x86: on x86_64, correct reading of PC RTC when update in progress in time_64.c
authorDavid P. Reed <dpreed@reed.com>
Thu, 15 Nov 2007 01:14:50 +0000 (20:14 -0500)
committerThomas Gleixner <tglx@apollo.(none)>
Sat, 17 Nov 2007 15:27:01 +0000 (16:27 +0100)
commitbbbd99955bfe84c9ae63f51db946a7bcd21f48be
treeef9c2f2a675523fba0ae84ae7d3514d804f0ec1e
parentc399da0d97e06803e51085ec076b63a3168aad1b
x86: on x86_64, correct reading of PC RTC when update in progress in time_64.c

Correct potentially unstable PC RTC time register reading in time_64.c

Stop the use of an incorrect technique for reading the standard PC RTC
timer, which is documented to "disconnect" time registers from the bus
while updates are in progress.  The use of UIP flag while interrupts
are disabled to protect a 244 microsecond window is one of the
Motorola spec sheet's documented ways to read the RTC time registers
reliably.

tglx: removed locking changes from original patch, as they gain nothing
(read_persistent_clock is only called during boot, suspend, resume - so
no hot path affected) and conflict with the paravirt locking scheme
(see 32bit code), which we do not want to complicate for no benefit.

Signed-off-by: David P. Reed <dpreed@reed.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
arch/x86/kernel/time_64.c