/* -*- 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;
  }