MIPS: Kernel hangs occasionally during boot.

The Kernel hangs occasionally during boot after "Calibrating delay loop..".
This is caused by the c0_compare_int_usable() routine in cevt-r4k.c
returning false which causes the system to disable the timer and hang later.
The false return happens because the routine is using a series of four calls
to irq_disable_hazard() as a delay while it waits for the timer changes to
propagate to the cp0 cause register. On newer MIPS cores, like the 74K, the
series of irq_disable_hazard() calls turn into ehb instructions and can take
as little as a few clock ticks for all 4 instructions. This is not enough of
a delay, so the routine thinks the timer is not working.  This fix uses up
to a max number of cycle counter ticks for the delay and uses
back_to_back_c0_hazard() instead of irq_disable_hazard() to handle the
hazard condition between cp0 writes and cp0 reads.
Signed-off-by: Al Cooper <alcooperx@gmail.com>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/2911/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/kernel/cevt-r4k.c

@@ -103,19 +103,10 @@ static int c0_compare_int_pending(void)
 
 /*
  * Compare interrupt can be routed and latched outside the core,
- * so a single execution hazard barrier may not be enough to give
- * it time to clear as seen in the Cause register.  4 time the
- * pipeline depth seems reasonably conservative, and empirically
- * works better in configurations with high CPU/bus clock ratios.
+ * so wait up to worst case number of cycle counter ticks for timer interrupt
+ * changes to propagate to the cause register.
  */
-
-#define compare_change_hazard() \
-	do { \
-		irq_disable_hazard(); \
-		irq_disable_hazard(); \
-		irq_disable_hazard(); \
-		irq_disable_hazard(); \
-	} while (0)
+#define COMPARE_INT_SEEN_TICKS 50
 
 int c0_compare_int_usable(void)
 {
@@ -126,8 +117,12 @@ int c0_compare_int_usable(void)
 	 * IP7 already pending?  Try to clear it by acking the timer.
 	 */
 	if (c0_compare_int_pending()) {
-		write_c0_compare(read_c0_count());
-		compare_change_hazard();
+		cnt = read_c0_count();
+		write_c0_compare(cnt);
+		back_to_back_c0_hazard();
+		while (read_c0_count() < (cnt  + COMPARE_INT_SEEN_TICKS))
+			if (!c0_compare_int_pending())
+				break;
 		if (c0_compare_int_pending())
 			return 0;
 	}
@@ -136,7 +131,7 @@ int c0_compare_int_usable(void)
 		cnt = read_c0_count();
 		cnt += delta;
 		write_c0_compare(cnt);
-		compare_change_hazard();
+		back_to_back_c0_hazard();
 		if ((int)(read_c0_count() - cnt) < 0)
 		    break;
 		/* increase delta if the timer was already expired */
@@ -145,12 +140,17 @@ int c0_compare_int_usable(void)
 	while ((int)(read_c0_count() - cnt) <= 0)
 		;	/* Wait for expiry  */
 
-	compare_change_hazard();
+	while (read_c0_count() < (cnt + COMPARE_INT_SEEN_TICKS))
+		if (c0_compare_int_pending())
+			break;
 	if (!c0_compare_int_pending())
 		return 0;
-
-	write_c0_compare(read_c0_count());
-	compare_change_hazard();
+	cnt = read_c0_count();
+	write_c0_compare(cnt);
+	back_to_back_c0_hazard();
+	while (read_c0_count() < (cnt + COMPARE_INT_SEEN_TICKS))
+		if (!c0_compare_int_pending())
+			break;
 	if (c0_compare_int_pending())
 		return 0;