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1 /* -*- linux-c -*- ------------------------------------------------------- *
2  *
3  *   Copyright (C) 1991, 1992 Linus Torvalds
4  *   Copyright 2007-2008 rPath, Inc. - All Rights Reserved
5  *   Copyright 2009 Intel Corporation; author H. Peter Anvin
6  *
7  *   This file is part of the Linux kernel, and is made available under
8  *   the terms of the GNU General Public License version 2.
9  *
10  * ----------------------------------------------------------------------- */
11 
12 /*
13  * Enable A20 gate (return -1 on failure)
14  */
15 
16 #include "boot.h"
17 
18 #define MAX_8042_LOOPS	100000
19 #define MAX_8042_FF	32
20 
empty_8042(void)21 static int empty_8042(void)
22 {
23 	u8 status;
24 	int loops = MAX_8042_LOOPS;
25 	int ffs   = MAX_8042_FF;
26 
27 	while (loops--) {
28 		io_delay();
29 
30 		status = inb(0x64);
31 		if (status == 0xff) {
32 			/* FF is a plausible, but very unlikely status */
33 			if (!--ffs)
34 				return -1; /* Assume no KBC present */
35 		}
36 		if (status & 1) {
37 			/* Read and discard input data */
38 			io_delay();
39 			(void)inb(0x60);
40 		} else if (!(status & 2)) {
41 			/* Buffers empty, finished! */
42 			return 0;
43 		}
44 	}
45 
46 	return -1;
47 }
48 
49 /* Returns nonzero if the A20 line is enabled.  The memory address
50    used as a test is the int $0x80 vector, which should be safe. */
51 
52 #define A20_TEST_ADDR	(4*0x80)
53 #define A20_TEST_SHORT  32
54 #define A20_TEST_LONG	2097152	/* 2^21 */
55 
a20_test(int loops)56 static int a20_test(int loops)
57 {
58 	int ok = 0;
59 	int saved, ctr;
60 
61 	set_fs(0x0000);
62 	set_gs(0xffff);
63 
64 	saved = ctr = rdfs32(A20_TEST_ADDR);
65 
66 	while (loops--) {
67 		wrfs32(++ctr, A20_TEST_ADDR);
68 		io_delay();	/* Serialize and make delay constant */
69 		ok = rdgs32(A20_TEST_ADDR+0x10) ^ ctr;
70 		if (ok)
71 			break;
72 	}
73 
74 	wrfs32(saved, A20_TEST_ADDR);
75 	return ok;
76 }
77 
78 /* Quick test to see if A20 is already enabled */
a20_test_short(void)79 static int a20_test_short(void)
80 {
81 	return a20_test(A20_TEST_SHORT);
82 }
83 
84 /* Longer test that actually waits for A20 to come on line; this
85    is useful when dealing with the KBC or other slow external circuitry. */
a20_test_long(void)86 static int a20_test_long(void)
87 {
88 	return a20_test(A20_TEST_LONG);
89 }
90 
enable_a20_bios(void)91 static void enable_a20_bios(void)
92 {
93 	struct biosregs ireg;
94 
95 	initregs(&ireg);
96 	ireg.ax = 0x2401;
97 	intcall(0x15, &ireg, NULL);
98 }
99 
enable_a20_kbc(void)100 static void enable_a20_kbc(void)
101 {
102 	empty_8042();
103 
104 	outb(0xd1, 0x64);	/* Command write */
105 	empty_8042();
106 
107 	outb(0xdf, 0x60);	/* A20 on */
108 	empty_8042();
109 
110 	outb(0xff, 0x64);	/* Null command, but UHCI wants it */
111 	empty_8042();
112 }
113 
enable_a20_fast(void)114 static void enable_a20_fast(void)
115 {
116 	u8 port_a;
117 
118 	port_a = inb(0x92);	/* Configuration port A */
119 	port_a |=  0x02;	/* Enable A20 */
120 	port_a &= ~0x01;	/* Do not reset machine */
121 	outb(port_a, 0x92);
122 }
123 
124 /*
125  * Actual routine to enable A20; return 0 on ok, -1 on failure
126  */
127 
128 #define A20_ENABLE_LOOPS 255	/* Number of times to try */
129 
enable_a20(void)130 int enable_a20(void)
131 {
132        int loops = A20_ENABLE_LOOPS;
133        int kbc_err;
134 
135        while (loops--) {
136 	       /* First, check to see if A20 is already enabled
137 		  (legacy free, etc.) */
138 	       if (a20_test_short())
139 		       return 0;
140 
141 	       /* Next, try the BIOS (INT 0x15, AX=0x2401) */
142 	       enable_a20_bios();
143 	       if (a20_test_short())
144 		       return 0;
145 
146 	       /* Try enabling A20 through the keyboard controller */
147 	       kbc_err = empty_8042();
148 
149 	       if (a20_test_short())
150 		       return 0; /* BIOS worked, but with delayed reaction */
151 
152 	       if (!kbc_err) {
153 		       enable_a20_kbc();
154 		       if (a20_test_long())
155 			       return 0;
156 	       }
157 
158 	       /* Finally, try enabling the "fast A20 gate" */
159 	       enable_a20_fast();
160 	       if (a20_test_long())
161 		       return 0;
162        }
163 
164        return -1;
165 }
166