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 } 162