tjh.dev
Linux kernel mirror (for testing)
git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
kernel
os
linux
1/* -*- linux-c -*- ------------------------------------------------------- *
2 *
3 * Copyright (C) 1991, 1992 Linus Torvalds
4 * Copyright 2007 rPath, Inc. - All Rights Reserved
5 *
6 * This file is part of the Linux kernel, and is made available under
7 * the terms of the GNU General Public License version 2.
8 *
9 * ----------------------------------------------------------------------- */
10
11/*
12 * Enable A20 gate (return -1 on failure)
13 */
14
15#include "boot.h"
16
17#define MAX_8042_LOOPS 100000
18
19static int empty_8042(void)
20{
21 u8 status;
22 int loops = MAX_8042_LOOPS;
23
24 while (loops--) {
25 io_delay();
26
27 status = inb(0x64);
28 if (status & 1) {
29 /* Read and discard input data */
30 io_delay();
31 (void)inb(0x60);
32 } else if (!(status & 2)) {
33 /* Buffers empty, finished! */
34 return 0;
35 }
36 }
37
38 return -1;
39}
40
41/* Returns nonzero if the A20 line is enabled. The memory address
42 used as a test is the int $0x80 vector, which should be safe. */
43
44#define A20_TEST_ADDR (4*0x80)
45#define A20_TEST_SHORT 32
46#define A20_TEST_LONG 2097152 /* 2^21 */
47
48static int a20_test(int loops)
49{
50 int ok = 0;
51 int saved, ctr;
52
53 set_fs(0x0000);
54 set_gs(0xffff);
55
56 saved = ctr = rdfs32(A20_TEST_ADDR);
57
58 while (loops--) {
59 wrfs32(++ctr, A20_TEST_ADDR);
60 io_delay(); /* Serialize and make delay constant */
61 ok = rdgs32(A20_TEST_ADDR+0x10) ^ ctr;
62 if (ok)
63 break;
64 }
65
66 wrfs32(saved, A20_TEST_ADDR);
67 return ok;
68}
69
70/* Quick test to see if A20 is already enabled */
71static int a20_test_short(void)
72{
73 return a20_test(A20_TEST_SHORT);
74}
75
76/* Longer test that actually waits for A20 to come on line; this
77 is useful when dealing with the KBC or other slow external circuitry. */
78static int a20_test_long(void)
79{
80 return a20_test(A20_TEST_LONG);
81}
82
83static void enable_a20_bios(void)
84{
85 asm volatile("pushfl; int $0x15; popfl"
86 : : "a" ((u16)0x2401));
87}
88
89static void enable_a20_kbc(void)
90{
91 empty_8042();
92
93 outb(0xd1, 0x64); /* Command write */
94 empty_8042();
95
96 outb(0xdf, 0x60); /* A20 on */
97 empty_8042();
98}
99
100static void enable_a20_fast(void)
101{
102 u8 port_a;
103
104 port_a = inb(0x92); /* Configuration port A */
105 port_a |= 0x02; /* Enable A20 */
106 port_a &= ~0x01; /* Do not reset machine */
107 outb(port_a, 0x92);
108}
109
110/*
111 * Actual routine to enable A20; return 0 on ok, -1 on failure
112 */
113
114#define A20_ENABLE_LOOPS 255 /* Number of times to try */
115
116int enable_a20(void)
117{
118 int loops = A20_ENABLE_LOOPS;
119
120#if defined(CONFIG_X86_ELAN)
121 /* Elan croaks if we try to touch the KBC */
122 enable_a20_fast();
123 while (!a20_test_long())
124 ;
125 return 0;
126#elif defined(CONFIG_X86_VOYAGER)
127 /* On Voyager, a20_test() is unsafe? */
128 enable_a20_kbc();
129 return 0;
130#else
131 while (loops--) {
132 /* First, check to see if A20 is already enabled
133 (legacy free, etc.) */
134 if (a20_test_short())
135 return 0;
136
137 /* Next, try the BIOS (INT 0x15, AX=0x2401) */
138 enable_a20_bios();
139 if (a20_test_short())
140 return 0;
141
142 /* Try enabling A20 through the keyboard controller */
143 empty_8042();
144 if (a20_test_short())
145 return 0; /* BIOS worked, but with delayed reaction */
146
147 enable_a20_kbc();
148 if (a20_test_long())
149 return 0;
150
151 /* Finally, try enabling the "fast A20 gate" */
152 enable_a20_fast();
153 if (a20_test_long())
154 return 0;
155 }
156
157 return -1;
158#endif
159}