[PATCH] x86_64: Add pmtimer support

There are unfortunately more and more multi processor Opteron systems which
don't have HPET timer support in the southbridge.  This covers in particular
Nvidia and VIA chipsets.  They also don't guarantee that the TSCs are
synchronized between CPUs; and especially with MP powernow the systems are
nearly unusable because the time gets very inconsistent between CPUs.

The timer code for x86-64 was originally written under the assumption that we
could fall back to the HPET timer on such systems.  But this doesn't work
there.

Another alternative is to use the ACPI PM timer as primary time source.  This
patch does that.  The kernel only uses PM timer when there is no other choice
because it has some disadvantages.

Ported over from i386.  It should be faster than the i386 version because I
dropped the "read three times" workaround, but is still considerable slower
than HPET and also does not work together with vsyscalls which have to be
disabled.

Cc: <mark.langsdorf@amd.com>
Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 14 insertions, 0 deletions

diff --git a/arch/x86_64/Kconfig b/arch/x86_64/Kconfig
index 44ee7f6acf7b..82cb2a3f127a 100644
--- a/arch/x86_64/Kconfig
+++ b/arch/x86_64/Kconfig
@@ -303,6 +303,20 @@ config HPET_TIMER
           as it is off-chip.  You can find the HPET spec at
           <http://www.intel.com/labs/platcomp/hpet/hpetspec.htm>.
 
+config X86_PM_TIMER
+        bool "PM timer"
+        default y
+        help
+          Support the ACPI PM timer for time keeping. This is slow,
+          but is useful on some chipsets without HPET on systems with more
+          than one CPU. On a single processor or single socket multi core
+          system it is normally not required.
+          When the PM timer is active 64bit vsyscalls are disabled
+          and should not be enabled (/proc/sys/kernel/vsyscall64 should
+          not be changed).
+          The kernel selects the PM timer only as a last resort, so it is
+          useful to enable just in case.
+
 config HPET_EMULATE_RTC
         bool "Provide RTC interrupt"
         depends on HPET_TIMER && RTC=y