A20 handling code

A20 handling code for the new x86 setup code.  This implements the same
algorithms as the assembly version.
Signed-off-by: NH. Peter Anvin <hpa@zytor.com>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

arch/i386/boot/a20.c

+/* -*- linux-c -*- ------------------------------------------------------- *
+ *
+ *   Copyright (C) 1991, 1992 Linus Torvalds
+ *   Copyright 2007 rPath, Inc. - All Rights Reserved
+ *
+ *   This file is part of the Linux kernel, and is made available under
+ *   the terms of the GNU General Public License version 2.
+ *
+ * ----------------------------------------------------------------------- */
+
+/*
+ * arch/i386/boot/a20.c
+ *
+ * Enable A20 gate (return -1 on failure)
+ */
+
+#include "boot.h"
+
+#define MAX_8042_LOOPS	100000
+
+static int empty_8042(void)
+{
+	u8 status;
+	int loops = MAX_8042_LOOPS;
+
+	while (loops--) {
+		io_delay();
+
+		status = inb(0x64);
+		if (status & 1) {
+			/* Read and discard input data */
+			io_delay();
+			(void)inb(0x60);
+		} else if (!(status & 2)) {
+			/* Buffers empty, finished! */
+			return 0;
+		}
+	}
+
+	return -1;
+}
+
+/* Returns nonzero if the A20 line is enabled.  The memory address
+   used as a test is the int $0x80 vector, which should be safe. */
+
+#define A20_TEST_ADDR	(4*0x80)
+#define A20_TEST_SHORT  32
+#define A20_TEST_LONG	2097152	/* 2^21 */
+
+static int a20_test(int loops)
+{
+	int ok = 0;
+	int saved, ctr;
+
+	set_fs(0x0000);
+	set_gs(0xffff);
+
+	saved = ctr = rdfs32(A20_TEST_ADDR);
+
+	while (loops--) {
+		wrfs32(++ctr, A20_TEST_ADDR);
+		io_delay();	/* Serialize and make delay constant */
+		ok = rdgs32(A20_TEST_ADDR+0x10) ^ ctr;
+		if (ok)
+			break;
+	}
+
+	wrfs32(saved, A20_TEST_ADDR);
+	return ok;
+}
+
+/* Quick test to see if A20 is already enabled */
+static int a20_test_short(void)
+{
+	return a20_test(A20_TEST_SHORT);
+}
+
+/* Longer test that actually waits for A20 to come on line; this
+   is useful when dealing with the KBC or other slow external circuitry. */
+static int a20_test_long(void)
+{
+	return a20_test(A20_TEST_LONG);
+}
+
+static void enable_a20_bios(void)
+{
+	asm volatile("pushfl; int $0x15; popfl"
+		     : : "a" ((u16)0x2401));
+}
+
+static void enable_a20_kbc(void)
+{
+	empty_8042();
+
+	outb(0xd1, 0x64);	/* Command write */
+	empty_8042();
+
+	outb(0xdf, 0x60);	/* A20 on */
+	empty_8042();
+}
+
+static void enable_a20_fast(void)
+{
+	u8 port_a;
+
+	port_a = inb(0x92);	/* Configuration port A */
+	port_a |=  0x02;	/* Enable A20 */
+	port_a &= ~0x01;	/* Do not reset machine */
+	outb(port_a, 0x92);
+}
+
+/*
+ * Actual routine to enable A20; return 0 on ok, -1 on failure
+ */
+
+#define A20_ENABLE_LOOPS 255	/* Number of times to try */
+
+int enable_a20(void)
+{
+	int loops = A20_ENABLE_LOOPS;
+
+#if defined(CONFIG_X86_ELAN)
+	/* Elan croaks if we try to touch the KBC */
+	enable_a20_fast();
+	while (!a20_test_long())
+		;
+	return 0;
+#elif defined(CONFIG_X86_VOYAGER)
+	/* On Voyager, a20_test() is unsafe? */
+	enable_a20_kbc();
+	return 0;
+#else
+	while (loops--) {
+		/* First, check to see if A20 is already enabled
+		   (legacy free, etc.) */
+		if (a20_test_short())
+			return 0;
+
+		/* Next, try the BIOS (INT 0x15, AX=0x2401) */
+		enable_a20_bios();
+		if (a20_test_short())
+			return 0;
+
+		/* Try enabling A20 through the keyboard controller */
+		empty_8042();
+		if (a20_test_short())
+			return 0; /* BIOS worked, but with delayed reaction */
+
+		enable_a20_kbc();
+		if (a20_test_long())
+			return 0;
+
+		/* Finally, try enabling the "fast A20 gate" */
+		enable_a20_fast();
+		if (a20_test_long())
+			return 0;
+	}
+
+	return -1;
+#endif
+}