arch/x86/boot/a20.c

   1 // SPDX-License-Identifier: GPL-2.0-only
   2 /* -*- linux-c -*- ------------------------------------------------------- *
   3  *
   4  *   Copyright (C) 1991, 1992 Linus Torvalds
   5  *   Copyright 2007-2008 rPath, Inc. - All Rights Reserved
   6  *   Copyright 2009 Intel Corporation; author H. Peter Anvin
   7  *
   8  * ----------------------------------------------------------------------- */
   9
  10 /*
  11  * Enable A20 gate (return -1 on failure)
  12  */
  13
  14 #include "boot.h"
  15
  16 #define MAX_8042_LOOPS  100000
  17 #define MAX_8042_FF     32
  18
  19 static int empty_8042(void)
  20 {
  21         u8 status;
  22         int loops = MAX_8042_LOOPS;
  23         int ffs   = MAX_8042_FF;
  24
  25         while (loops--) {
  26                 io_delay();
  27
  28                 status = inb(0x64);
  29                 if (status == 0xff) {
  30                         /* FF is a plausible, but very unlikely status */
  31                         if (!--ffs)
  32                                 return -1; /* Assume no KBC present */
  33                 }
  34                 if (status & 1) {
  35                         /* Read and discard input data */
  36                         io_delay();
  37                         (void)inb(0x60);
  38                 } else if (!(status & 2)) {
  39                         /* Buffers empty, finished! */
  40                         return 0;
  41                 }
  42         }
  43
  44         return -1;
  45 }
  46
  47 /* Returns nonzero if the A20 line is enabled.  The memory address
  48    used as a test is the int $0x80 vector, which should be safe. */
  49
  50 #define A20_TEST_ADDR   (4*0x80)
  51 #define A20_TEST_SHORT  32
  52 #define A20_TEST_LONG   2097152 /* 2^21 */
  53
  54 static int a20_test(int loops)
  55 {
  56         int ok = 0;
  57         int saved, ctr;
  58
  59         set_fs(0x0000);
  60         set_gs(0xffff);
  61
  62         saved = ctr = rdfs32(A20_TEST_ADDR);
  63
  64         while (loops--) {
  65                 wrfs32(++ctr, A20_TEST_ADDR);
  66                 io_delay();     /* Serialize and make delay constant */
  67                 ok = rdgs32(A20_TEST_ADDR+0x10) ^ ctr;
  68                 if (ok)
  69                         break;
  70         }
  71
  72         wrfs32(saved, A20_TEST_ADDR);
  73         return ok;
  74 }
  75
  76 /* Quick test to see if A20 is already enabled */
  77 static int a20_test_short(void)
  78 {
  79         return a20_test(A20_TEST_SHORT);
  80 }
  81
  82 /* Longer test that actually waits for A20 to come on line; this
  83    is useful when dealing with the KBC or other slow external circuitry. */
  84 static int a20_test_long(void)
  85 {
  86         return a20_test(A20_TEST_LONG);
  87 }
  88
  89 static void enable_a20_bios(void)
  90 {
  91         struct biosregs ireg;
  92
  93         initregs(&ireg);
  94         ireg.ax = 0x2401;
  95         intcall(0x15, &ireg, NULL);
  96 }
  97
  98 static void enable_a20_kbc(void)
  99 {
 100         empty_8042();
 101
 102         outb(0xd1, 0x64);       /* Command write */
 103         empty_8042();
 104
 105         outb(0xdf, 0x60);       /* A20 on */
 106         empty_8042();
 107
 108         outb(0xff, 0x64);       /* Null command, but UHCI wants it */
 109         empty_8042();
 110 }
 111
 112 static void enable_a20_fast(void)
 113 {
 114         u8 port_a;
 115
 116         port_a = inb(0x92);     /* Configuration port A */
 117         port_a |=  0x02;        /* Enable A20 */
 118         port_a &= ~0x01;        /* Do not reset machine */
 119         outb(port_a, 0x92);
 120 }
 121
 122 /*
 123  * Actual routine to enable A20; return 0 on ok, -1 on failure
 124  */
 125
 126 #define A20_ENABLE_LOOPS 255    /* Number of times to try */
 127
 128 int enable_a20(void)
 129 {
 130        int loops = A20_ENABLE_LOOPS;
 131        int kbc_err;
 132
 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                kbc_err = empty_8042();
 146
 147                if (a20_test_short())
 148                        return 0; /* BIOS worked, but with delayed reaction */
 149
 150                if (!kbc_err) {
 151                        enable_a20_kbc();
 152                        if (a20_test_long())
 153                                return 0;
 154                }
 155
 156                /* Finally, try enabling the "fast A20 gate" */
 157                enable_a20_fast();
 158                if (a20_test_long())
 159                        return 0;
 160        }
 161
 162        return -1;
 163 }