1 /* -*- linux-c -*- ------------------------------------------------------- *
2 *
3 * Copyright (C) 1991, 1992 Linus Torvalds
4 * Copyright 2007-2008 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
empty_8042(void)19 static 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
a20_test(int loops)48 static 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 */
a20_test_short(void)71 static 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. */
a20_test_long(void)78 static int a20_test_long(void)
79 {
80 return a20_test(A20_TEST_LONG);
81 }
82
enable_a20_bios(void)83 static void enable_a20_bios(void)
84 {
85 asm volatile("pushfl; int $0x15; popfl"
86 : : "a" ((u16)0x2401));
87 }
88
enable_a20_kbc(void)89 static 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 outb(0xff, 0x64); /* Null command, but UHCI wants it */
100 empty_8042();
101 }
102
enable_a20_fast(void)103 static void enable_a20_fast(void)
104 {
105 u8 port_a;
106
107 port_a = inb(0x92); /* Configuration port A */
108 port_a |= 0x02; /* Enable A20 */
109 port_a &= ~0x01; /* Do not reset machine */
110 outb(port_a, 0x92);
111 }
112
113 /*
114 * Actual routine to enable A20; return 0 on ok, -1 on failure
115 */
116
117 #define A20_ENABLE_LOOPS 255 /* Number of times to try */
118
enable_a20(void)119 int enable_a20(void)
120 {
121 #if defined(CONFIG_X86_ELAN)
122 /* Elan croaks if we try to touch the KBC */
123 enable_a20_fast();
124 while (!a20_test_long())
125 ;
126 return 0;
127 #elif defined(CONFIG_X86_VOYAGER)
128 /* On Voyager, a20_test() is unsafe? */
129 enable_a20_kbc();
130 return 0;
131 #else
132 int loops = A20_ENABLE_LOOPS;
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 }
162