Messages in this thread | | | Date | Wed, 08 Oct 2008 16:25:12 -0400 | From | Chris Snook <> | Subject | Re: x86_32 tsc/pit and hrtimers |
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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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