Merge branch 'x86-timers-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/linux-2.6-tip

* 'x86-timers-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/linux-2.6-tip:
x86: Hpet: Avoid the comparator readback penalty

arch/x86/kernel/hpet.c

···
 			   struct clock_event_device *evt, int timer)
 {
 	u32 cnt;
+	s32 res;
 
 	cnt = hpet_readl(HPET_COUNTER);
 	cnt += (u32) delta;
 	hpet_writel(cnt, HPET_Tn_CMP(timer));
 
 	/*
-	 * 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.
-	 *
-	 * 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.
-	 *
-	 * 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.
+	 * HPETs are a complete disaster. The compare register is
+	 * based on a equal comparison and neither provides a less
+	 * than or equal functionality (which would require to take
+	 * the wraparound into account) nor a simple count down event
+	 * mode. Further the write to the comparator register is
+	 * delayed internally up to two HPET clock cycles in certain
+	 * chipsets (ATI, ICH9,10). We worked around that by reading
+	 * back the compare register, but that required another
+	 * workaround for ICH9,10 chips where the first readout after
+	 * write can return the old stale value. We already have a
+	 * minimum delta of 5us enforced, but a NMI or SMI hitting
+	 * between the counter readout and the comparator write can
+	 * move us behind that point easily. Now instead of reading
+	 * the compare register back several times, we make the ETIME
+	 * decision based on the following: Return ETIME if the
+	 * counter value after the write is less than 8 HPET cycles
+	 * away from the event or if the counter is already ahead of
+	 * the event.
 	 */
-	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");
-	}
+	res = (s32)(cnt - hpet_readl(HPET_COUNTER));
 
-	return (s32)(hpet_readl(HPET_COUNTER) - cnt) >= 0 ? -ETIME : 0;
+	return res < 8 ? -ETIME : 0;
 }
 
 static void hpet_legacy_set_mode(enum clock_event_mode mode,