Commit 5a8a8128 authored by H. Peter Anvin's avatar H. Peter Anvin Committed by Linus Torvalds
Browse files

A20 handling code



A20 handling code for the new x86 setup code.  This implements the same
algorithms as the assembly version.
Signed-off-by: default avatarH. Peter Anvin <hpa@zytor.com>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 5be86566
/* -*- linux-c -*- ------------------------------------------------------- *
*
* Copyright (C) 1991, 1992 Linus Torvalds
* Copyright 2007 rPath, Inc. - All Rights Reserved
*
* This file is part of the Linux kernel, and is made available under
* the terms of the GNU General Public License version 2.
*
* ----------------------------------------------------------------------- */
/*
* arch/i386/boot/a20.c
*
* Enable A20 gate (return -1 on failure)
*/
#include "boot.h"
#define MAX_8042_LOOPS 100000
static int empty_8042(void)
{
u8 status;
int loops = MAX_8042_LOOPS;
while (loops--) {
io_delay();
status = inb(0x64);
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)
{
asm volatile("pushfl; int $0x15; popfl"
: : "a" ((u16)0x2401));
}
static void enable_a20_kbc(void)
{
empty_8042();
outb(0xd1, 0x64); /* Command write */
empty_8042();
outb(0xdf, 0x60); /* A20 on */
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;
#if defined(CONFIG_X86_ELAN)
/* Elan croaks if we try to touch the KBC */
enable_a20_fast();
while (!a20_test_long())
;
return 0;
#elif defined(CONFIG_X86_VOYAGER)
/* On Voyager, a20_test() is unsafe? */
enable_a20_kbc();
return 0;
#else
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 */
empty_8042();
if (a20_test_short())
return 0; /* BIOS worked, but with delayed reaction */
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;
#endif
}
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