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 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 asm volatile("pushfl; int $0x15; popfl" 94 : : "a" ((u16)0x2401)); 95 } 96 97 static void enable_a20_kbc(void) 98 { 99 empty_8042(); 100 101 outb(0xd1, 0x64); /* Command write */ 102 empty_8042(); 103 104 outb(0xdf, 0x60); /* A20 on */ 105 empty_8042(); 106 107 outb(0xff, 0x64); /* Null command, but UHCI wants it */ 108 empty_8042(); 109 } 110 111 static void enable_a20_fast(void) 112 { 113 u8 port_a; 114 115 port_a = inb(0x92); /* Configuration port A */ 116 port_a |= 0x02; /* Enable A20 */ 117 port_a &= ~0x01; /* Do not reset machine */ 118 outb(port_a, 0x92); 119 } 120 121 /* 122 * Actual routine to enable A20; return 0 on ok, -1 on failure 123 */ 124 125 #define A20_ENABLE_LOOPS 255 /* Number of times to try */ 126 127 int enable_a20(void) 128 { 129 int loops = A20_ENABLE_LOOPS; 130 int kbc_err; 131 132 while (loops--) { 133 /* First, check to see if A20 is already enabled 134 (legacy free, etc.) */ 135 if (a20_test_short()) 136 return 0; 137 138 /* Next, try the BIOS (INT 0x15, AX=0x2401) */ 139 enable_a20_bios(); 140 if (a20_test_short()) 141 return 0; 142 143 /* Try enabling A20 through the keyboard controller */ 144 kbc_err = empty_8042(); 145 146 if (a20_test_short()) 147 return 0; /* BIOS worked, but with delayed reaction */ 148 149 if (!kbc_err) { 150 enable_a20_kbc(); 151 if (a20_test_long()) 152 return 0; 153 } 154 155 /* Finally, try enabling the "fast A20 gate" */ 156 enable_a20_fast(); 157 if (a20_test_long()) 158 return 0; 159 } 160 161 return -1; 162 } 163