355 lines
12 KiB
ReStructuredText
355 lines
12 KiB
ReStructuredText
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.. SPDX-License-Identifier: GPL-2.0
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=========================================
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MTRR (Memory Type Range Register) control
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=========================================
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:Authors: - Richard Gooch <rgooch@atnf.csiro.au> - 3 Jun 1999
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- Luis R. Rodriguez <mcgrof@do-not-panic.com> - April 9, 2015
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Phasing out MTRR use
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====================
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MTRR use is replaced on modern x86 hardware with PAT. Direct MTRR use by
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drivers on Linux is now completely phased out, device drivers should use
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arch_phys_wc_add() in combination with ioremap_wc() to make MTRR effective on
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non-PAT systems while a no-op but equally effective on PAT enabled systems.
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Even if Linux does not use MTRRs directly, some x86 platform firmware may still
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set up MTRRs early before booting the OS. They do this as some platform
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firmware may still have implemented access to MTRRs which would be controlled
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and handled by the platform firmware directly. An example of platform use of
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MTRRs is through the use of SMI handlers, one case could be for fan control,
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the platform code would need uncachable access to some of its fan control
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registers. Such platform access does not need any Operating System MTRR code in
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place other than mtrr_type_lookup() to ensure any OS specific mapping requests
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are aligned with platform MTRR setup. If MTRRs are only set up by the platform
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firmware code though and the OS does not make any specific MTRR mapping
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requests mtrr_type_lookup() should always return MTRR_TYPE_INVALID.
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For details refer to Documentation/arch/x86/pat.rst.
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.. tip::
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On Intel P6 family processors (Pentium Pro, Pentium II and later)
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the Memory Type Range Registers (MTRRs) may be used to control
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processor access to memory ranges. This is most useful when you have
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a video (VGA) card on a PCI or AGP bus. Enabling write-combining
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allows bus write transfers to be combined into a larger transfer
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before bursting over the PCI/AGP bus. This can increase performance
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of image write operations 2.5 times or more.
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The Cyrix 6x86, 6x86MX and M II processors have Address Range
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Registers (ARRs) which provide a similar functionality to MTRRs. For
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these, the ARRs are used to emulate the MTRRs.
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The AMD K6-2 (stepping 8 and above) and K6-3 processors have two
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MTRRs. These are supported. The AMD Athlon family provide 8 Intel
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style MTRRs.
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The Centaur C6 (WinChip) has 8 MCRs, allowing write-combining. These
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are supported.
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The VIA Cyrix III and VIA C3 CPUs offer 8 Intel style MTRRs.
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The CONFIG_MTRR option creates a /proc/mtrr file which may be used
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to manipulate your MTRRs. Typically the X server should use
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this. This should have a reasonably generic interface so that
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similar control registers on other processors can be easily
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supported.
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There are two interfaces to /proc/mtrr: one is an ASCII interface
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which allows you to read and write. The other is an ioctl()
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interface. The ASCII interface is meant for administration. The
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ioctl() interface is meant for C programs (i.e. the X server). The
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interfaces are described below, with sample commands and C code.
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Reading MTRRs from the shell
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============================
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::
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% cat /proc/mtrr
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reg00: base=0x00000000 ( 0MB), size= 128MB: write-back, count=1
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reg01: base=0x08000000 ( 128MB), size= 64MB: write-back, count=1
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Creating MTRRs from the C-shell::
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# echo "base=0xf8000000 size=0x400000 type=write-combining" >! /proc/mtrr
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or if you use bash::
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# echo "base=0xf8000000 size=0x400000 type=write-combining" >| /proc/mtrr
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And the result thereof::
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% cat /proc/mtrr
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reg00: base=0x00000000 ( 0MB), size= 128MB: write-back, count=1
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reg01: base=0x08000000 ( 128MB), size= 64MB: write-back, count=1
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reg02: base=0xf8000000 (3968MB), size= 4MB: write-combining, count=1
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This is for video RAM at base address 0xf8000000 and size 4 megabytes. To
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find out your base address, you need to look at the output of your X
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server, which tells you where the linear framebuffer address is. A
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typical line that you may get is::
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(--) S3: PCI: 968 rev 0, Linear FB @ 0xf8000000
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Note that you should only use the value from the X server, as it may
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move the framebuffer base address, so the only value you can trust is
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that reported by the X server.
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To find out the size of your framebuffer (what, you don't actually
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know?), the following line will tell you::
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(--) S3: videoram: 4096k
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That's 4 megabytes, which is 0x400000 bytes (in hexadecimal).
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A patch is being written for XFree86 which will make this automatic:
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in other words the X server will manipulate /proc/mtrr using the
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ioctl() interface, so users won't have to do anything. If you use a
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commercial X server, lobby your vendor to add support for MTRRs.
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Creating overlapping MTRRs
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==========================
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::
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%echo "base=0xfb000000 size=0x1000000 type=write-combining" >/proc/mtrr
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%echo "base=0xfb000000 size=0x1000 type=uncachable" >/proc/mtrr
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And the results::
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% cat /proc/mtrr
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reg00: base=0x00000000 ( 0MB), size= 64MB: write-back, count=1
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reg01: base=0xfb000000 (4016MB), size= 16MB: write-combining, count=1
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reg02: base=0xfb000000 (4016MB), size= 4kB: uncachable, count=1
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Some cards (especially Voodoo Graphics boards) need this 4 kB area
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excluded from the beginning of the region because it is used for
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registers.
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NOTE: You can only create type=uncachable region, if the first
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region that you created is type=write-combining.
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Removing MTRRs from the C-shel
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==============================
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::
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% echo "disable=2" >! /proc/mtrr
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or using bash::
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% echo "disable=2" >| /proc/mtrr
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Reading MTRRs from a C program using ioctl()'s
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==============================================
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::
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/* mtrr-show.c
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Source file for mtrr-show (example program to show MTRRs using ioctl()'s)
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Copyright (C) 1997-1998 Richard Gooch
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This program is free software; you can redistribute it and/or modify
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it under the terms of the GNU General Public License as published by
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the Free Software Foundation; either version 2 of the License, or
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(at your option) any later version.
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This program is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with this program; if not, write to the Free Software
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Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
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Richard Gooch may be reached by email at rgooch@atnf.csiro.au
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The postal address is:
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Richard Gooch, c/o ATNF, P. O. Box 76, Epping, N.S.W., 2121, Australia.
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*/
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/*
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This program will use an ioctl() on /proc/mtrr to show the current MTRR
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settings. This is an alternative to reading /proc/mtrr.
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Written by Richard Gooch 17-DEC-1997
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Last updated by Richard Gooch 2-MAY-1998
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*/
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#include <stdio.h>
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#include <stdlib.h>
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#include <string.h>
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#include <sys/types.h>
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#include <sys/stat.h>
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#include <fcntl.h>
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#include <sys/ioctl.h>
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#include <errno.h>
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#include <asm/mtrr.h>
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#define TRUE 1
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#define FALSE 0
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#define ERRSTRING strerror (errno)
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static char *mtrr_strings[MTRR_NUM_TYPES] =
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{
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"uncachable", /* 0 */
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"write-combining", /* 1 */
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"?", /* 2 */
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"?", /* 3 */
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"write-through", /* 4 */
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"write-protect", /* 5 */
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"write-back", /* 6 */
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};
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int main ()
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{
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int fd;
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struct mtrr_gentry gentry;
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if ( ( fd = open ("/proc/mtrr", O_RDONLY, 0) ) == -1 )
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{
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if (errno == ENOENT)
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{
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fputs ("/proc/mtrr not found: not supported or you don't have a PPro?\n",
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stderr);
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exit (1);
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}
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fprintf (stderr, "Error opening /proc/mtrr\t%s\n", ERRSTRING);
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exit (2);
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}
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for (gentry.regnum = 0; ioctl (fd, MTRRIOC_GET_ENTRY, &gentry) == 0;
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++gentry.regnum)
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{
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if (gentry.size < 1)
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{
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fprintf (stderr, "Register: %u disabled\n", gentry.regnum);
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continue;
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}
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fprintf (stderr, "Register: %u base: 0x%lx size: 0x%lx type: %s\n",
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gentry.regnum, gentry.base, gentry.size,
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mtrr_strings[gentry.type]);
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}
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if (errno == EINVAL) exit (0);
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fprintf (stderr, "Error doing ioctl(2) on /dev/mtrr\t%s\n", ERRSTRING);
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exit (3);
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} /* End Function main */
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Creating MTRRs from a C programme using ioctl()'s
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=================================================
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::
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/* mtrr-add.c
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Source file for mtrr-add (example programme to add an MTRRs using ioctl())
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Copyright (C) 1997-1998 Richard Gooch
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This program is free software; you can redistribute it and/or modify
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it under the terms of the GNU General Public License as published by
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the Free Software Foundation; either version 2 of the License, or
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(at your option) any later version.
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This program is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with this program; if not, write to the Free Software
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Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
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Richard Gooch may be reached by email at rgooch@atnf.csiro.au
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The postal address is:
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Richard Gooch, c/o ATNF, P. O. Box 76, Epping, N.S.W., 2121, Australia.
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*/
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/*
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This programme will use an ioctl() on /proc/mtrr to add an entry. The first
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available mtrr is used. This is an alternative to writing /proc/mtrr.
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Written by Richard Gooch 17-DEC-1997
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Last updated by Richard Gooch 2-MAY-1998
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*/
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#include <stdio.h>
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#include <string.h>
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#include <stdlib.h>
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#include <unistd.h>
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#include <sys/types.h>
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#include <sys/stat.h>
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#include <fcntl.h>
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#include <sys/ioctl.h>
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#include <errno.h>
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#include <asm/mtrr.h>
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#define TRUE 1
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#define FALSE 0
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#define ERRSTRING strerror (errno)
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static char *mtrr_strings[MTRR_NUM_TYPES] =
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{
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"uncachable", /* 0 */
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"write-combining", /* 1 */
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"?", /* 2 */
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"?", /* 3 */
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"write-through", /* 4 */
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"write-protect", /* 5 */
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"write-back", /* 6 */
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};
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int main (int argc, char **argv)
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{
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int fd;
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struct mtrr_sentry sentry;
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if (argc != 4)
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{
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fprintf (stderr, "Usage:\tmtrr-add base size type\n");
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exit (1);
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}
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sentry.base = strtoul (argv[1], NULL, 0);
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sentry.size = strtoul (argv[2], NULL, 0);
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for (sentry.type = 0; sentry.type < MTRR_NUM_TYPES; ++sentry.type)
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{
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if (strcmp (argv[3], mtrr_strings[sentry.type]) == 0) break;
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}
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if (sentry.type >= MTRR_NUM_TYPES)
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{
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fprintf (stderr, "Illegal type: \"%s\"\n", argv[3]);
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exit (2);
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}
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if ( ( fd = open ("/proc/mtrr", O_WRONLY, 0) ) == -1 )
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{
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if (errno == ENOENT)
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{
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fputs ("/proc/mtrr not found: not supported or you don't have a PPro?\n",
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stderr);
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exit (3);
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}
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fprintf (stderr, "Error opening /proc/mtrr\t%s\n", ERRSTRING);
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exit (4);
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}
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if (ioctl (fd, MTRRIOC_ADD_ENTRY, &sentry) == -1)
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{
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fprintf (stderr, "Error doing ioctl(2) on /dev/mtrr\t%s\n", ERRSTRING);
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exit (5);
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}
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fprintf (stderr, "Sleeping for 5 seconds so you can see the new entry\n");
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sleep (5);
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close (fd);
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fputs ("I've just closed /proc/mtrr so now the new entry should be gone\n",
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stderr);
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} /* End Function main */
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