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a20.c

/* -*- 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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