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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 | // SPDX-License-Identifier: GPL-2.0-only /* -*- linux-c -*- ------------------------------------------------------- * * * Copyright (C) 1991, 1992 Linus Torvalds * Copyright 2007-2008 rPath, Inc. - All Rights Reserved * Copyright 2009 Intel Corporation; author H. Peter Anvin * * ----------------------------------------------------------------------- */ /* * Enable A20 gate (return -1 on failure) */ #include "boot.h" #define MAX_8042_LOOPS 100000 #define MAX_8042_FF 32 static int empty_8042(void) { u8 status; int loops = MAX_8042_LOOPS; int ffs = MAX_8042_FF; while (loops--) { io_delay(); status = inb(0x64); if (status == 0xff) { /* FF is a plausible, but very unlikely status */ if (!--ffs) return -1; /* Assume no KBC present */ } if (status & 1) { /* Read and discard input data */ io_delay(); (void)inb(0x60); } else if (!(status & 2)) { /* Buffers empty, finished! */ return 0; } } return -1; } /* Returns nonzero if the A20 line is enabled. The memory address used as a test is the int $0x80 vector, which should be safe. */ #define A20_TEST_ADDR (4*0x80) #define A20_TEST_SHORT 32 #define A20_TEST_LONG 2097152 /* 2^21 */ static int a20_test(int loops) { int ok = 0; int saved, ctr; set_fs(0x0000); set_gs(0xffff); saved = ctr = rdfs32(A20_TEST_ADDR); while (loops--) { wrfs32(++ctr, A20_TEST_ADDR); io_delay(); /* Serialize and make delay constant */ ok = rdgs32(A20_TEST_ADDR+0x10) ^ ctr; if (ok) break; } wrfs32(saved, A20_TEST_ADDR); return ok; } /* Quick test to see if A20 is already enabled */ static int a20_test_short(void) { return a20_test(A20_TEST_SHORT); } /* Longer test that actually waits for A20 to come on line; this is useful when dealing with the KBC or other slow external circuitry. */ static int a20_test_long(void) { return a20_test(A20_TEST_LONG); } static void enable_a20_bios(void) { struct biosregs ireg; initregs(&ireg); ireg.ax = 0x2401; intcall(0x15, &ireg, NULL); } static void enable_a20_kbc(void) { empty_8042(); outb(0xd1, 0x64); /* Command write */ empty_8042(); outb(0xdf, 0x60); /* A20 on */ empty_8042(); outb(0xff, 0x64); /* Null command, but UHCI wants it */ empty_8042(); } static void enable_a20_fast(void) { u8 port_a; port_a = inb(0x92); /* Configuration port A */ port_a |= 0x02; /* Enable A20 */ port_a &= ~0x01; /* Do not reset machine */ outb(port_a, 0x92); } /* * Actual routine to enable A20; return 0 on ok, -1 on failure */ #define A20_ENABLE_LOOPS 255 /* Number of times to try */ int enable_a20(void) { int loops = A20_ENABLE_LOOPS; int kbc_err; while (loops--) { /* First, check to see if A20 is already enabled (legacy free, etc.) */ if (a20_test_short()) return 0; /* Next, try the BIOS (INT 0x15, AX=0x2401) */ enable_a20_bios(); if (a20_test_short()) return 0; /* Try enabling A20 through the keyboard controller */ kbc_err = empty_8042(); if (a20_test_short()) return 0; /* BIOS worked, but with delayed reaction */ if (!kbc_err) { enable_a20_kbc(); if (a20_test_long()) return 0; } /* Finally, try enabling the "fast A20 gate" */ enable_a20_fast(); if (a20_test_long()) return 0; } return -1; } |