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
19static 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
54static 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 */
77static 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. */
84static int a20_test_long(void)
85{
86	return a20_test(A20_TEST_LONG);
87}
88
89static 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
98static 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
112static 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
128int 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