERRSTRING strerror (errno)
 274
 275 static char *mtrr_strings[MTRR_NUM_TYPES] =
 276 {
 277     "uncachable",               /* 0 */
 278     "write-combining",          /* 1 */
 279     "?",                        /* 2 */
 280     "?",                        /* 3 */
 281     "write-through",            /* 4 */
 282     "write-protect",            /* 5 */
 283     "write-back",               /* 6 */
 284 };
 285
 286 int main (int argc, char **argv)
 287 {
 288     int fd;
 289     struct mtrr_sentry sentry;
 290
 291     if (argc != 4)
 292     {
 293         fprintf (stderr, "Usage:\tmtrr-add base size type\n");
 294         exit (1);
 295     }
 296     sentry.base = strtoul (argv[1], NULL, 0);
 297     sentry.size = strtoul (argv[2], NULL, 0);
 298     for (sentry.type = 0; sentry.type < MTRR_NUM_TYPES; ++sentry.type)
 299     {
 300         if (strcmp (argv[3], mtrr_strings[sentry.type]) == 0) break;
 301     }
 302     if (sentry.type >= MTRR_NUM_TYPES)
 303     {
 304         fprintf (stderr, "Illegal type: \"%s\"\n", argv[3]);
 305         exit (2);
 306     }
 307     if ( ( fd = open ("/proc/mtrr", O_WRONLY, 0) ) == -1 )
 308     {
 309         if (errno == ENOENT)
 310         {
 311             fputs ("/proc/mtrr not found: not supported or you don't have a PPro?\n",
 312                    stderr);
 313             exit (3);
 314         }
 315         fprintf (stderr, "Error opening /proc/mtrr\t%s\n", ERRSTRING);
 316         exit (4);
 317     }
 318     if (ioctl (fd, MTRRIOC_ADD_ENTRY, &sentry) == -1)
 319     {
 320         fprintf (stderr, "Error doing ioctl(2) on /dev/mtrr\t%s\n", ERRSTRING);
 321         exit (5);
 322     }
 323     fprintf (stderr, "Sleeping for 5 seconds so you can see the new entry\n");
 324     sleep (5);
 325     close (fd);
 326     fputs ("I've just closed /proc/mtrr so now the new entry should be gone\n",
 327            stderr);
 328 }   /*  End Function main  */
 329 ===============================================================================