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    SubjectRe: x86_32 tsc/pit and hrtimers
    Jeff Hansen wrote:
    > Linus, Ingo, All,
    > I've been struggling with hrtimer support in 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
    > 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
    > 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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