1 files changed, 15 insertions, 4 deletions

diff --git a/arch/x86/kernel/hpet.c b/arch/x86/kernel/hpet.c
index 7f024ff47d1d..ba6e65884603 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;
 }