Re: x86_32 tsc/pit and hrtimers

[Chris Snook]

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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