xref: /linux/arch/x86/boot/a20.c (revision 75bf465f0bc33e9b776a46d6a1b9b990f5fb7c37)
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 
empty_8042(void)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 
a20_test(int loops)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 */
a20_test_short(void)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. */
a20_test_long(void)84 static int a20_test_long(void)
85 {
86 	return a20_test(A20_TEST_LONG);
87 }
88 
enable_a20_bios(void)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 
enable_a20_kbc(void)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 
enable_a20_fast(void)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 
enable_a20(void)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