root/arch/x86/boot/a20.c

DEFINITIONS

1. empty_8042
2. a20_test
3. a20_test_short
4. a20_test_long
5. enable_a20_bios
6. enable_a20_kbc
7. enable_a20_fast
8. enable_a20

/* -*- linux-c -*- ------------------------------------------------------- *
 *
 *   Copyright (C) 1991, 1992 Linus Torvalds
 *   Copyright 2007-2008 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.
 *
 * ----------------------------------------------------------------------- */

/*
 * 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();

        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)
{
#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
       int loops = A20_ENABLE_LOOPS;
        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
}