[PATCH] i386: clockevents fix breakage on Geode/Cyrix PIT implementations

[PATCH] i386: clockevents fix breakage on Geode/Cyrix PIT implementations

[Thomas Gleixner]

The PIT has no dedicated mode for shut down. The only way to disable PIT
is to put it into one shot mode. AMD implementations of PIT on Geode
(also observed on Cyrix) are confused by an "empty" transition from
CLOCK_EVT_MODE_UNUSED to CLOCK_EVT_MODE_SHUTDOWN, which puts the PIT
into one shot mode momentarily.

I realized after staring helpless at the bug report
http://bugzilla.kernel.org/show_bug.cgi?id=8027 for quite a while, that
the only change, which might influence the bogomips calibration, is the
above transition during the PIT initialization.

Avoiding the unnecessary switch to oneshot and later to periodic mode
fixes the weird bogomips value and also the resulting slowness.

The fix is confirmed on OLPC and another Geode based box.

Note: this is unrelated to the Dual Core problem discussed here:
http://lkml.org/lkml/2007/3/17/48

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

diff --git a/arch/i386/kernel/i8253.c b/arch/i386/kernel/i8253.c
index 5cbb776..10cef5c 100644
--- a/arch/i386/kernel/i8253.c
+++ b/arch/i386/kernel/i8253.c
@@ -47,9 +47,17 @@ static void init_pit_timer(enum clock_event_mode mode,
 		outb(LATCH >> 8 , PIT_CH0);	/* MSB */
 		break;
 
-	case CLOCK_EVT_MODE_ONESHOT:
+	/*
+	 * Avoid unnecessary state transitions, as it confuses
+	 * Geode / Cyrix based boxen.
+	 */
 	case CLOCK_EVT_MODE_SHUTDOWN:
+		if (evt->mode == CLOCK_EVT_MODE_UNUSED)
+			break;
 	case CLOCK_EVT_MODE_UNUSED:
+		if (evt->mode == CLOCK_EVT_MODE_SHUTDOWN)
+			break;
+	case CLOCK_EVT_MODE_ONESHOT:
 		/* One shot setup */
 		outb_p(0x38, PIT_MODE);
 		udelay(10);