1.. SPDX-License-Identifier: GPL-2.0 2 3========================================= 4MTRR (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 11Phasing out MTRR use 12==================== 13 14MTRR use is replaced on modern x86 hardware with PAT. Direct MTRR use by 15drivers on Linux is now completely phased out, device drivers should use 16arch_phys_wc_add() in combination with ioremap_wc() to make MTRR effective on 17non-PAT systems while a no-op but equally effective on PAT enabled systems. 18 19Even if Linux does not use MTRRs directly, some x86 platform firmware may still 20set up MTRRs early before booting the OS. They do this as some platform 21firmware may still have implemented access to MTRRs which would be controlled 22and handled by the platform firmware directly. An example of platform use of 23MTRRs is through the use of SMI handlers, one case could be for fan control, 24the platform code would need uncachable access to some of its fan control 25registers. Such platform access does not need any Operating System MTRR code in 26place other than mtrr_type_lookup() to ensure any OS specific mapping requests 27are aligned with platform MTRR setup. If MTRRs are only set up by the platform 28firmware code though and the OS does not make any specific MTRR mapping 29requests mtrr_type_lookup() should always return MTRR_TYPE_INVALID. 30 31For 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 61There are two interfaces to /proc/mtrr: one is an ASCII interface 62which allows you to read and write. The other is an ioctl() 63interface. The ASCII interface is meant for administration. The 64ioctl() interface is meant for C programs (i.e. the X server). The 65interfaces are described below, with sample commands and C code. 66 67 68Reading 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 76Creating MTRRs from the C-shell:: 77 78 # echo "base=0xf8000000 size=0x400000 type=write-combining" >! /proc/mtrr 79 80or if you use bash:: 81 82 # echo "base=0xf8000000 size=0x400000 type=write-combining" >| /proc/mtrr 83 84And 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 91This is for video RAM at base address 0xf8000000 and size 4 megabytes. To 92find out your base address, you need to look at the output of your X 93server, which tells you where the linear framebuffer address is. A 94typical line that you may get is:: 95 96 (--) S3: PCI: 968 rev 0, Linear FB @ 0xf8000000 97 98Note that you should only use the value from the X server, as it may 99move the framebuffer base address, so the only value you can trust is 100that reported by the X server. 101 102To find out the size of your framebuffer (what, you don't actually 103know?), the following line will tell you:: 104 105 (--) S3: videoram: 4096k 106 107That's 4 megabytes, which is 0x400000 bytes (in hexadecimal). 108A patch is being written for XFree86 which will make this automatic: 109in other words the X server will manipulate /proc/mtrr using the 110ioctl() interface, so users won't have to do anything. If you use a 111commercial X server, lobby your vendor to add support for MTRRs. 112 113 114Creating 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 121And 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 128Some cards (especially Voodoo Graphics boards) need this 4 kB area 129excluded from the beginning of the region because it is used for 130registers. 131 132NOTE: You can only create type=uncachable region, if the first 133region that you created is type=write-combining. 134 135 136Removing MTRRs from the C-shel 137============================== 138:: 139 140 % echo "disable=2" >! /proc/mtrr 141 142or using bash:: 143 144 % echo "disable=2" >| /proc/mtrr 145 146 147Reading 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 246Creating 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 */ 355