a20.c
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/* -*- linux-c -*- ------------------------------------------------------- *
*
* Copyright (C) 1991, 1992 Linus Torvalds
* Copyright 2007-2008 rPath, Inc. - All Rights Reserved
* Copyright 2009 Intel Corporation; author H. Peter Anvin
*
* This file is part of the Linux kernel, and is made available under
* the terms of the GNU General Public License version 2.
*
* ----------------------------------------------------------------------- */
/*
* 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;
}