x86: hpet: Make WARN_ON understandable

commit 18ed61da985c57eea3fe8038b13fa2837c9b3c3f upstream.

Andrew complained rightly that the WARN_ON in hpet_next_event() is
confusing and the code comment not really helpful.

Change it to WARN_ONCE and print the reason in clear text. Change the
comment to explain what kind of hardware wreckage we deal with.

Pointed-out-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Venki Pallipadi <venkatesh.pallipadi@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

diff --git a/arch/x86/kernel/hpet.c b/arch/x86/kernel/hpet.c
index 58778736496a37170a63e56019a040267098d183..4dda49c68a35bf703bd40bb3b3eac22459d5e740 100644 (file)
--- a/arch/x86/kernel/hpet.c
+++ b/arch/x86/kernel/hpet.c
@@ -385,11 +385,22 @@ 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((u32)hpet_readl(HPET_Tn_CMP(timer)) != cnt);
+       WARN_ONCE((u32)hpet_readl(HPET_Tn_CMP(timer)) != cnt,
+                 KERN_WARNING "hpet: compare register read back failed.\n");
 
        return (s32)((u32)hpet_readl(HPET_COUNTER) - cnt) >= 0 ? -ETIME : 0;
 }