PcAtChipsetPkg/HpetTimerDxe: Fix race condition in SetTimerPeriod()
authorMichael Kinney <michael.d.kinney@intel.com>
Wed, 26 Oct 2016 22:26:03 +0000 (15:26 -0700)
committerMichael Kinney <michael.d.kinney@intel.com>
Fri, 28 Oct 2016 04:46:42 +0000 (21:46 -0700)
commit70830df6dc72b4fea5e5e9eef07f0039a8a77afe
tree4bfa832aca95815faebba21a46860c5bfa8dcae2
parenta6f5a5b51f0f4e6915f1ac2b799574fd4591c440
PcAtChipsetPkg/HpetTimerDxe: Fix race condition in SetTimerPeriod()

https://bugzilla.tianocore.org/show_bug.cgi?id=182

The function TimerDriverSetTimerPeriod() disables the HPET timer
while the HPET timer HW is reprogrammed with a new timer period.
However, the MMIO write to disable the HPET timer HW can be
delayed and an HPET timer interrupt may be processed in the middle
of reprogramming the HPET timer HW and this may produced unexpected
results.

The fix is to raise TPL to TPL_HIGH_LEVEL in
TimerDriverSetTimerPeriod() during the time the HPET timer HW is
reprogrammed.  This guarantees that no timer interrupts are
processed during reprogramming.

The TimerDriverGenerateSoftInterrupt() function in this same
driver also raises TPL to TPL_HIGH_LEVEL, so this fix matches
the logic that is already used in another function for the same
reason.

Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Michael Kinney <michael.d.kinney@intel.com>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>
PcAtChipsetPkg/HpetTimerDxe/HpetTimer.c