[tip:x86/urgent] x86, hpet: Fix bug in RTC emulation

From: tip-bot for Alok Kataria
Date: Thu Apr 01 2010 - 18:59:11 EST

Commit-ID: b4a5e8a1deca7e61ebaffb37344766b0f0e9f327
Gitweb: http://git.kernel.org/tip/b4a5e8a1deca7e61ebaffb37344766b0f0e9f327
Author: Alok Kataria <akataria@xxxxxxxxxx>
AuthorDate: Thu, 11 Mar 2010 14:00:16 -0800
Committer: H. Peter Anvin <hpa@xxxxxxxxx>
CommitDate: Thu, 1 Apr 2010 15:21:48 -0700

x86, hpet: Fix bug in RTC emulation

We think there exists a bug in the HPET code that emulates the RTC.

In the normal case, when the RTC frequency is set, the rtc driver tells
the hpet code about it here:

int hpet_set_periodic_freq(unsigned long freq)
uint64_t clc;

if (!is_hpet_enabled())
return 0;

hpet_pie_limit = DEFAULT_RTC_INT_FREQ / freq;
else {
clc = (uint64_t) hpet_clockevent.mult * NSEC_PER_SEC;
do_div(clc, freq);
clc >>= hpet_clockevent.shift;
hpet_pie_delta = (unsigned long) clc;
return 1;

If freq is set to 64Hz (DEFAULT_RTC_INT_FREQ) or lower, then
hpet_pie_limit (a static) is set to non-zero. Then, on every one-shot
HPET interrupt, hpet_rtc_timer_reinit is called to compute the next
timeout. Well, that function has this logic:

if (!(hpet_rtc_flags & RTC_PIE) || hpet_pie_limit)
delta = hpet_default_delta;
delta = hpet_pie_delta;

Since hpet_pie_limit is not 0, hpet_default_delta is used. That
corresponds to 64Hz.

Now, if you set a different rtc frequency, you'll take the else path
through hpet_set_periodic_freq, but unfortunately no one resets
hpet_pie_limit back to 0.

Boom....now you are stuck with 64Hz RTC interrupts forever.

The patch below just resets the hpet_pie_limit value when requested freq
is greater than DEFAULT_RTC_INT_FREQ, which we think fixes this problem.

Signed-off-by: Alok N Kataria <akataria@xxxxxxxxxx>
LKML-Reference: <201003112200.o2BM0Hre012875@xxxxxxxxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Daniel Hecht <dhecht@xxxxxxxxxx>
Cc: Venkatesh Pallipadi <venkatesh.pallipadi@xxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: H. Peter Anvin <hpa@xxxxxxxxx>
arch/x86/kernel/hpet.c | 1 +
1 files changed, 1 insertions(+), 0 deletions(-)

diff --git a/arch/x86/kernel/hpet.c b/arch/x86/kernel/hpet.c
index 3d422da..2bda5f0 100644
--- a/arch/x86/kernel/hpet.c
+++ b/arch/x86/kernel/hpet.c
@@ -1149,6 +1149,7 @@ int hpet_set_periodic_freq(unsigned long freq)
do_div(clc, freq);
clc >>= hpet_clockevent.shift;
hpet_pie_delta = clc;
+ hpet_pie_limit = 0;
return 1;
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