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gerrit-public.fairphone.software / kernel / msm-5.4 / 5a8a8128bc218ebd067c660912d838344b05c608 / . / arch / i386 / boot / a20.c
blob: 31348d054fca6129db01e2fd1e6f3600dc3038dd [file] [log] [blame]
H. Peter Anvin5a8a8122007-07-11 12:18:42 -0700[diff] [blame^]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 * arch/i386/boot/a20.c
13 *
14 * Enable A20 gate (return -1 on failure)
15 */
16
17#include "boot.h"
18
19#define MAX_8042_LOOPS 100000
20
21static int empty_8042(void)
22{
23 u8 status;
24 int loops = MAX_8042_LOOPS;
25
26 while (loops--) {
27 io_delay();
28
29 status = inb(0x64);
30 if (status & 1) {
31 /* Read and discard input data */
32 io_delay();
33 (void)inb(0x60);
34 } else if (!(status & 2)) {
35 /* Buffers empty, finished! */
36 return 0;
37 }
38 }
39
40 return -1;
41}
42
43/* Returns nonzero if the A20 line is enabled. The memory address
44 used as a test is the int $0x80 vector, which should be safe. */
45
46#define A20_TEST_ADDR (4*0x80)
47#define A20_TEST_SHORT 32
48#define A20_TEST_LONG 2097152 /* 2^21 */
49
50static int a20_test(int loops)
51{
52 int ok = 0;
53 int saved, ctr;
54
55 set_fs(0x0000);
56 set_gs(0xffff);
57
58 saved = ctr = rdfs32(A20_TEST_ADDR);
59
60 while (loops--) {
61 wrfs32(++ctr, A20_TEST_ADDR);
62 io_delay(); /* Serialize and make delay constant */
63 ok = rdgs32(A20_TEST_ADDR+0x10) ^ ctr;
64 if (ok)
65 break;
66 }
67
68 wrfs32(saved, A20_TEST_ADDR);
69 return ok;
70}
71
72/* Quick test to see if A20 is already enabled */
73static int a20_test_short(void)
74{
75 return a20_test(A20_TEST_SHORT);
76}
77
78/* Longer test that actually waits for A20 to come on line; this
79 is useful when dealing with the KBC or other slow external circuitry. */
80static int a20_test_long(void)
81{
82 return a20_test(A20_TEST_LONG);
83}
84
85static void enable_a20_bios(void)
86{
87 asm volatile("pushfl; int $0x15; popfl"
88 : : "a" ((u16)0x2401));
89}
90
91static void enable_a20_kbc(void)
92{
93 empty_8042();
94
95 outb(0xd1, 0x64); /* Command write */
96 empty_8042();
97
98 outb(0xdf, 0x60); /* A20 on */
99 empty_8042();
100}
101
102static void enable_a20_fast(void)
103{
104 u8 port_a;
105
106 port_a = inb(0x92); /* Configuration port A */
107 port_a |= 0x02; /* Enable A20 */
108 port_a &= ~0x01; /* Do not reset machine */
109 outb(port_a, 0x92);
110}
111
112/*
113 * Actual routine to enable A20; return 0 on ok, -1 on failure
114 */
115
116#define A20_ENABLE_LOOPS 255 /* Number of times to try */
117
118int enable_a20(void)
119{
120 int loops = A20_ENABLE_LOOPS;
121
122#if defined(CONFIG_X86_ELAN)
123 /* Elan croaks if we try to touch the KBC */
124 enable_a20_fast();
125 while (!a20_test_long())
126 ;
127 return 0;
128#elif defined(CONFIG_X86_VOYAGER)
129 /* On Voyager, a20_test() is unsafe? */
130 enable_a20_kbc();
131 return 0;
132#else
133 while (loops--) {
134 /* First, check to see if A20 is already enabled
135 (legacy free, etc.) */
136 if (a20_test_short())
137 return 0;
138
139 /* Next, try the BIOS (INT 0x15, AX=0x2401) */
140 enable_a20_bios();
141 if (a20_test_short())
142 return 0;
143
144 /* Try enabling A20 through the keyboard controller */
145 empty_8042();
146 if (a20_test_short())
147 return 0; /* BIOS worked, but with delayed reaction */
148
149 enable_a20_kbc();
150 if (a20_test_long())
151 return 0;
152
153 /* Finally, try enabling the "fast A20 gate" */
154 enable_a20_fast();
155 if (a20_test_long())
156 return 0;
157 }
158
159 return -1;
160#endif
161}