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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 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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      Last update: 2008-10-08 22:27    [W:0.027 / U:5.444 seconds]
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