sparc64: Add some more commentary to __raw_local_irq_save()

Suggested by Peter Zijlstra
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

arch/sparc/include/asm/irqflags_64.h

@@ -80,6 +80,13 @@ static inline unsigned long __raw_local_irq_save(void)
 
 	/* Disable interrupts to PIL_NORMAL_MAX unless we already
 	 * are using PIL_NMI, in which case PIL_NMI is retained.
+	 *
+	 * The only values we ever program into the %pil are 0,
+	 * PIL_NORMAL_MAX and PIL_NMI.
+	 *
+	 * Since PIL_NMI is the largest %pil value and all bits are
+	 * set in it (0xf), it doesn't matter what PIL_NORMAL_MAX
+	 * actually is.
 	 */
 	__asm__ __volatile__(
 		"rdpr	%%pil, %0\n\t"