FreeBSD/Linux Kernel Cross Reference
sys/Documentation/mtrr.txt

     MTRR (Memory Type Range Register) control
     3 Jun 1999
     Richard Gooch
     <rgooch@atnf.csiro.au>
     
       On Intel P6 family processors (Pentium Pro, Pentium II and later)
       the Memory Type Range Registers (MTRRs) may be used to control
       processor access to memory ranges. This is most useful when you have
       a video (VGA) card on a PCI or AGP bus. Enabling write-combining
      allows bus write transfers to be combined into a larger transfer
      before bursting over the PCI/AGP bus. This can increase performance
      of image write operations 2.5 times or more.
    
      The Cyrix 6x86, 6x86MX and M II processors have Address Range
      Registers (ARRs) which provide a similar functionality to MTRRs. For
      these, the ARRs are used to emulate the MTRRs.
    
      The AMD K6-2 (stepping 8 and above) and K6-3 processors have two
      MTRRs. These are supported.  The AMD Athlon family provide 8 Intel
      style MTRRs.
      
      The Centaur C6 (WinChip) has 8 MCRs, allowing write-combining. These
      are supported.
    
      The VIA Cyrix III and VIA C3 CPUs offer 8 Intel style MTRRs.
    
      The CONFIG_MTRR option creates a /proc/mtrr file which may be used
      to manipulate your MTRRs. Typically the X server should use
      this. This should have a reasonably generic interface so that
      similar control registers on other processors can be easily
      supported.
    
    
    There are two interfaces to /proc/mtrr: one is an ASCII interface
    which allows you to read and write. The other is an ioctl()
    interface. The ASCII interface is meant for administration. The
    ioctl() interface is meant for C programs (i.e. the X server). The
    interfaces are described below, with sample commands and C code.
    
    ===============================================================================
    Reading MTRRs from the shell:
    
    % cat /proc/mtrr
    reg00: base=0x00000000 (   0MB), size= 128MB: write-back, count=1
    reg01: base=0x08000000 ( 128MB), size=  64MB: write-back, count=1
    ===============================================================================
    Creating MTRRs from the C-shell:
    # echo "base=0xf8000000 size=0x400000 type=write-combining" >! /proc/mtrr
    or if you use bash:
    # echo "base=0xf8000000 size=0x400000 type=write-combining" >| /proc/mtrr
    
    And the result thereof:
    % cat /proc/mtrr
    reg00: base=0x00000000 (   0MB), size= 128MB: write-back, count=1
    reg01: base=0x08000000 ( 128MB), size=  64MB: write-back, count=1
    reg02: base=0xf8000000 (3968MB), size=   4MB: write-combining, count=1
    
    This is for video RAM at base address 0xf8000000 and size 4 megabytes. To
    find out your base address, you need to look at the output of your X
    server, which tells you where the linear framebuffer address is. A
    typical line that you may get is:
    
    (--) S3: PCI: 968 rev 0, Linear FB @ 0xf8000000
    
    Note that you should only use the value from the X server, as it may
    move the framebuffer base address, so the only value you can trust is
    that reported by the X server.
    
    To find out the size of your framebuffer (what, you don't actually
    know?), the following line will tell you:
    
    (--) S3: videoram:  4096k
    
    That's 4 megabytes, which is 0x400000 bytes (in hexadecimal).
    A patch is being written for XFree86 which will make this automatic:
    in other words the X server will manipulate /proc/mtrr using the
    ioctl() interface, so users won't have to do anything. If you use a
    commercial X server, lobby your vendor to add support for MTRRs.
    ===============================================================================
    Creating overlapping MTRRs:
    
    %echo "base=0xfb000000 size=0x1000000 type=write-combining" >/proc/mtrr
    %echo "base=0xfb000000 size=0x1000 type=uncachable" >/proc/mtrr
    
    And the results: cat /proc/mtrr
    reg00: base=0x00000000 (   0MB), size=  64MB: write-back, count=1
    reg01: base=0xfb000000 (4016MB), size=  16MB: write-combining, count=1
    reg02: base=0xfb000000 (4016MB), size=   4kB: uncachable, count=1
    
    Some cards (especially Voodoo Graphics boards) need this 4 kB area 
    excluded from the beginning of the region because it is used for
    registers.
    
    NOTE: You can only create type=uncachable region, if the first
    region that you created is type=write-combining.
    ===============================================================================
    Removing MTRRs from the C-shell:
    % echo "disable=2" >! /proc/mtrr
    or using bash:
   % echo "disable=2" >| /proc/mtrr
   ===============================================================================
   Reading MTRRs from a C program using ioctl()'s:
   
   /*  mtrr-show.c
   
       Source file for mtrr-show (example program to show MTRRs using ioctl()'s)
   
       Copyright (C) 1997-1998  Richard Gooch
   
       This program is free software; you can redistribute it and/or modify
       it under the terms of the GNU General Public License as published by
       the Free Software Foundation; either version 2 of the License, or
       (at your option) any later version.
   
       This program is distributed in the hope that it will be useful,
       but WITHOUT ANY WARRANTY; without even the implied warranty of
       MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
       GNU General Public License for more details.
   
       You should have received a copy of the GNU General Public License
       along with this program; if not, write to the Free Software
       Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
   
       Richard Gooch may be reached by email at  rgooch@atnf.csiro.au
       The postal address is:
         Richard Gooch, c/o ATNF, P. O. Box 76, Epping, N.S.W., 2121, Australia.
   */
   
   /*
       This program will use an ioctl() on /proc/mtrr to show the current MTRR
       settings. This is an alternative to reading /proc/mtrr.
   
   
       Written by      Richard Gooch   17-DEC-1997
   
       Last updated by Richard Gooch   2-MAY-1998
   
   
   */
   #include <stdio.h>
   #include <stdlib.h>
   #include <string.h>
   #include <sys/types.h>
   #include <sys/stat.h>
   #include <fcntl.h>
   #include <sys/ioctl.h>
   #include <errno.h>
   #include <asm/mtrr.h>
   
   #define TRUE 1
   #define FALSE 0
   #define ERRSTRING strerror (errno)
   
   static char *mtrr_strings[MTRR_NUM_TYPES] =
   {
       "uncachable",               /* 0 */
       "write-combining",          /* 1 */
       "?",                        /* 2 */
       "?",                        /* 3 */
       "write-through",            /* 4 */
       "write-protect",            /* 5 */
       "write-back",               /* 6 */
   };
   
   int main ()
   {
       int fd;
       struct mtrr_gentry gentry;
   
       if ( ( fd = open ("/proc/mtrr", O_RDONLY, 0) ) == -1 )
       {
           if (errno == ENOENT)
           {
               fputs ("/proc/mtrr not found: not supported or you don't have a PPro?\n",
                      stderr);
               exit (1);
           }
           fprintf (stderr, "Error opening /proc/mtrr\t%s\n", ERRSTRING);
           exit (2);
       }
       for (gentry.regnum = 0; ioctl (fd, MTRRIOC_GET_ENTRY, &gentry) == 0;
            ++gentry.regnum)
       {
           if (gentry.size < 1)
           {
               fprintf (stderr, "Register: %u disabled\n", gentry.regnum);
               continue;
           }
           fprintf (stderr, "Register: %u base: 0x%lx size: 0x%lx type: %s\n",
                    gentry.regnum, gentry.base, gentry.size,
                    mtrr_strings[gentry.type]);
       }
       if (errno == EINVAL) exit (0);
       fprintf (stderr, "Error doing ioctl(2) on /dev/mtrr\t%s\n", ERRSTRING);
       exit (3);
   }   /*  End Function main  */
   ===============================================================================
   Creating MTRRs from a C programme using ioctl()'s:
   
   /*  mtrr-add.c
   
       Source file for mtrr-add (example programme to add an MTRRs using ioctl())
   
       Copyright (C) 1997-1998  Richard Gooch
   
       This program is free software; you can redistribute it and/or modify
       it under the terms of the GNU General Public License as published by
       the Free Software Foundation; either version 2 of the License, or
       (at your option) any later version.
   
       This program is distributed in the hope that it will be useful,
       but WITHOUT ANY WARRANTY; without even the implied warranty of
       MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
       GNU General Public License for more details.
   
       You should have received a copy of the GNU General Public License
       along with this program; if not, write to the Free Software
       Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
   
       Richard Gooch may be reached by email at  rgooch@atnf.csiro.au
       The postal address is:
         Richard Gooch, c/o ATNF, P. O. Box 76, Epping, N.S.W., 2121, Australia.
   */
   
   /*
       This programme will use an ioctl() on /proc/mtrr to add an entry. The first
       available mtrr is used. This is an alternative to writing /proc/mtrr.
   
   
       Written by      Richard Gooch   17-DEC-1997
   
       Last updated by Richard Gooch   2-MAY-1998
   
   
   */
   #include <stdio.h>
   #include <string.h>
   #include <stdlib.h>
   #include <unistd.h>
   #include <sys/types.h>
   #include <sys/stat.h>
   #include <fcntl.h>
   #include <sys/ioctl.h>
   #include <errno.h>
   #include <asm/mtrr.h>
   
   #define TRUE 1
   #define FALSE 0
   #define ERRSTRING strerror (errno)
   
   static char *mtrr_strings[MTRR_NUM_TYPES] =
   {
       "uncachable",               /* 0 */
       "write-combining",          /* 1 */
       "?",                        /* 2 */
       "?",                        /* 3 */
       "write-through",            /* 4 */
       "write-protect",            /* 5 */
       "write-back",               /* 6 */
   };
   
   int main (int argc, char **argv)
   {
       int fd;
       struct mtrr_sentry sentry;
   
       if (argc != 4)
       {
           fprintf (stderr, "Usage:\tmtrr-add base size type\n");
           exit (1);
       }
       sentry.base = strtoul (argv[1], NULL, 0);
       sentry.size = strtoul (argv[2], NULL, 0);
       for (sentry.type = 0; sentry.type < MTRR_NUM_TYPES; ++sentry.type)
       {
           if (strcmp (argv[3], mtrr_strings[sentry.type]) == 0) break;
       }
       if (sentry.type >= MTRR_NUM_TYPES)
       {
           fprintf (stderr, "Illegal type: \"%s\"\n", argv[3]);
           exit (2);
       }
       if ( ( fd = open ("/proc/mtrr", O_WRONLY, 0) ) == -1 )
       {
           if (errno == ENOENT)
           {
               fputs ("/proc/mtrr not found: not supported or you don't have a PPro?\n",
                      stderr);
               exit (3);
           }
           fprintf (stderr, "Error opening /proc/mtrr\t%s\n", ERRSTRING);
           exit (4);
       }
       if (ioctl (fd, MTRRIOC_ADD_ENTRY, &sentry) == -1)
       {
           fprintf (stderr, "Error doing ioctl(2) on /dev/mtrr\t%s\n", ERRSTRING);
           exit (5);
       }
       fprintf (stderr, "Sleeping for 5 seconds so you can see the new entry\n");
       sleep (5);
       close (fd);
       fputs ("I've just closed /proc/mtrr so now the new entry should be gone\n",
              stderr);
   }   /*  End Function main  */
   ===============================================================================

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