arch/x86/boot/memory.c

   1 // SPDX-License-Identifier: GPL-2.0-only
   2 /* -*- linux-c -*- ------------------------------------------------------- *
   3  *
   4  *   Copyright (C) 1991, 1992 Linus Torvalds
   5  *   Copyright 2007 rPath, Inc. - All Rights Reserved
   6  *   Copyright 2009 Intel Corporation; author H. Peter Anvin
   7  *
   8  * ----------------------------------------------------------------------- */
   9
  10 /*
  11  * Memory detection code
  12  */
  13
  14 #include "boot.h"
  15
  16 #define SMAP    0x534d4150      /* ASCII "SMAP" */
  17
  18 static void detect_memory_e820(void)
  19 {
  20         int count = 0;
  21         struct biosregs ireg, oreg;
  22         struct boot_e820_entry *desc = boot_params.e820_table;
  23         static struct boot_e820_entry buf; /* static so it is zeroed */
  24
  25         initregs(&ireg);
  26         ireg.ax  = 0xe820;
  27         ireg.cx  = sizeof(buf);
  28         ireg.edx = SMAP;
  29         ireg.di  = (size_t)&buf;
  30
  31         /*
  32          * Note: at least one BIOS is known which assumes that the
  33          * buffer pointed to by one e820 call is the same one as
  34          * the previous call, and only changes modified fields.  Therefore,
  35          * we use a temporary buffer and copy the results entry by entry.
  36          *
  37          * This routine deliberately does not try to account for
  38          * ACPI 3+ extended attributes.  This is because there are
  39          * BIOSes in the field which report zero for the valid bit for
  40          * all ranges, and we don't currently make any use of the
  41          * other attribute bits.  Revisit this if we see the extended
  42          * attribute bits deployed in a meaningful way in the future.
  43          */
  44
  45         do {
  46                 intcall(0x15, &ireg, &oreg);
  47                 ireg.ebx = oreg.ebx; /* for next iteration... */
  48
  49                 /* BIOSes which terminate the chain with CF = 1 as opposed
  50                    to %ebx = 0 don't always report the SMAP signature on
  51                    the final, failing, probe. */
  52                 if (oreg.eflags & X86_EFLAGS_CF)
  53                         break;
  54
  55                 /* Some BIOSes stop returning SMAP in the middle of
  56                    the search loop.  We don't know exactly how the BIOS
  57                    screwed up the map at that point, we might have a
  58                    partial map, the full map, or complete garbage, so
  59                    just return failure. */
  60                 if (oreg.eax != SMAP) {
  61                         count = 0;
  62                         break;
  63                 }
  64
  65                 *desc++ = buf;
  66                 count++;
  67         } while (ireg.ebx && count < ARRAY_SIZE(boot_params.e820_table));
  68
  69         boot_params.e820_entries = count;
  70 }
  71
  72 static void detect_memory_e801(void)
  73 {
  74         struct biosregs ireg, oreg;
  75
  76         initregs(&ireg);
  77         ireg.ax = 0xe801;
  78         intcall(0x15, &ireg, &oreg);
  79
  80         if (oreg.eflags & X86_EFLAGS_CF)
  81                 return;
  82
  83         /* Do we really need to do this? */
  84         if (oreg.cx || oreg.dx) {
  85                 oreg.ax = oreg.cx;
  86                 oreg.bx = oreg.dx;
  87         }
  88
  89         if (oreg.ax > 15*1024) {
  90                 return; /* Bogus! */
  91         } else if (oreg.ax == 15*1024) {
  92                 boot_params.alt_mem_k = (oreg.bx << 6) + oreg.ax;
  93         } else {
  94                 /*
  95                  * This ignores memory above 16MB if we have a memory
  96                  * hole there.  If someone actually finds a machine
  97                  * with a memory hole at 16MB and no support for
  98                  * 0E820h they should probably generate a fake e820
  99                  * map.
 100                  */
 101                 boot_params.alt_mem_k = oreg.ax;
 102         }
 103 }
 104
 105 static void detect_memory_88(void)
 106 {
 107         struct biosregs ireg, oreg;
 108
 109         initregs(&ireg);
 110         ireg.ah = 0x88;
 111         intcall(0x15, &ireg, &oreg);
 112
 113         boot_params.screen_info.ext_mem_k = oreg.ax;
 114 }
 115
 116 void detect_memory(void)
 117 {
 118         detect_memory_e820();
 119
 120         detect_memory_e801();
 121
 122         detect_memory_88();
 123 }