1/* -*- linux-c -*- ------------------------------------------------------- *
2 *
3 *   Copyright (C) 1991, 1992 Linus Torvalds
4 *   Copyright 2007-2008 rPath, Inc. - All Rights Reserved
5 *   Copyright 2009 Intel Corporation; author H. Peter Anvin
6 *
7 *   This file is part of the Linux kernel, and is made available under
8 *   the terms of the GNU General Public License version 2.
9 *
10 * ----------------------------------------------------------------------- */
11
12/*
13 * Enable A20 gate (return -1 on failure)
14 */
15
16#include "boot.h"
17
18#define MAX_8042_LOOPS	100000
19#define MAX_8042_FF	32
20
21static int empty_8042(void)
22{
23	u8 status;
24	int loops = MAX_8042_LOOPS;
25	int ffs   = MAX_8042_FF;
26
27	while (loops--) {
28		io_delay();
29
30		status = inb(0x64);
31		if (status == 0xff) {
32			/* FF is a plausible, but very unlikely status */
33			if (!--ffs)
34				return -1; /* Assume no KBC present */
35		}
36		if (status & 1) {
37			/* Read and discard input data */
38			io_delay();
39			(void)inb(0x60);
40		} else if (!(status & 2)) {
41			/* Buffers empty, finished! */
42			return 0;
43		}
44	}
45
46	return -1;
47}
48
49/* Returns nonzero if the A20 line is enabled.  The memory address
50   used as a test is the int $0x80 vector, which should be safe. */
51
52#define A20_TEST_ADDR	(4*0x80)
53#define A20_TEST_SHORT  32
54#define A20_TEST_LONG	2097152	/* 2^21 */
55
56static int a20_test(int loops)
57{
58	int ok = 0;
59	int saved, ctr;
60
61	set_fs(0x0000);
62	set_gs(0xffff);
63
64	saved = ctr = rdfs32(A20_TEST_ADDR);
65
66	while (loops--) {
67		wrfs32(++ctr, A20_TEST_ADDR);
68		io_delay();	/* Serialize and make delay constant */
69		ok = rdgs32(A20_TEST_ADDR+0x10) ^ ctr;
70		if (ok)
71			break;
72	}
73
74	wrfs32(saved, A20_TEST_ADDR);
75	return ok;
76}
77
78/* Quick test to see if A20 is already enabled */
79static int a20_test_short(void)
80{
81	return a20_test(A20_TEST_SHORT);
82}
83
84/* Longer test that actually waits for A20 to come on line; this
85   is useful when dealing with the KBC or other slow external circuitry. */
86static int a20_test_long(void)
87{
88	return a20_test(A20_TEST_LONG);
89}
90
91static void enable_a20_bios(void)
92{
93	struct biosregs ireg;
94
95	initregs(&ireg);
96	ireg.ax = 0x2401;
97	intcall(0x15, &ireg, NULL);
98}
99
100static void enable_a20_kbc(void)
101{
102	empty_8042();
103
104	outb(0xd1, 0x64);	/* Command write */
105	empty_8042();
106
107	outb(0xdf, 0x60);	/* A20 on */
108	empty_8042();
109
110	outb(0xff, 0x64);	/* Null command, but UHCI wants it */
111	empty_8042();
112}
113
114static void enable_a20_fast(void)
115{
116	u8 port_a;
117
118	port_a = inb(0x92);	/* Configuration port A */
119	port_a |=  0x02;	/* Enable A20 */
120	port_a &= ~0x01;	/* Do not reset machine */
121	outb(port_a, 0x92);
122}
123
124/*
125 * Actual routine to enable A20; return 0 on ok, -1 on failure
126 */
127
128#define A20_ENABLE_LOOPS 255	/* Number of times to try */
129
130int enable_a20(void)
131{
132       int loops = A20_ENABLE_LOOPS;
133       int kbc_err;
134
135       while (loops--) {
136	       /* First, check to see if A20 is already enabled
137		  (legacy free, etc.) */
138	       if (a20_test_short())
139		       return 0;
140
141	       /* Next, try the BIOS (INT 0x15, AX=0x2401) */
142	       enable_a20_bios();
143	       if (a20_test_short())
144		       return 0;
145
146	       /* Try enabling A20 through the keyboard controller */
147	       kbc_err = empty_8042();
148
149	       if (a20_test_short())
150		       return 0; /* BIOS worked, but with delayed reaction */
151
152	       if (!kbc_err) {
153		       enable_a20_kbc();
154		       if (a20_test_long())
155			       return 0;
156	       }
157
158	       /* Finally, try enabling the "fast A20 gate" */
159	       enable_a20_fast();
160	       if (a20_test_long())
161		       return 0;
162       }
163
164       return -1;
165}
166