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From: Andi Kleen on 4 Feb 2010 14:30

[AK: Sorry, here's the version that actually compiles. Forgot to do
the final commit to git]

HPET: Drop WARN_ON for mismatch on HPET CMP readback

At least one Intel chipset seems to always return a constant value
when reading back the HPET CMP register. This triggers the WARN_ON_ONCE
on each boot.

In addition the WARN_ON was buggy: it has a side-effect and
actually needed code could be optimized out if someone disabled
CONFIG_BUG.

So far there's no indication that miscompare on reading
points to actual problem.

So simply drop the WARN_ON_ONCE.

Based on discussions with Thomas Gleixner.

Signed-off-by: Andi Kleen <ak(a)linux.intel.com>

diff --git a/arch/x86/kernel/hpet.c b/arch/x86/kernel/hpet.c
index ad80a1c..c7eb16f 100644
--- a/arch/x86/kernel/hpet.c
+++ b/arch/x86/kernel/hpet.c
@@ -394,14 +394,11 @@ static int hpet_next_event(unsigned long delta,
* at that point and we would wait for the next hpet interrupt
* forever. We found out that reading the CMP register back
* forces the transfer so we can rely on the comparison with
- * the counter register below. If the read back from the
- * compare register does not match the value we programmed
- * then we might have a real hardware problem. We can not do
- * much about it here, but at least alert the user/admin with
- * a prominent warning.
+ * the counter register below.
+ * But don't actually check the read-back value. Some Intel chipsets
+ * return always the same value.
*/
- WARN_ONCE(hpet_readl(HPET_Tn_CMP(timer)) != cnt,
- KERN_WARNING "hpet: compare register read back failed.\n");
+ (void)hpet_readl(HPET_Tn_CMP(timer));

return (s32)(hpet_readl(HPET_COUNTER) - cnt) >= 0 ? -ETIME : 0;
}
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