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SubjectRe: x86_32 tsc/pit and hrtimers
Chris,

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"?

-Jeff

---------------------------------------------------
"If someone's gotta do it, it might as well be me."

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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