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SubjectRe: x86_32 tsc/pit and hrtimers
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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