arch/i386/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  * arch/i386/boot/a20.c
  13  *
  14  * Enable A20 gate (return -1 on failure)
  15  */
  16
  17 #include "boot.h"
  18
  19 #define MAX_8042_LOOPS  100000
  20
  21 static int empty_8042(void)
  22 {
  23         u8 status;
  24         int loops = MAX_8042_LOOPS;
  25
  26         while (loops--) {
  27                 io_delay();
  28
  29                 status = inb(0x64);
  30                 if (status & 1) {
  31                         /* Read and discard input data */
  32                         io_delay();
  33                         (void)inb(0x60);
  34                 } else if (!(status & 2)) {
  35                         /* Buffers empty, finished! */
  36                         return 0;
  37                 }
  38         }
  39
  40         return -1;
  41 }
  42
  43 /* Returns nonzero if the A20 line is enabled.  The memory address
  44    used as a test is the int $0x80 vector, which should be safe. */
  45
  46 #define A20_TEST_ADDR   (4*0x80)
  47 #define A20_TEST_SHORT  32
  48 #define A20_TEST_LONG   2097152 /* 2^21 */
  49
  50 static int a20_test(int loops)
  51 {
  52         int ok = 0;
  53         int saved, ctr;
  54
  55         set_fs(0x0000);
  56         set_gs(0xffff);
  57
  58         saved = ctr = rdfs32(A20_TEST_ADDR);
  59
  60         while (loops--) {
  61                 wrfs32(++ctr, A20_TEST_ADDR);
  62                 io_delay();     /* Serialize and make delay constant */
  63                 ok = rdgs32(A20_TEST_ADDR+0x10) ^ ctr;
  64                 if (ok)
  65                         break;
  66         }
  67
  68         wrfs32(saved, A20_TEST_ADDR);
  69         return ok;
  70 }
  71
  72 /* Quick test to see if A20 is already enabled */
  73 static int a20_test_short(void)
  74 {
  75         return a20_test(A20_TEST_SHORT);
  76 }
  77
  78 /* Longer test that actually waits for A20 to come on line; this
  79    is useful when dealing with the KBC or other slow external circuitry. */
  80 static int a20_test_long(void)
  81 {
  82         return a20_test(A20_TEST_LONG);
  83 }
  84
  85 static void enable_a20_bios(void)
  86 {
  87         asm volatile("pushfl; int $0x15; popfl"
  88                      : : "a" ((u16)0x2401));
  89 }
  90
  91 static void enable_a20_kbc(void)
  92 {
  93         empty_8042();
  94
  95         outb(0xd1, 0x64);       /* Command write */
  96         empty_8042();
  97
  98         outb(0xdf, 0x60);       /* A20 on */
  99         empty_8042();
 100 }
 101
 102 static void enable_a20_fast(void)
 103 {
 104         u8 port_a;
 105
 106         port_a = inb(0x92);     /* Configuration port A */
 107         port_a |=  0x02;        /* Enable A20 */
 108         port_a &= ~0x01;        /* Do not reset machine */
 109         outb(port_a, 0x92);
 110 }
 111
 112 /*
 113  * Actual routine to enable A20; return 0 on ok, -1 on failure
 114  */
 115
 116 #define A20_ENABLE_LOOPS 255    /* Number of times to try */
 117
 118 int enable_a20(void)
 119 {
 120         int loops = A20_ENABLE_LOOPS;
 121
 122 #if defined(CONFIG_X86_ELAN)
 123         /* Elan croaks if we try to touch the KBC */
 124         enable_a20_fast();
 125         while (!a20_test_long())
 126                 ;
 127         return 0;
 128 #elif defined(CONFIG_X86_VOYAGER)
 129         /* On Voyager, a20_test() is unsafe? */
 130         enable_a20_kbc();
 131         return 0;
 132 #else
 133         while (loops--) {
 134                 /* First, check to see if A20 is already enabled
 135                    (legacy free, etc.) */
 136                 if (a20_test_short())
 137                         return 0;
 138
 139                 /* Next, try the BIOS (INT 0x15, AX=0x2401) */
 140                 enable_a20_bios();
 141                 if (a20_test_short())
 142                         return 0;
 143
 144                 /* Try enabling A20 through the keyboard controller */
 145                 empty_8042();
 146                 if (a20_test_short())
 147                         return 0; /* BIOS worked, but with delayed reaction */
 148
 149                 enable_a20_kbc();
 150                 if (a20_test_long())
 151                         return 0;
 152
 153                 /* Finally, try enabling the "fast A20 gate" */
 154                 enable_a20_fast();
 155                 if (a20_test_long())
 156                         return 0;
 157         }
 158
 159         return -1;
 160 #endif
 161 }