x86 setup: a20: early timeout for a nonexistent keyboard controller

When probing the keyboard controller to enable A20, if we get FF back
(which is *possible* as a valid status word, but is extremely
unlikely) then bail after much fewer iterations than we otherwise
would, and abort the attempt to access the KBC.

This hopefully should make it work a lot better for embedded platforms
which don't have a KBC and where the BIOS doesn't implement
INT 15h AX=2401h (and doesn't boot with A20 already enabled.)

If this works, it will be the one remaining use of CONFIG_X86_ELAN as
anything other than a processor type optimization option.
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

arch/x86/boot/a20.c

@@ -2,6 +2,7 @@
  *
  *   Copyright (C) 1991, 1992 Linus Torvalds
  *   Copyright 2007-2008 rPath, Inc. - All Rights Reserved
+ *   Copyright 2009 Intel Corporation
  *
  *   This file is part of the Linux kernel, and is made available under
  *   the terms of the GNU General Public License version 2.
@@ -15,16 +16,23 @@
 #include "boot.h"
 
 #define MAX_8042_LOOPS	100000
+#define MAX_8042_FF	32
 
 static int empty_8042(void)
 {
 	u8 status;
 	int loops = MAX_8042_LOOPS;
+	int ffs   = MAX_8042_FF;
 
 	while (loops--) {
 		io_delay();
 
 		status = inb(0x64);
+		if (status == 0xff) {
+			/* FF is a plausible, but very unlikely status */
+			if (!--ffs)
+				return -1; /* Assume no KBC present */
+		}
 		if (status & 1) {
 			/* Read and discard input data */
 			io_delay();
@@ -118,44 +126,43 @@ static void enable_a20_fast(void)
 
 int enable_a20(void)
 {
-#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)
+#ifdef CONFIG_X86_VOYAGER
 	/* On Voyager, a20_test() is unsafe? */
 	enable_a20_kbc();
 	return 0;
 #else
        int loops = A20_ENABLE_LOOPS;
-	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;
+       int kbc_err;
+
+       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 */
+	       kbc_err = empty_8042();
+
+	       if (a20_test_short())
+		       return 0; /* BIOS worked, but with delayed reaction */
+	
+	       if (!kbc_err) {
+		       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
 }