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 21static 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 56static 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 */ 79static 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. */ 86static int a20_test_long(void) 87{ 88 return a20_test(A20_TEST_LONG); 89} 90 91static 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 100static 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 114static 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 130int 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} 166