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