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-rw-r--r--arch/x86/kernel/hpet.c31
1 files changed, 17 insertions, 14 deletions
diff --git a/arch/x86/kernel/hpet.c b/arch/x86/kernel/hpet.c
index 351f9c0fea1..410fdb3f193 100644
--- a/arch/x86/kernel/hpet.c
+++ b/arch/x86/kernel/hpet.c
@@ -35,7 +35,6 @@
unsigned long hpet_address;
u8 hpet_blockid; /* OS timer block num */
u8 hpet_msi_disable;
-u8 hpet_readback_cmp;
#ifdef CONFIG_PCI_MSI
static unsigned long hpet_num_timers;
@@ -395,23 +394,27 @@ 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.
+ * 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.
*
- * That works fine on those ATI chipsets, but on newer Intel
- * chipsets (ICH9...) this triggers due to an erratum: Reading
- * the comparator immediately following a write is returning
- * the old value.
+ * An erratum on some chipsets (ICH9,..), results in
+ * comparator read immediately following a write returning old
+ * value. Workaround for this is to read this value second
+ * time, when first read returns old value.
*
- * We restrict the read back to the affected ATI chipsets (set
- * by quirks) and also run it with hpet=verbose for debugging
- * purposes.
+ * In fact the write to the comparator register is delayed up
+ * to two HPET cycles so the workaround we tried to restrict
+ * the readback to those known to be borked ATI chipsets
+ * failed miserably. So we give up on optimizations forever
+ * and penalize all HPET incarnations unconditionally.
*/
- if (hpet_readback_cmp || hpet_verbose) {
- u32 cmp = hpet_readl(HPET_Tn_CMP(timer));
-
- if (cmp != cnt)
+ if (unlikely((u32)hpet_readl(HPET_Tn_CMP(timer)) != cnt)) {
+ if (hpet_readl(HPET_Tn_CMP(timer)) != cnt)
printk_once(KERN_WARNING
- "hpet: compare register read back failed.\n");
+ "hpet: compare register read back failed.\n");
}
return (s32)(hpet_readl(HPET_COUNTER) - cnt) >= 0 ? -ETIME : 0;