[PATCH] x86_64: Calibrate APIC timer using PM timer

On some broken motherboards (at least one NForce3 based AMD64 laptop)
the PIT timer runs at a incorrect frequency.  This patch adds a new
option "apicpmtimer" that allows to use the APIC timer and calibrate it
using the PMTimer.  It requires the earlier patch that allows to run the
main timer from the APIC.

Specifying apicpmtimer implies apicmaintimer.

The option defaults to off for now.

I tested it on a few systems and the resulting APIC timer frequencies
were usually a bit off, but always <1%, which should be tolerable.

TBD figure out heuristic to enable this automatically on the affected
systems TBD perhaps do it on all NForce3s or using DMI?

Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 5 insertions, 0 deletions

diff --git a/Documentation/x86_64/boot-options.txt b/Documentation/x86_64/boot-options.txt
index 654ea4fccff8..153740f460a6 100644
--- a/Documentation/x86_64/boot-options.txt
+++ b/Documentation/x86_64/boot-options.txt
@@ -47,6 +47,11 @@ APICs
    noapicmaintimer  Don't do time keeping using the APIC timer.
                  Useful when this option was auto selected, but doesn't work.
 
+   apicpmtimer
+                 Do APIC timer calibration using the pmtimer. Implies
+                 apicmaintimer. Useful when your PIT timer is totally
+                 broken.
+
 Early Console
 
    syntax: earlyprintk=vga