linux-kernel - [tip:timers/core] timekeeping: Move reset of cycle

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Message-ID: <tip-1be396794897f80bfc8774719ba60309a9e3d374@git.kernel.org>
Date:	Sat, 15 Aug 2009 09:01:56 GMT
From:	tip-bot for Martin Schwidefsky <schwidefsky@...ibm.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org, dwalker@...o99.com, hpa@...or.com,
	mingo@...hat.com, johnstul@...ibm.com, schwidefsky@...ibm.com,
	tglx@...utronix.de
Subject: [tip:timers/core] timekeeping: Move reset of cycle_last for tsc clocksource to tsc

Commit-ID:  1be396794897f80bfc8774719ba60309a9e3d374
Gitweb:     http://git.kernel.org/tip/1be396794897f80bfc8774719ba60309a9e3d374
Author:     Martin Schwidefsky <schwidefsky@...ibm.com>
AuthorDate: Fri, 14 Aug 2009 15:47:20 +0200
Committer:  Thomas Gleixner <tglx@...utronix.de>
CommitDate: Sat, 15 Aug 2009 10:55:45 +0200

timekeeping: Move reset of cycle_last for tsc clocksource to tsc

change_clocksource resets the cycle_last value to zero then sets it to
a value read from the clocksource. The reset to zero is required only
for the TSC clocksource to make the read_tsc function work after a
resume. The reason is that the TSC read function uses cycle_last to
detect backwards going TSCs. In the resume case cycle_last contains
the TSC value from the last update before the suspend. On resume the
TSC starts counting from 0 again and would trip over the cycle_last
comparison.

This is subtle and surprising. Move the reset to a resume function in
the tsc code.

Signed-off-by: Martin Schwidefsky <schwidefsky@...ibm.com>
Acked-by: Thomas Gleixner <tglx@...utronix.de>
Acked-by: John Stultz <johnstul@...ibm.com>
Cc: Daniel Walker <dwalker@...o99.com>
LKML-Reference: <20090814134808.142191175@...ibm.com>
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>


---
 arch/x86/kernel/tsc.c     |    6 ++++++
 kernel/time/timekeeping.c |    1 -
 2 files changed, 6 insertions(+), 1 deletions(-)

diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index 71f4368..9684254 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -744,10 +744,16 @@ static cycle_t __vsyscall_fn vread_tsc(void)
 }
 #endif
 
+static void resume_tsc(void)
+{
+	clocksource_tsc.cycle_last = 0;
+}
+
 static struct clocksource clocksource_tsc = {
 	.name                   = "tsc",
 	.rating                 = 300,
 	.read                   = read_tsc,
+	.resume			= resume_tsc,
 	.mask                   = CLOCKSOURCE_MASK(64),
 	.shift                  = 22,
 	.flags                  = CLOCK_SOURCE_IS_CONTINUOUS |
diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 016a259..b567301 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -295,7 +295,6 @@ static void change_clocksource(void)
 	if (old->disable)
 		old->disable(old);
 
-	clock->cycle_last = 0;
 	clock->cycle_last = clock->read(clock);
 	clock->error = 0;
 	clock->xtime_nsec = 0;
--
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