arch/x86/boot/a20.c

   1 /* -*- linux-c -*- ------------------------------------------------------- *
   2  *
   3  *   Copyright (C) 1991, 1992 Linus Torvalds
   4  *   Copyright 2007 rPath, Inc. - All Rights Reserved
   5  *
   6  *   This file is part of the Linux kernel, and is made available under
   7  *   the terms of the GNU General Public License version 2.
   8  *
   9  * ----------------------------------------------------------------------- */
  10
  11 /*
  12  * Enable A20 gate (return -1 on failure)
  13  */
  14
  15 #include "boot.h"
  16
  17 #define MAX_8042_LOOPS  100000
  18
  19 static int empty_8042(void)
  20 {
  21         u8 status;
  22         int loops = MAX_8042_LOOPS;
  23
  24         while (loops--) {
  25                 io_delay();
  26
  27                 status = inb(0x64);
  28                 if (status & 1) {
  29                         /* Read and discard input data */
  30                         io_delay();
  31                         (void)inb(0x60);
  32                 } else if (!(status & 2)) {
  33                         /* Buffers empty, finished! */
  34                         return 0;
  35                 }
  36         }
  37
  38         return -1;
  39 }
  40
  41 /* Returns nonzero if the A20 line is enabled.  The memory address
  42    used as a test is the int $0x80 vector, which should be safe. */
  43
  44 #define A20_TEST_ADDR   (4*0x80)
  45 #define A20_TEST_SHORT  32
  46 #define A20_TEST_LONG   2097152 /* 2^21 */
  47
  48 static int a20_test(int loops)
  49 {
  50         int ok = 0;
  51         int saved, ctr;
  52
  53         set_fs(0x0000);
  54         set_gs(0xffff);
  55
  56         saved = ctr = rdfs32(A20_TEST_ADDR);
  57
  58         while (loops--) {
  59                 wrfs32(++ctr, A20_TEST_ADDR);
  60                 io_delay();     /* Serialize and make delay constant */
  61                 ok = rdgs32(A20_TEST_ADDR+0x10) ^ ctr;
  62                 if (ok)
  63                         break;
  64         }
  65
  66         wrfs32(saved, A20_TEST_ADDR);
  67         return ok;
  68 }
  69
  70 /* Quick test to see if A20 is already enabled */
  71 static int a20_test_short(void)
  72 {
  73         return a20_test(A20_TEST_SHORT);
  74 }
  75
  76 /* Longer test that actually waits for A20 to come on line; this
  77    is useful when dealing with the KBC or other slow external circuitry. */
  78 static int a20_test_long(void)
  79 {
  80         return a20_test(A20_TEST_LONG);
  81 }
  82
  83 static void enable_a20_bios(void)
  84 {
  85         asm volatile("pushfl; int $0x15; popfl"
  86                      : : "a" ((u16)0x2401));
  87 }
  88
  89 static void enable_a20_kbc(void)
  90 {
  91         empty_8042();
  92
  93         outb(0xd1, 0x64);       /* Command write */
  94         empty_8042();
  95
  96         outb(0xdf, 0x60);       /* A20 on */
  97         empty_8042();
  98 }
  99
 100 static void enable_a20_fast(void)
 101 {
 102         u8 port_a;
 103
 104         port_a = inb(0x92);     /* Configuration port A */
 105         port_a |=  0x02;        /* Enable A20 */
 106         port_a &= ~0x01;        /* Do not reset machine */
 107         outb(port_a, 0x92);
 108 }
 109
 110 /*
 111  * Actual routine to enable A20; return 0 on ok, -1 on failure
 112  */
 113
 114 #define A20_ENABLE_LOOPS 255    /* Number of times to try */
 115
 116 int enable_a20(void)
 117 {
 118 #if defined(CONFIG_X86_ELAN)
 119         /* Elan croaks if we try to touch the KBC */
 120         enable_a20_fast();
 121         while (!a20_test_long())
 122                 ;
 123         return 0;
 124 #elif defined(CONFIG_X86_VOYAGER)
 125         /* On Voyager, a20_test() is unsafe? */
 126         enable_a20_kbc();
 127         return 0;
 128 #else
 129        int loops = A20_ENABLE_LOOPS;
 130         while (loops--) {
 131                 /* First, check to see if A20 is already enabled
 132                    (legacy free, etc.) */
 133                 if (a20_test_short())
 134                         return 0;
 135
 136                 /* Next, try the BIOS (INT 0x15, AX=0x2401) */
 137                 enable_a20_bios();
 138                 if (a20_test_short())
 139                         return 0;
 140
 141                 /* Try enabling A20 through the keyboard controller */
 142                 empty_8042();
 143                 if (a20_test_short())
 144                         return 0; /* BIOS worked, but with delayed reaction */
 145
 146                 enable_a20_kbc();
 147                 if (a20_test_long())
 148                         return 0;
 149
 150                 /* Finally, try enabling the "fast A20 gate" */
 151                 enable_a20_fast();
 152                 if (a20_test_long())
 153                         return 0;
 154         }
 155
 156         return -1;
 157 #endif
 158 }