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Message-ID: <Pine.LNX.4.64.0810081542300.14815@ren>
Date: Wed, 8 Oct 2008 15:43:30 -0600 (MDT)
From: Jeff Hansen <x@...fhansen.com>
To: Chris Snook <csnook@...hat.com>
cc: torvalds@...ux-foundation.org, linux-kernel@...r.kernel.org,
mingo@...e.hu
Subject: [PATCH] Re: x86_32 tsc/pit and hrtimers
[HRTIMER]: Add highres=noverify option to bypass clocksource verification
This disregards the CLOCK_SOURCE_MUST_VERIFY flag on all clocksources.
This is particularly useful on legacy x86_32 systems that have no ACPI,
LAPIC, or HPET timers, where only TSC and PIT are available.
Thanks to Chris Snook for suggesting this.
---
include/linux/clocksource.h | 2 ++
kernel/hrtimer.c | 2 ++
kernel/time/clocksource.c | 3 ++-
3 files changed, 6 insertions(+), 1 deletions(-)
diff --git a/include/linux/clocksource.h b/include/linux/clocksource.h
index 55e434f..90ae835 100644
--- a/include/linux/clocksource.h
+++ b/include/linux/clocksource.h
@@ -104,6 +104,8 @@ extern struct clocksource *clock; /* current clocksource */
#define CLOCK_SOURCE_WATCHDOG 0x10
#define CLOCK_SOURCE_VALID_FOR_HRES 0x20
+extern int clocksource_noverify;
+
/* simplify initialization of mask field */
#define CLOCKSOURCE_MASK(bits) (cycle_t)((bits) < 64 ? ((1ULL<<(bits))-1) : -1)
diff --git a/kernel/hrtimer.c b/kernel/hrtimer.c
index ab80515..2fdf59f 100644
--- a/kernel/hrtimer.c
+++ b/kernel/hrtimer.c
@@ -476,6 +476,8 @@ static int __init setup_hrtimer_hres(char *str)
hrtimer_hres_enabled = 0;
else if (!strcmp(str, "on"))
hrtimer_hres_enabled = 1;
+ else if (!strcmp(str, "noverify"))
+ clocksource_noverify = 1;
else
return 0;
return 1;
diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
index dadde53..d3c14c1 100644
--- a/kernel/time/clocksource.c
+++ b/kernel/time/clocksource.c
@@ -54,6 +54,7 @@ static LIST_HEAD(clocksource_list);
static DEFINE_SPINLOCK(clocksource_lock);
static char override_name[32];
static int finished_booting;
+int clocksource_noverify;
/* clocksource_done_booting - Called near the end of core bootup
*
@@ -165,7 +166,7 @@ static void clocksource_check_watchdog(struct clocksource *cs)
unsigned long flags;
spin_lock_irqsave(&watchdog_lock, flags);
- if (cs->flags & CLOCK_SOURCE_MUST_VERIFY) {
+ if (!clocksource_noverify && cs->flags & CLOCK_SOURCE_MUST_VERIFY) {
int started = !list_empty(&watchdog_list);
list_add(&cs->wd_list, &watchdog_list);
--
1.5.6.4
On Wed, 8 Oct 2008, Chris Snook wrote:
> Jeff Hansen wrote:
>> This worked perfectly! I second adding a kernel option that forces
>> trusting the TSC. I can make a patch if you'd like. Should the option be
>> something like "trusttsc" or "tsc=noverify"?
>
> How about a "highres=noverify" flag, to disable all CLOCK_SOURCE_MUST_VERIFY
> checking? People might want to use this with other timers too.
>
> -- Chris
>
>> On Wed, 8 Oct 2008, Chris Snook wrote:
>>
>> > Jeff Hansen wrote:
>> > > Linus, Ingo, All,
>> > >
>> > > I've been struggling with hrtimer support in 2.6.26.5 on an older
>> > > x86_32/i386 system, and I'm wondering if there are any easy fixes
>> > > that you
>> > > (or anyone else) would suggest.
>> > >
>> > > Basically, this system does not print out the message:
>> > >
>> > > "Switched to high resolution mode on CPU 0"
>> > >
>> > > indicating that one-shot, hrtimers, etc. won't work, since high
>> > > resolution
>> > > mode has not been enabled. I've verified that hrtimers started with
>> > > hrtimer_start do not have the expected resolution further than 1/HZ.
>> > >
>> > > This system does not have LAPIC, ACPI, or HPET, so really the only
>> > > clocksources I can use are TSC and PIT. This should be fine (in
>> > > theory,
>> > > unless it wasn't designed like that), but apparently the clocksource
>> > > flags
>> > > are not initialized in such a way that one of them ever gets marked
>> > > as
>> > > CLOCK_SOURCE_VALID_FOR_HRES.
>> > >
>> > > The flow of the flags on each of these clocksources is as follows:
>> > >
>> > > 1) The flags on the TSC clocksource are CLOCK_SOURCE_IS_CONTINUOUS |
>> > > CLOCK_SOURCE_MUST_VERIFY, which causes PIT to be used as the
>> > > watchdog
>> > > clocksource. (see kernel/time/clocksource.c:~171)
>> > > 2) Around line 122 in kernel/time/clocksource.c, where most
>> > > clocksources'
>> > > flags usually get ORed with CLOCK_SOURCE_VALID_FOR_HRES, the
>> > > PIT's do
>> > > not because it is not CLOCK_SOURCE_IS_CONTINUOUS, and the TSC's
>> > > do not
>> > > also because the PIT (as the watchdog) is not
>> > > CLOCK_SOURCE_IS_CONTINUOUS.
>> > >
>> > > I get the same results on a new laptop booting into 32-bit Linux with
>> > > hpet
>> > > and acpi disabled.
>> > >
>> > > Can you please tell me if this is supposed to work, and I just have a
>> > > poorly configured kernel; or if TSC/PIT drivers were not designed to
>> > > work
>> > > this way in the first place. If it wasn't designed to do this, do
>> > > you
>> > > have any tips on implementing this, since I'll be needing to do that?
>> > >
>> > > -Jeff Hansen
>> >
>> > This is not supposed to work, but it might be worthwhile to add a boot
>> > option to force the kernel to trust the TSC, as hardware that lacks any
>> > high-res timers also tends to be primitive enough that the TSC can be
>> > trusted, if it exists. If you patch out the CLOCK_SOURCE_MUST_VERIFY
>> > flag on the TSC, do you get correctly-functioning high-res timers on
>> > this system?
>> >
>> > -- Chris
>> >
>> >
>> >
>
>
>
--
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