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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 ===============================================================================