Messages in this thread | | | Date | Wed, 8 Oct 2008 13:46:17 -0600 (MDT) | From | Jeff Hansen <> | Subject | Re: x86_32 tsc/pit and hrtimers |
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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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