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 } 164