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Based on kernel version 4.8. Page generated on 2016-10-06 23:19 EST.

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