1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
|
/* -*- 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;
}
|
