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

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Message-ID: <48ED1728.5060708@redhat.com>
Date:	Wed, 08 Oct 2008 16:25:12 -0400
From:	Chris Snook <csnook@...hat.com>
To:	Jeff Hansen <x@...fhansen.com>
CC:	torvalds@...ux-foundation.org, linux-kernel@...r.kernel.org,
	mingo@...e.hu
Subject: Re: 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 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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