1 .. SPDX-License-Identifier: GPL-2.0
2
3 =========================================
4 MTRR (Memory Type Range Register) control
5 =========================================
6
7 :Authors: - Richard Gooch <rgooch@atnf.csiro.au> - 3 Jun 1999
8 - Luis R. Rodriguez <mcgrof@do-not-panic.com> - April 9, 2015
9
10
11 Phasing out MTRR use
12 ====================
13
14 MTRR use is replaced on modern x86 hardware with PAT. Direct MTRR use by
15 drivers on Linux is now completely phased out, device drivers should use
16 arch_phys_wc_add() in combination with ioremap_wc() to make MTRR effective on
17 non-PAT systems while a no-op but equally effective on PAT enabled systems.
18
19 Even if Linux does not use MTRRs directly, some x86 platform firmware may still
20 set up MTRRs early before booting the OS. They do this as some platform
21 firmware may still have implemented access to MTRRs which would be controlled
22 and handled by the platform firmware directly. An example of platform use of
23 MTRRs is through the use of SMI handlers, one case could be for fan control,
24 the platform code would need uncachable access to some of its fan control
25 registers. Such platform access does not need any Operating System MTRR code in
26 place other than mtrr_type_lookup() to ensure any OS specific mapping requests
27 are aligned with platform MTRR setup. If MTRRs are only set up by the platform
28 firmware code though and the OS does not make any specific MTRR mapping
29 requests mtrr_type_lookup() should always return MTRR_TYPE_INVALID.
30
31 For details refer to Documentation/arch/x86/pat.rst.
32
33 .. tip::
34 On Intel P6 family processors (Pentium Pro, Pentium II and later)
35 the Memory Type Range Registers (MTRRs) may be used to control
36 processor access to memory ranges. This is most useful when you have
37 a video (VGA) card on a PCI or AGP bus. Enabling write-combining
38 allows bus write transfers to be combined into a larger transfer
39 before bursting over the PCI/AGP bus. This can increase performance
40 of image write operations 2.5 times or more.
41
42 The Cyrix 6x86, 6x86MX and M II processors have Address Range
43 Registers (ARRs) which provide a similar functionality to MTRRs. For
44 these, the ARRs are used to emulate the MTRRs.
45
46 The AMD K6-2 (stepping 8 and above) and K6-3 processors have two
47 MTRRs. These are supported. The AMD Athlon family provide 8 Intel
48 style MTRRs.
49
50 The Centaur C6 (WinChip) has 8 MCRs, allowing write-combining. These
51 are supported.
52
53 The VIA Cyrix III and VIA C3 CPUs offer 8 Intel style MTRRs.
54
55 The CONFIG_MTRR option creates a /proc/mtrr file which may be used
56 to manipulate your MTRRs. Typically the X server should use
57 this. This should have a reasonably generic interface so that
58 similar control registers on other processors can be easily
59 supported.
60
61 There are two interfaces to /proc/mtrr: one is an ASCII interface
62 which allows you to read and write. The other is an ioctl()
63 interface. The ASCII interface is meant for administration. The
64 ioctl() interface is meant for C programs (i.e. the X server). The
65 interfaces are described below, with sample commands and C code.
66
67
68 Reading MTRRs from the shell
69 ============================
70 ::
71
72 % cat /proc/mtrr
73 reg00: base=0x00000000 ( 0MB), size= 128MB: write-back, count=1
74 reg01: base=0x08000000 ( 128MB), size= 64MB: write-back, count=1
75
76 Creating MTRRs from the C-shell::
77
78 # echo "base=0xf8000000 size=0x400000 type=write-combining" >! /proc/mtrr
79
80 or if you use bash::
81
82 # echo "base=0xf8000000 size=0x400000 type=write-combining" >| /proc/mtrr
83
84 And the result thereof::
85
86 % cat /proc/mtrr
87 reg00: base=0x00000000 ( 0MB), size= 128MB: write-back, count=1
88 reg01: base=0x08000000 ( 128MB), size= 64MB: write-back, count=1
89 reg02: base=0xf8000000 (3968MB), size= 4MB: write-combining, count=1
90
91 This is for video RAM at base address 0xf8000000 and size 4 megabytes. To
92 find out your base address, you need to look at the output of your X
93 server, which tells you where the linear framebuffer address is. A
94 typical line that you may get is::
95
96 (--) S3: PCI: 968 rev 0, Linear FB @ 0xf8000000
97
98 Note that you should only use the value from the X server, as it may
99 move the framebuffer base address, so the only value you can trust is
100 that reported by the X server.
101
102 To find out the size of your framebuffer (what, you don't actually
103 know?), the following line will tell you::
104
105 (--) S3: videoram: 4096k
106
107 That's 4 megabytes, which is 0x400000 bytes (in hexadecimal).
108 A patch is being written for XFree86 which will make this automatic:
109 in other words the X server will manipulate /proc/mtrr using the
110 ioctl() interface, so users won't have to do anything. If you use a
111 commercial X server, lobby your vendor to add support for MTRRs.
112
113
114 Creating overlapping MTRRs
115 ==========================
116 ::
117
118 %echo "base=0xfb000000 size=0x1000000 type=write-combining" >/proc/mtrr
119 %echo "base=0xfb000000 size=0x1000 type=uncachable" >/proc/mtrr
120
121 And the results::
122
123 % cat /proc/mtrr
124 reg00: base=0x00000000 ( 0MB), size= 64MB: write-back, count=1
125 reg01: base=0xfb000000 (4016MB), size= 16MB: write-combining, count=1
126 reg02: base=0xfb000000 (4016MB), size= 4kB: uncachable, count=1
127
128 Some cards (especially Voodoo Graphics boards) need this 4 kB area
129 excluded from the beginning of the region because it is used for
130 registers.
131
132 NOTE: You can only create type=uncachable region, if the first
133 region that you created is type=write-combining.
134
135
136 Removing MTRRs from the C-shel
137 ==============================
138 ::
139
140 % echo "disable=2" >! /proc/mtrr
141
142 or using bash::
143
144 % echo "disable=2" >| /proc/mtrr
145
146
147 Reading MTRRs from a C program using ioctl()'s
148 ==============================================
149 ::
150
151 /* mtrr-show.c
152
153 Source file for mtrr-show (example program to show MTRRs using ioctl()'s)
154
155 Copyright (C) 1997-1998 Richard Gooch
156
157 This program is free software; you can redistribute it and/or modify
158 it under the terms of the GNU General Public License as published by
159 the Free Software Foundation; either version 2 of the License, or
160 (at your option) any later version.
161
162 This program is distributed in the hope that it will be useful,
163 but WITHOUT ANY WARRANTY; without even the implied warranty of
164 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
165 GNU General Public License for more details.
166
167 You should have received a copy of the GNU General Public License
168 along with this program; if not, write to the Free Software
169 Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
170
171 Richard Gooch may be reached by email at rgooch@atnf.csiro.au
172 The postal address is:
173 Richard Gooch, c/o ATNF, P. O. Box 76, Epping, N.S.W., 2121, Australia.
174 */
175
176 /*
177 This program will use an ioctl() on /proc/mtrr to show the current MTRR
178 settings. This is an alternative to reading /proc/mtrr.
179
180
181 Written by Richard Gooch 17-DEC-1997
182
183 Last updated by Richard Gooch 2-MAY-1998
184
185
186 */
187 #include <stdio.h>
188 #include <stdlib.h>
189 #include <string.h>
190 #include <sys/types.h>
191 #include <sys/stat.h>
192 #include <fcntl.h>
193 #include <sys/ioctl.h>
194 #include <errno.h>
195 #include <asm/mtrr.h>
196
197 #define TRUE 1
198 #define FALSE 0
199 #define ERRSTRING strerror (errno)
200
201 static char *mtrr_strings[MTRR_NUM_TYPES] =
202 {
203 "uncachable", /* 0 */
204 "write-combining", /* 1 */
205 "?", /* 2 */
206 "?", /* 3 */
207 "write-through", /* 4 */
208 "write-protect", /* 5 */
209 "write-back", /* 6 */
210 };
211
212 int main ()
213 {
214 int fd;
215 struct mtrr_gentry gentry;
216
217 if ( ( fd = open ("/proc/mtrr", O_RDONLY, 0) ) == -1 )
218 {
219 if (errno == ENOENT)
220 {
221 fputs ("/proc/mtrr not found: not supported or you don't have a PPro?\n",
222 stderr);
223 exit (1);
224 }
225 fprintf (stderr, "Error opening /proc/mtrr\t%s\n", ERRSTRING);
226 exit (2);
227 }
228 for (gentry.regnum = 0; ioctl (fd, MTRRIOC_GET_ENTRY, &gentry) == 0;
229 ++gentry.regnum)
230 {
231 if (gentry.size < 1)
232 {
233 fprintf (stderr, "Register: %u disabled\n", gentry.regnum);
234 continue;
235 }
236 fprintf (stderr, "Register: %u base: 0x%lx size: 0x%lx type: %s\n",
237 gentry.regnum, gentry.base, gentry.size,
238 mtrr_strings[gentry.type]);
239 }
240 if (errno == EINVAL) exit (0);
241 fprintf (stderr, "Error doing ioctl(2) on /dev/mtrr\t%s\n", ERRSTRING);
242 exit (3);
243 } /* End Function main */
244
245
246 Creating MTRRs from a C programme using ioctl()'s
247 =================================================
248 ::
249
250 /* mtrr-add.c
251
252 Source file for mtrr-add (example programme to add an MTRRs using ioctl())
253
254 Copyright (C) 1997-1998 Richard Gooch
255
256 This program is free software; you can redistribute it and/or modify
257 it under the terms of the GNU General Public License as published by
258 the Free Software Foundation; either version 2 of the License, or
259 (at your option) any later version.
260
261 This program is distributed in the hope that it will be useful,
262 but WITHOUT ANY WARRANTY; without even the implied warranty of
263 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
264 GNU General Public License for more details.
265
266 You should have received a copy of the GNU General Public License
267 along with this program; if not, write to the Free Software
268 Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
269
270 Richard Gooch may be reached by email at rgooch@atnf.csiro.au
271 The postal address is:
272 Richard Gooch, c/o ATNF, P. O. Box 76, Epping, N.S.W., 2121, Australia.
273 */
274
275 /*
276 This programme will use an ioctl() on /proc/mtrr to add an entry. The first
277 available mtrr is used. This is an alternative to writing /proc/mtrr.
278
279
280 Written by Richard Gooch 17-DEC-1997
281
282 Last updated by Richard Gooch 2-MAY-1998
283
284
285 */
286 #include <stdio.h>
287 #include <string.h>
288 #include <stdlib.h>
289 #include <unistd.h>
290 #include <sys/types.h>
291 #include <sys/stat.h>
292 #include <fcntl.h>
293 #include <sys/ioctl.h>
294 #include <errno.h>
295 #include <asm/mtrr.h>
296
297 #define TRUE 1
298 #define FALSE 0
299 #define ERRSTRING strerror (errno)
300
301 static char *mtrr_strings[MTRR_NUM_TYPES] =
302 {
303 "uncachable", /* 0 */
304 "write-combining", /* 1 */
305 "?", /* 2 */
306 "?", /* 3 */
307 "write-through", /* 4 */
308 "write-protect", /* 5 */
309 "write-back", /* 6 */
310 };
311
312 int main (int argc, char **argv)
313 {
314 int fd;
315 struct mtrr_sentry sentry;
316
317 if (argc != 4)
318 {
319 fprintf (stderr, "Usage:\tmtrr-add base size type\n");
320 exit (1);
321 }
322 sentry.base = strtoul (argv[1], NULL, 0);
323 sentry.size = strtoul (argv[2], NULL, 0);
324 for (sentry.type = 0; sentry.type < MTRR_NUM_TYPES; ++sentry.type)
325 {
326 if (strcmp (argv[3], mtrr_strings[sentry.type]) == 0) break;
327 }
328 if (sentry.type >= MTRR_NUM_TYPES)
329 {
330 fprintf (stderr, "Illegal type: \"%s\"\n", argv[3]);
331 exit (2);
332 }
333 if ( ( fd = open ("/proc/mtrr", O_WRONLY, 0) ) == -1 )
334 {
335 if (errno == ENOENT)
336 {
337 fputs ("/proc/mtrr not found: not supported or you don't have a PPro?\n",
338 stderr);
339 exit (3);
340 }
341 fprintf (stderr, "Error opening /proc/mtrr\t%s\n", ERRSTRING);
342 exit (4);
343 }
344 if (ioctl (fd, MTRRIOC_ADD_ENTRY, &sentry) == -1)
345 {
346 fprintf (stderr, "Error doing ioctl(2) on /dev/mtrr\t%s\n", ERRSTRING);
347 exit (5);
348 }
349 fprintf (stderr, "Sleeping for 5 seconds so you can see the new entry\n");
350 sleep (5);
351 close (fd);
352 fputs ("I've just closed /proc/mtrr so now the new entry should be gone\n",
353 stderr);
354 } /* End Function main */
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