Gitweb:     
http://git.kernel.org/git/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=5a8a8128bc218ebd067c660912d838344b05c608
Commit:     5a8a8128bc218ebd067c660912d838344b05c608
Parent:     5be865661516263d90317a6b35b588a2d7c3cb55
Author:     H. Peter Anvin <[EMAIL PROTECTED]>
AuthorDate: Wed Jul 11 12:18:42 2007 -0700
Committer:  Linus Torvalds <[EMAIL PROTECTED]>
CommitDate: Thu Jul 12 10:55:55 2007 -0700

    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 <[EMAIL PROTECTED]>
    Signed-off-by: Linus Torvalds <[EMAIL PROTECTED]>
---
 arch/i386/boot/a20.c |  161 ++++++++++++++++++++++++++++++++++++++++++++++++++
 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 0000000..31348d0
--- /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
+}
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