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(v: ++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(v: 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(regs: &ireg); |
94 | ireg.ax = 0x2401; |
95 | intcall(int_no: 0x15, ireg: &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 | } |
164 | |