linux-kernel - Re: x86_32 tsc/pit and hrtimers

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Date:	Wed, 8 Oct 2008 13:46:17 -0600 (MDT)
From:	Jeff Hansen <x@...fhansen.com>
To:	Chris Snook <csnook@...hat.com>
cc:	torvalds@...ux-foundation.org, linux-kernel@...r.kernel.org,
	mingo@...e.hu
Subject: Re: x86_32 tsc/pit and hrtimers

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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