[PATCH] x86_64: Explain why HPET T0_CMP register is written twice

After writing the CFG register, the first value written to the T0_CMP
register is the value at which next interrupt should be triggered, every
value after that sets the period of the interrupt. For that reason, the code
needs to write the value twice - to set both the phase and period.

[AK: I had already figured it out by myself, but it's still useful
to have a comment for this.]

Signed-off-by: Vojtech Pavlik <vojtech@suse.cz>
Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

diff --git a/arch/x86_64/kernel/time.c b/arch/x86_64/kernel/time.c
index 51afb07bc14ed551d0bea62cd7862a8964e39c97..ee622ac0376b993689c05732bae8ab5dda7d764b 100644 (file)
--- a/arch/x86_64/kernel/time.c
+++ b/arch/x86_64/kernel/time.c
@@ -795,8 +795,8 @@ static int hpet_timer_stop_set_go(unsigned long tick)
        if (hpet_use_timer) {
                hpet_writel(HPET_TN_ENABLE | HPET_TN_PERIODIC | HPET_TN_SETVAL |
                    HPET_TN_32BIT, HPET_T0_CFG);
-               hpet_writel(hpet_tick, HPET_T0_CMP);
-               hpet_writel(hpet_tick, HPET_T0_CMP); /* AK: why twice? */
+               hpet_writel(hpet_tick, HPET_T0_CMP); /* next interrupt */
+               hpet_writel(hpet_tick, HPET_T0_CMP); /* period */
                cfg |= HPET_CFG_LEGACY;
        }
 /*