[PATCH] Make c0_compare_int_usable more bullet proof

[Atsushi Nemoto <anemo@xxxxxxxxxxxxx>]

Use write_c0_compare(read_c0_count()) to clear interrupt.
And use delta value based on its speed for faster probing.

Signed-off-by: Atsushi Nemoto <anemo@xxxxxxxxxxxxx>
---
diff --git a/arch/mips/kernel/cevt-r4k.c b/arch/mips/kernel/cevt-r4k.c
index a915e56..46a896f 100644
--- a/arch/mips/kernel/cevt-r4k.c
+++ b/arch/mips/kernel/cevt-r4k.c
@@ -179,19 +179,21 @@ static int c0_compare_int_pending(void)
 
 static int c0_compare_int_usable(void)
 {
-	const unsigned int delta = 0x300000;
+	unsigned int delta;
 	unsigned int cnt;
 
 	/*
 	 * IP7 already pending?  Try to clear it by acking the timer.
 	 */
 	if (c0_compare_int_pending()) {
-		write_c0_compare(read_c0_compare());
+		write_c0_compare(read_c0_count());
 		irq_disable_hazard();
 		if (c0_compare_int_pending())
 			return 0;
 	}
 
+	delta = read_c0_count();
+	delta = ((read_c0_count() - delta) ?: 1) << 8;
 	cnt = read_c0_count();
 	cnt += delta;
 	write_c0_compare(cnt);
@@ -202,7 +204,7 @@ static int c0_compare_int_usable(void)
 	if (!c0_compare_int_pending())
 		return 0;
 
-	write_c0_compare(read_c0_compare());
+	write_c0_compare(read_c0_count());
 	irq_disable_hazard();
 	if (c0_compare_int_pending())
 		return 0;