arch/x86/boot/a20.c

   1 /* -*- linux-c -*- ------------------------------------------------------- *
   2  *
   3  *   Copyright (C) 1991, 1992 Linus Torvalds
   4  *   Copyright 2007-2008 rPath, Inc. - All Rights Reserved
   5  *   Copyright 2009 Intel Corporation; author H. Peter Anvin
   6  *
   7  *   This file is part of the Linux kernel, and is made available under
   8  *   the terms of the GNU General Public License version 2.
   9  *
  10  * ----------------------------------------------------------------------- */
  11
  12 /*
  13  * Enable A20 gate (return -1 on failure)
  14  */
  15
  16 #include "boot.h"
  17
  18 #define MAX_8042_LOOPS  100000
  19 #define MAX_8042_FF     32
  20
  21 static int empty_8042(void)
  22 {
  23         u8 status;
  24         int loops = MAX_8042_LOOPS;
  25         int ffs   = MAX_8042_FF;
  26
  27         while (loops--) {
  28                 io_delay();
  29
  30                 status = inb(0x64);
  31                 if (status == 0xff) {
  32                         /* FF is a plausible, but very unlikely status */
  33                         if (!--ffs)
  34                                 return -1; /* Assume no KBC present */
  35                 }
  36                 if (status & 1) {
  37                         /* Read and discard input data */
  38                         io_delay();
  39                         (void)inb(0x60);
  40                 } else if (!(status & 2)) {
  41                         /* Buffers empty, finished! */
  42                         return 0;
  43                 }
  44         }
  45
  46         return -1;
  47 }
  48
  49 /* Returns nonzero if the A20 line is enabled.  The memory address
  50    used as a test is the int $0x80 vector, which should be safe. */
  51
  52 #define A20_TEST_ADDR   (4*0x80)
  53 #define A20_TEST_SHORT  32
  54 #define A20_TEST_LONG   2097152 /* 2^21 */
  55
  56 static int a20_test(int loops)
  57 {
  58         int ok = 0;
  59         int saved, ctr;
  60
  61         set_fs(0x0000);
  62         set_gs(0xffff);
  63
  64         saved = ctr = rdfs32(A20_TEST_ADDR);
  65
  66         while (loops--) {
  67                 wrfs32(++ctr, A20_TEST_ADDR);
  68                 io_delay();     /* Serialize and make delay constant */
  69                 ok = rdgs32(A20_TEST_ADDR+0x10) ^ ctr;
  70                 if (ok)
  71                         break;
  72         }
  73
  74         wrfs32(saved, A20_TEST_ADDR);
  75         return ok;
  76 }
  77
  78 /* Quick test to see if A20 is already enabled */
  79 static int a20_test_short(void)
  80 {
  81         return a20_test(A20_TEST_SHORT);
  82 }
  83
  84 /* Longer test that actually waits for A20 to come on line; this
  85    is useful when dealing with the KBC or other slow external circuitry. */
  86 static int a20_test_long(void)
  87 {
  88         return a20_test(A20_TEST_LONG);
  89 }
  90
  91 static void enable_a20_bios(void)
  92 {
  93         struct biosregs ireg;
  94
  95         initregs(&ireg);
  96         ireg.ax = 0x2401;
  97         intcall(0x15, &ireg, NULL);
  98 }
  99
 100 static void enable_a20_kbc(void)
 101 {
 102         empty_8042();
 103
 104         outb(0xd1, 0x64);       /* Command write */
 105         empty_8042();
 106
 107         outb(0xdf, 0x60);       /* A20 on */
 108         empty_8042();
 109
 110         outb(0xff, 0x64);       /* Null command, but UHCI wants it */
 111         empty_8042();
 112 }
 113
 114 static void enable_a20_fast(void)
 115 {
 116         u8 port_a;
 117
 118         port_a = inb(0x92);     /* Configuration port A */
 119         port_a |=  0x02;        /* Enable A20 */
 120         port_a &= ~0x01;        /* Do not reset machine */
 121         outb(port_a, 0x92);
 122 }
 123
 124 /*
 125  * Actual routine to enable A20; return 0 on ok, -1 on failure
 126  */
 127
 128 #define A20_ENABLE_LOOPS 255    /* Number of times to try */
 129
 130 int enable_a20(void)
 131 {
 132        int loops = A20_ENABLE_LOOPS;
 133        int kbc_err;
 134
 135        while (loops--) {
 136                /* First, check to see if A20 is already enabled
 137                   (legacy free, etc.) */
 138                if (a20_test_short())
 139                        return 0;
 140
 141                /* Next, try the BIOS (INT 0x15, AX=0x2401) */
 142                enable_a20_bios();
 143                if (a20_test_short())
 144                        return 0;
 145
 146                /* Try enabling A20 through the keyboard controller */
 147                kbc_err = empty_8042();
 148
 149                if (a20_test_short())
 150                        return 0; /* BIOS worked, but with delayed reaction */
 151
 152                if (!kbc_err) {
 153                        enable_a20_kbc();
 154                        if (a20_test_long())
 155                                return 0;
 156                }
 157
 158                /* Finally, try enabling the "fast A20 gate" */
 159                enable_a20_fast();
 160                if (a20_test_long())
 161                        return 0;
 162        }
 163
 164        return -1;
 165 }