1 // SPDX-License-Identifier: GPL-2.0-only
2 /* -*- linux-c -*- ------------------------------------------------------- *
3 *
4 * Copyright (C) 1991, 1992 Linus Torvalds
5 * Copyright 2007-2008 rPath, Inc. - All Rights Reserved
6 * Copyright 2009 Intel Corporation; author H. Peter Anvin
7 *
8 * ----------------------------------------------------------------------- */
9
10 /*
11 * Enable A20 gate (return -1 on failure)
12 */
13
14 #include "boot.h"
15
16 #define MAX_8042_LOOPS 100000
17 #define MAX_8042_FF 32
18
19 static int empty_8042(void)
20 {
21 u8 status;
22 int loops = MAX_8042_LOOPS;
23 int ffs = MAX_8042_FF;
24
25 while (loops--) {
26 io_delay();
27
28 status = inb(0x64);
29 if (status == 0xff) {
30 /* FF is a plausible, but very unlikely status */
31 if (!--ffs)
32 return -1; /* Assume no KBC present */
33 }
34 if (status & 1) {
35 /* Read and discard input data */
36 io_delay();
37 (void)inb(0x60);
38 } else if (!(status & 2)) {
39 /* Buffers empty, finished! */
40 return 0;
41 }
42 }
43
44 return -1;
45 }
46
47 /* Returns nonzero if the A20 line is enabled. The memory address
48 used as a test is the int $0x80 vector, which should be safe. */
49
50 #define A20_TEST_ADDR (4*0x80)
51 #define A20_TEST_SHORT 32
52 #define A20_TEST_LONG 2097152 /* 2^21 */
53
54 static int a20_test(int loops)
55 {
56 int ok = 0;
57 int saved, ctr;
58
59 set_fs(0x0000);
60 set_gs(0xffff);
61
62 saved = ctr = rdfs32(A20_TEST_ADDR);
63
64 while (loops--) {
65 wrfs32(++ctr, A20_TEST_ADDR);
66 io_delay(); /* Serialize and make delay constant */
67 ok = rdgs32(A20_TEST_ADDR+0x10) ^ ctr;
68 if (ok)
69 break;
70 }
71
72 wrfs32(saved, A20_TEST_ADDR);
73 return ok;
74 }
75
76 /* Quick test to see if A20 is already enabled */
77 static int a20_test_short(void)
78 {
79 return a20_test(A20_TEST_SHORT);
80 }
81
82 /* Longer test that actually waits for A20 to come on line; this
83 is useful when dealing with the KBC or other slow external circuitry. */
84 static int a20_test_long(void)
85 {
86 return a20_test(A20_TEST_LONG);
87 }
88
89 static void enable_a20_bios(void)
90 {
91 struct biosregs ireg;
92
93 initregs(&ireg);
94 ireg.ax = 0x2401;
95 intcall(0x15, &ireg, NULL);
96 }
97
98 static void enable_a20_kbc(void)
99 {
100 empty_8042();
101
102 outb(0xd1, 0x64); /* Command write */
103 empty_8042();
104
105 outb(0xdf, 0x60); /* A20 on */
106 empty_8042();
107
108 outb(0xff, 0x64); /* Null command, but UHCI wants it */
109 empty_8042();
110 }
111
112 static void enable_a20_fast(void)
113 {
114 u8 port_a;
115
116 port_a = inb(0x92); /* Configuration port A */
117 port_a |= 0x02; /* Enable A20 */
118 port_a &= ~0x01; /* Do not reset machine */
119 outb(port_a, 0x92);
120 }
121
122 /*
123 * Actual routine to enable A20; return 0 on ok, -1 on failure
124 */
125
126 #define A20_ENABLE_LOOPS 255 /* Number of times to try */
127
128 int enable_a20(void)
129 {
130 int loops = A20_ENABLE_LOOPS;
131 int kbc_err;
132
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 kbc_err = empty_8042();
146
147 if (a20_test_short())
148 return 0; /* BIOS worked, but with delayed reaction */
149
150 if (!kbc_err) {
151 enable_a20_kbc();
152 if (a20_test_long())
153 return 0;
154 }
155
156 /* Finally, try enabling the "fast A20 gate" */
157 enable_a20_fast();
158 if (a20_test_long())
159 return 0;
160 }
161
162 return -1;
163 }