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-rw-r--r--arch/x86/kernel/hpet.c19
1 files changed, 15 insertions, 4 deletions
diff --git a/arch/x86/kernel/hpet.c b/arch/x86/kernel/hpet.c
index 7f024ff47d1..ba6e6588460 100644
--- a/arch/x86/kernel/hpet.c
+++ b/arch/x86/kernel/hpet.c
@@ -384,11 +384,22 @@ static int hpet_next_event(unsigned long delta,
hpet_writel(cnt, HPET_Tn_CMP(timer));
/*
- * We need to read back the CMP register to make sure that
- * what we wrote hit the chip before we compare it to the
- * counter.
+ * We need to read back the CMP register on certain HPET
+ * implementations (ATI chipsets) which seem to delay the
+ * transfer of the compare register into the internal compare
+ * logic. With small deltas this might actually be too late as
+ * the counter could already be higher than the compare value
+ * 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.
*/
- WARN_ON_ONCE(hpet_readl(HPET_Tn_CMP(timer)) != cnt);
+ WARN_ONCE(hpet_readl(HPET_Tn_CMP(timer)) != cnt,
+ KERN_WARNING "hpet: compare register read back failed.\n");
return (s32)(hpet_readl(HPET_COUNTER) - cnt) >= 0 ? -ETIME : 0;
}