On advice of hpa: be more patient with the KBC and SCPA methods; retry

in a long loop before giving up on them.  Record method which worked
and default to that method on next attempt.

src/arch/i386/firmware/pcbios/gateA20.c

@@ -24,11 +24,21 @@
 
 #define SCP_A		0x92		/* System Control Port A */
 
-#define A20_MAX_RETRIES	32
-
 enum { Disable_A20 = 0x2400, Enable_A20 = 0x2401, Query_A20_Status = 0x2402,
 	Query_A20_Support = 0x2403 };
 
+enum a20_methods {
+	A20_UNKNOWN = 0,
+	A20_INT15,
+	A20_KBC,
+	A20_SCPA,
+};
+
+#define A20_MAX_RETRIES		32
+#define A20_INT15_RETRIES	32
+#define A20_KBC_RETRIES		(2^21)
+#define A20_SCPA_RETRIES	(2^21)
+
 /**
  * Drain keyboard controller
  */
@@ -47,29 +57,32 @@ static void empty_8042 ( void ) {
 /**
  * Fast test to see if gate A20 is already set
  *
+ * @v retries		Number of times to retry before giving up
  * @ret set		Gate A20 is set
  */
-static int gateA20_is_set ( void ) {
+static int gateA20_is_set ( int retries ) {
 	static uint32_t test_pattern = 0xdeadbeef;
 	physaddr_t test_pattern_phys = virt_to_phys ( &test_pattern );
 	physaddr_t verify_pattern_phys = ( test_pattern_phys ^ 0x100000 );
 	userptr_t verify_pattern_user = phys_to_user ( verify_pattern_phys );
 	uint32_t verify_pattern;
 
-	/* Check for difference */
-	copy_from_user ( &verify_pattern, verify_pattern_user, 0,
-			 sizeof ( verify_pattern ) );
-	if ( verify_pattern != test_pattern )
-		return 1;
+	do {
+		/* Check for difference */
+		copy_from_user ( &verify_pattern, verify_pattern_user, 0,
+				 sizeof ( verify_pattern ) );
+		if ( verify_pattern != test_pattern )
+			return 1;
 
-	/* Invert pattern and retest, just to be sure */
-	test_pattern ^= 0xffffffff;
-	copy_from_user ( &verify_pattern, verify_pattern_user, 0,
-			 sizeof ( verify_pattern ) );
-	if ( verify_pattern != test_pattern )
-		return 1;
+		/* Avoid false negatives */
+		test_pattern++;
 
-	/* Pattern matched both times; gate A20 is not set */
+		SLOW_DOWN_IO;
+
+		/* Always retry at least once, to avoid false negatives */
+	} while ( retries-- >= 0 );
+
+	/* Pattern matched every time; gate A20 is not set */
 	return 0;
 }
 
@@ -83,6 +96,7 @@ static int gateA20_is_set ( void ) {
  */
 void gateA20_set ( void ) {
 	static char reentry_guard = 0;
+	static int a20_method = A20_UNKNOWN;
 	unsigned int discard_a;
 	unsigned int scp_a;
 	int retries = 0;
@@ -93,41 +107,51 @@ void gateA20_set ( void ) {
 	reentry_guard = 1;
 
 	/* Fast check to see if gate A20 is already enabled */
-	if ( gateA20_is_set() )
+	if ( gateA20_is_set ( 0 ) )
 		goto out;
 
 	for ( ; retries < A20_MAX_RETRIES ; retries++ ) {
-
-		/* Try INT 15 method first */
-		__asm__ __volatile__ ( REAL_CODE ( "int $0x15" )
-				       : "=a" ( discard_a )
-				       : "a" ( Enable_A20 ) );
-		if ( gateA20_is_set() ) {
-			DBG ( "Enabled gate A20 using BIOS\n" );
-			goto out;
-		}
-
-		/* Try keyboard controller method */
-		empty_8042();
-		outb ( KC_CMD_WOUT, K_CMD );
-		empty_8042();
-		outb ( KB_SET_A20, K_RDWR );
-		empty_8042();
-		if ( gateA20_is_set() ) {
-			DBG ( "Enabled gate A20 using keyboard controller\n" );
-			goto out;
-		}
-
-		/* Try "Fast gate A20" method */
-		scp_a = inb ( SCP_A );
-		scp_a &= ~0x01; /* Avoid triggering a reset */
-		scp_a |= 0x02; /* Enable A20 */
-		SLOW_DOWN_IO;
-		outb ( scp_a, SCP_A );
-		SLOW_DOWN_IO;
-		if ( gateA20_is_set() ) {
-			DBG ( "Enabled gate A20 using Fast Gate A20\n" );
-			goto out;
+		switch ( a20_method ) {
+		case A20_UNKNOWN:
+		case A20_INT15:
+			/* Try INT 15 method */
+			__asm__ __volatile__ ( REAL_CODE ( "int $0x15" )
+					       : "=a" ( discard_a )
+					       : "a" ( Enable_A20 ) );
+			if ( gateA20_is_set ( A20_INT15_RETRIES ) ) {
+				DBG ( "Enabled gate A20 using BIOS\n" );
+				a20_method = A20_INT15;
+				goto out;
+			}
+			/* fall through */
+		case A20_KBC:
+			/* Try keyboard controller method */
+			empty_8042();
+			outb ( KC_CMD_WOUT, K_CMD );
+			empty_8042();
+			outb ( KB_SET_A20, K_RDWR );
+			empty_8042();
+			if ( gateA20_is_set ( A20_KBC_RETRIES ) ) {
+				DBG ( "Enabled gate A20 using "
+				      "keyboard controller\n" );
+				a20_method = A20_KBC;
+				goto out;
+			}
+			/* fall through */
+		case A20_SCPA:
+			/* Try "Fast gate A20" method */
+			scp_a = inb ( SCP_A );
+			scp_a &= ~0x01; /* Avoid triggering a reset */
+			scp_a |= 0x02; /* Enable A20 */
+			SLOW_DOWN_IO;
+			outb ( scp_a, SCP_A );
+			SLOW_DOWN_IO;
+			if ( gateA20_is_set ( A20_SCPA_RETRIES ) ) {
+				DBG ( "Enabled gate A20 using "
+				      "Fast Gate A20\n" );
+				a20_method = A20_SCPA;
+				goto out;
+			}
 		}
 	}