[内核文档]MTRR (Memory Type Range Register) 控制
MTRR (Memory Type Range Register) control
在Intel P6系列的处理器上（Pentium Pro, Pentium II and later），内存类型范围寄存器（MTRRs）可以控制处理器对内存区域的访问。 如果系统中存在基于PCI或AGP总线的视频卡（VGA），MTRR将具有非常显著的作用。通过使能写捆绑（Write-combining），可以在总线传输开始前，将若干个总线写传输捆绑成一次较大的写传输操作。这可以将图像写操作的性能提高2.5倍或者更多。
Cyrix 6x86, 6x86MX 以及 M II处理器具有地址范围寄存器（ARRs：Address Range Registers），ARRs可以提供与MTRR类似的功能，能够用来模拟MTRRs。
AMD K6-2（stepping 8及以上）与K6-3处理器拥有2个MTRR。AMD Athlon系列的处理器拥有8个与Intel类似的MTRRs。
Centaur C6（WinChip）拥有8个MCRs，运行写捆绑（WC）。
VIA Cyrix III 以及 VIA C3 CPU能够提供8个类似于Intel的处理器。
选中配置选项CONFIG_MTRR后，内核将创建文件/proc/mtrr，通过此文件，能够对MTRR进行操作。X server通常就是这样做的。当然了，今后会有更加通用的接口，因而其它处理器上类似的控制寄存器也能够被轻易的支持。
文件/proc/mtrr具有2个接口：一个是ASCII接口，此接口可读可写。ASCII接口适用于管理。另一个是ioctl()接口，该接口适用于C程序（如，X Server）。下面我们将通过一些命令与C代码，详细地介绍这些接口。
在Shell中读取MTRRs：
% cat /proc/mtrr
reg00: base=0x00000000 ( 0MB), size= 128MB: write-back, count=1
reg01: base=0x08000000 ( 128MB), size= 64MB: write-back, count=1
利用C-shell创建MTRRs：
# echo "base=0xf8000000 size=0x400000 type=write-combining" >! /proc/mtrr
或者使用bash：
# echo "base=0xf8000000 size=0x400000 type=write-combining" >| /proc/mtrr
结果将变为：
% 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
新增加的区域就是Video RAM，基地址为0xf8000000，大小为4MB。在X Server的输出信息中，可以找到基地址，即线性缓存地址（linear framebuffer address）。包含基地址的输出信息可能如下：
(--) S3: PCI: 968 rev 0, Linear FB @ 0xf8000000
注意：X Server可能会修改framebuffer的基地址，因此只有X Server报告出的地址是可信的。
包含FrameBuffer大小的输出信息可能如下：
(--) S3: videoram: 4096k
此处为4MB，即0x400000字节。
一个XFree86的补丁可以自动地实现上述操作：也就是说X Server将通过ioctl()接口操作/proc/mtrr文件，用户不再需要自己处理了。如果使用商业的X Server，可以请生产商提供对MTRR的支持。
创建重叠的MTRRs：
%echo "base=0xfb000000 size=0x1000000 type=write-combining" >/proc/mtrr
%echo "base=0xfb000000 size=0x1000 type=uncachable" >/proc/mtrr
MTRR的配置变为：
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
某些板卡（特别是Voodoo图形卡）在Framebuffer区域之前需要4KB的空间，用来映射寄存器。
注意：如果创建的第一个区域是type=write-combining的，就可以只创建type=uncachable的区域。
在C-shell中移除MTRRs:
% echo "disable=2" >! /proc/mtrr
或者使用bash:
% echo "disable=2" >| /proc/mtrr
在C程序中使用ioctl()读取MTRRs：
/* 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 [email protected]
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 */
在C程序中利用ioctl()创建MTRRs：
/* 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 [email protected]
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 */