A20 handling code

A20 handling code for the new x86 setup code.  This implements the same
algorithms as the assembly version.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 161 insertions, 0 deletions

diff --git a/arch/i386/boot/a20.c b/arch/i386/boot/a20.c
new file mode 100644
index 000000000000..31348d054fca
--- /dev/null
+++ b/arch/i386/boot/a20.c
@@ -0,0 +1,161 @@
+/* -*- 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
+}