10. MTRR (Memory Type Range Register) control¶
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10.1. Phasing out MTRR use¶
MTRR use is replaced on modern x86 hardware with PAT. Direct MTRR use bydrivers on Linux is now completely phased out, device drivers should usearch_phys_wc_add() in combination with ioremap_wc() to make MTRR effective onnon-PAT systems while a no-op but equally effective on PAT enabled systems.
Even if Linux does not use MTRRs directly, some x86 platform firmware may stillset up MTRRs early before booting the OS. They do this as some platformfirmware may still have implemented access to MTRRs which would be controlledand handled by the platform firmware directly. An example of platform use ofMTRRs is through the use of SMI handlers, one case could be for fan control,the platform code would need uncachable access to some of its fan controlregisters. Such platform access does not need any Operating System MTRR code inplace other than mtrr_type_lookup() to ensure any OS specific mapping requestsare aligned with platform MTRR setup. If MTRRs are only set up by the platformfirmware code though and the OS does not make any specific MTRR mappingrequests mtrr_type_lookup() should always return MTRR_TYPE_INVALID.
For details refer toPAT (Page Attribute Table).
Tip
On Intel P6 family processors (Pentium Pro, Pentium II and later)the Memory Type Range Registers (MTRRs) may be used to controlprocessor access to memory ranges. This is most useful when you havea video (VGA) card on a PCI or AGP bus. Enabling write-combiningallows bus write transfers to be combined into a larger transferbefore bursting over the PCI/AGP bus. This can increase performanceof image write operations 2.5 times or more.
The Cyrix 6x86, 6x86MX and M II processors have Address RangeRegisters (ARRs) which provide a similar functionality to MTRRs. Forthese, the ARRs are used to emulate the MTRRs.
The AMD K6-2 (stepping 8 and above) and K6-3 processors have twoMTRRs. These are supported. The AMD Athlon family provide 8 Intelstyle MTRRs.
The Centaur C6 (WinChip) has 8 MCRs, allowing write-combining. Theseare 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 usedto manipulate your MTRRs. Typically the X server should usethis. This should have a reasonably generic interface so thatsimilar control registers on other processors can be easilysupported.
There are two interfaces to /proc/mtrr: one is an ASCII interfacewhich allows you to read and write. The other is an ioctl()interface. The ASCII interface is meant for administration. Theioctl() interface is meant for C programs (i.e. the X server). Theinterfaces are described below, with sample commands and C code.
10.2. Reading MTRRs from the shell¶
% cat /proc/mtrrreg00: base=0x00000000 ( 0MB), size= 128MB: write-back, count=1reg01: 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/mtrrreg00: base=0x00000000 ( 0MB), size= 128MB: write-back, count=1reg01: base=0x08000000 ( 128MB), size= 64MB: write-back, count=1reg02: base=0xf8000000 (3968MB), size= 4MB: write-combining, count=1
This is for video RAM at base address 0xf8000000 and size 4 megabytes. Tofind out your base address, you need to look at the output of your Xserver, which tells you where the linear framebuffer address is. Atypical 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 maymove the framebuffer base address, so the only value you can trust isthat reported by the X server.
To find out the size of your framebuffer (what, you don’t actuallyknow?), 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 theioctl() interface, so users won’t have to do anything. If you use acommercial X server, lobby your vendor to add support for MTRRs.
10.3. 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/mtrrreg00: base=0x00000000 ( 0MB), size= 64MB: write-back, count=1reg01: base=0xfb000000 (4016MB), size= 16MB: write-combining, count=1reg02: base=0xfb000000 (4016MB), size= 4kB: uncachable, count=1
Some cards (especially Voodoo Graphics boards) need this 4 kB areaexcluded from the beginning of the region because it is used forregisters.
NOTE: You can only create type=uncachable region, if the firstregion that you created is type=write-combining.
10.4. Removing MTRRs from the C-shel¶
% echo "disable=2" >! /proc/mtrr
or using bash:
% echo "disable=2" >| /proc/mtrr
10.5. 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 */10.6. 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 */