x86: HPET: Chose a paranoid safe value for the ETIME check

commit 995bd3bb5 (x86: Hpet: Avoid the comparator readback penalty)
chose 8 HPET cycles as a safe value for the ETIME check, as we had the
confirmation that the posted write to the comparator register is
delayed by two HPET clock cycles on Intel chipsets which showed
readback problems.

After that patch hit mainline we got reports from machines with newer
AMD chipsets which seem to have an even longer delay. See
http://thread.gmane.org/gmane.linux.kernel/1054283 and
http://thread.gmane.org/gmane.linux.kernel/1069458 for further
information.

Boris tried to come up with an ACPI based selection of the minimum
HPET cycles, but this failed on a couple of test machines. And of
course we did not get any useful information from the hardware folks.

For now our only option is to chose a paranoid high and safe value for
the minimum HPET cycles used by the ETIME check. Adjust the minimum ns
value for the HPET clockevent accordingly.

Reported-Bistected-and-Tested-by: Markus Trippelsdorf <markus@trippelsdorf.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
LKML-Reference: <alpine.LFD.2.00.1012131222420.2653@localhost6.localdomain6>
Cc: Simon Kirby <sim@hostway.ca>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Andreas Herrmann <Andreas.Herrmann3@amd.com>
Cc: John Stultz <johnstul@us.ibm.com>

arch/x86/kernel/hpet.c

@@ -27,6 +27,9 @@
 #define HPET_DEV_FSB_CAP		0x1000
 #define HPET_DEV_PERI_CAP		0x2000
 
+#define HPET_MIN_CYCLES			128
+#define HPET_MIN_PROG_DELTA		(HPET_MIN_CYCLES + (HPET_MIN_CYCLES >> 1))
+
 #define EVT_TO_HPET_DEV(evt) container_of(evt, struct hpet_dev, evt)
 
 /*
@@ -299,8 +302,9 @@ static void hpet_legacy_clockevent_register(void)
 	/* Calculate the min / max delta */
 	hpet_clockevent.max_delta_ns = clockevent_delta2ns(0x7FFFFFFF,
 							   &hpet_clockevent);
-	/* 5 usec minimum reprogramming delta. */
-	hpet_clockevent.min_delta_ns = 5000;
+	/* Setup minimum reprogramming delta. */
+	hpet_clockevent.min_delta_ns = clockevent_delta2ns(HPET_MIN_PROG_DELTA,
+							   &hpet_clockevent);
 
 	/*
 	 * Start hpet with the boot cpu mask and make it
@@ -393,22 +397,24 @@ static int hpet_next_event(unsigned long delta,
 	 * 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
+	 * chipsets (ATI, ICH9,10). Some newer AMD chipsets have even
+	 * longer delays. 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 had a minimum
+	 * programming 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
+	 * counter value after the write is less than HPET_MIN_CYCLES
 	 * away from the event or if the counter is already ahead of
-	 * the event.
+	 * the event. The minimum programming delta for the generic
+	 * clockevents code is set to 1.5 * HPET_MIN_CYCLES.
 	 */
 	res = (s32)(cnt - hpet_readl(HPET_COUNTER));
 
-	return res < 8 ? -ETIME : 0;
+	return res < HPET_MIN_CYCLES ? -ETIME : 0;
 }
 
 static void hpet_legacy_set_mode(enum clock_event_mode mode,