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163 lines
3.4 KiB
163 lines
3.4 KiB
// SPDX-License-Identifier: GPL-2.0-only |
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/* -*- linux-c -*- ------------------------------------------------------- * |
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* |
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* Copyright (C) 1991, 1992 Linus Torvalds |
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* Copyright 2007-2008 rPath, Inc. - All Rights Reserved |
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* Copyright 2009 Intel Corporation; author H. Peter Anvin |
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* |
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* ----------------------------------------------------------------------- */ |
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/* |
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* Enable A20 gate (return -1 on failure) |
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*/ |
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#include "boot.h" |
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#define MAX_8042_LOOPS 100000 |
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#define MAX_8042_FF 32 |
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static int empty_8042(void) |
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{ |
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u8 status; |
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int loops = MAX_8042_LOOPS; |
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int ffs = MAX_8042_FF; |
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while (loops--) { |
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io_delay(); |
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status = inb(0x64); |
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if (status == 0xff) { |
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/* FF is a plausible, but very unlikely status */ |
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if (!--ffs) |
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return -1; /* Assume no KBC present */ |
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} |
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if (status & 1) { |
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/* Read and discard input data */ |
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io_delay(); |
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(void)inb(0x60); |
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} else if (!(status & 2)) { |
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/* Buffers empty, finished! */ |
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return 0; |
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} |
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} |
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return -1; |
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} |
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/* Returns nonzero if the A20 line is enabled. The memory address |
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used as a test is the int $0x80 vector, which should be safe. */ |
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#define A20_TEST_ADDR (4*0x80) |
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#define A20_TEST_SHORT 32 |
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#define A20_TEST_LONG 2097152 /* 2^21 */ |
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static int a20_test(int loops) |
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{ |
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int ok = 0; |
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int saved, ctr; |
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set_fs(0x0000); |
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set_gs(0xffff); |
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saved = ctr = rdfs32(A20_TEST_ADDR); |
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while (loops--) { |
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wrfs32(++ctr, A20_TEST_ADDR); |
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io_delay(); /* Serialize and make delay constant */ |
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ok = rdgs32(A20_TEST_ADDR+0x10) ^ ctr; |
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if (ok) |
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break; |
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} |
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wrfs32(saved, A20_TEST_ADDR); |
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return ok; |
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} |
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/* Quick test to see if A20 is already enabled */ |
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static int a20_test_short(void) |
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{ |
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return a20_test(A20_TEST_SHORT); |
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} |
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/* Longer test that actually waits for A20 to come on line; this |
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is useful when dealing with the KBC or other slow external circuitry. */ |
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static int a20_test_long(void) |
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{ |
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return a20_test(A20_TEST_LONG); |
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} |
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static void enable_a20_bios(void) |
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{ |
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struct biosregs ireg; |
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initregs(&ireg); |
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ireg.ax = 0x2401; |
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intcall(0x15, &ireg, NULL); |
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} |
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static void enable_a20_kbc(void) |
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{ |
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empty_8042(); |
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outb(0xd1, 0x64); /* Command write */ |
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empty_8042(); |
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outb(0xdf, 0x60); /* A20 on */ |
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empty_8042(); |
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outb(0xff, 0x64); /* Null command, but UHCI wants it */ |
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empty_8042(); |
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} |
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static void enable_a20_fast(void) |
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{ |
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u8 port_a; |
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port_a = inb(0x92); /* Configuration port A */ |
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port_a |= 0x02; /* Enable A20 */ |
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port_a &= ~0x01; /* Do not reset machine */ |
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outb(port_a, 0x92); |
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} |
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/* |
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* Actual routine to enable A20; return 0 on ok, -1 on failure |
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*/ |
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#define A20_ENABLE_LOOPS 255 /* Number of times to try */ |
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int enable_a20(void) |
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{ |
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int loops = A20_ENABLE_LOOPS; |
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int kbc_err; |
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while (loops--) { |
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/* First, check to see if A20 is already enabled |
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(legacy free, etc.) */ |
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if (a20_test_short()) |
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return 0; |
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/* Next, try the BIOS (INT 0x15, AX=0x2401) */ |
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enable_a20_bios(); |
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if (a20_test_short()) |
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return 0; |
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/* Try enabling A20 through the keyboard controller */ |
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kbc_err = empty_8042(); |
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if (a20_test_short()) |
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return 0; /* BIOS worked, but with delayed reaction */ |
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if (!kbc_err) { |
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enable_a20_kbc(); |
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if (a20_test_long()) |
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return 0; |
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} |
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/* Finally, try enabling the "fast A20 gate" */ |
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enable_a20_fast(); |
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if (a20_test_long()) |
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return 0; |
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} |
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return -1; |
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}
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