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;
}