1 /* -*- linux-c -*- ------------------------------------------------------- * 2 * 3 * Copyright (C) 1991, 1992 Linus Torvalds 4 * Copyright 2007-2008 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 * Enable A20 gate (return -1 on failure) 13 */ 14 15 #include "boot.h" 16 17 #define MAX_8042_LOOPS 100000 18 19 static int empty_8042(void) 20 { 21 u8 status; 22 int loops = MAX_8042_LOOPS; 23 24 while (loops--) { 25 io_delay(); 26 27 status = inb(0x64); 28 if (status & 1) { 29 /* Read and discard input data */ 30 io_delay(); 31 (void)inb(0x60); 32 } else if (!(status & 2)) { 33 /* Buffers empty, finished! */ 34 return 0; 35 } 36 } 37 38 return -1; 39 } 40 41 /* Returns nonzero if the A20 line is enabled. The memory address 42 used as a test is the int $0x80 vector, which should be safe. */ 43 44 #define A20_TEST_ADDR (4*0x80) 45 #define A20_TEST_SHORT 32 46 #define A20_TEST_LONG 2097152 /* 2^21 */ 47 48 static int a20_test(int loops) 49 { 50 int ok = 0; 51 int saved, ctr; 52 53 set_fs(0x0000); 54 set_gs(0xffff); 55 56 saved = ctr = rdfs32(A20_TEST_ADDR); 57 58 while (loops--) { 59 wrfs32(++ctr, A20_TEST_ADDR); 60 io_delay(); /* Serialize and make delay constant */ 61 ok = rdgs32(A20_TEST_ADDR+0x10) ^ ctr; 62 if (ok) 63 break; 64 } 65 66 wrfs32(saved, A20_TEST_ADDR); 67 return ok; 68 } 69 70 /* Quick test to see if A20 is already enabled */ 71 static int a20_test_short(void) 72 { 73 return a20_test(A20_TEST_SHORT); 74 } 75 76 /* Longer test that actually waits for A20 to come on line; this 77 is useful when dealing with the KBC or other slow external circuitry. */ 78 static int a20_test_long(void) 79 { 80 return a20_test(A20_TEST_LONG); 81 } 82 83 static void enable_a20_bios(void) 84 { 85 asm volatile("pushfl; int $0x15; popfl" 86 : : "a" ((u16)0x2401)); 87 } 88 89 static void enable_a20_kbc(void) 90 { 91 empty_8042(); 92 93 outb(0xd1, 0x64); /* Command write */ 94 empty_8042(); 95 96 outb(0xdf, 0x60); /* A20 on */ 97 empty_8042(); 98 99 outb(0xff, 0x64); /* Null command, but UHCI wants it */ 100 empty_8042(); 101 } 102 103 static void enable_a20_fast(void) 104 { 105 u8 port_a; 106 107 port_a = inb(0x92); /* Configuration port A */ 108 port_a |= 0x02; /* Enable A20 */ 109 port_a &= ~0x01; /* Do not reset machine */ 110 outb(port_a, 0x92); 111 } 112 113 /* 114 * Actual routine to enable A20; return 0 on ok, -1 on failure 115 */ 116 117 #define A20_ENABLE_LOOPS 255 /* Number of times to try */ 118 119 int enable_a20(void) 120 { 121 #if defined(CONFIG_X86_ELAN) 122 /* Elan croaks if we try to touch the KBC */ 123 enable_a20_fast(); 124 while (!a20_test_long()) 125 ; 126 return 0; 127 #elif defined(CONFIG_X86_VOYAGER) 128 /* On Voyager, a20_test() is unsafe? */ 129 enable_a20_kbc(); 130 return 0; 131 #else 132 int loops = A20_ENABLE_LOOPS; 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