linux-kernel - Re: [PATCH RFC] x86, tsc: Allow for high latency in quick_pit

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Date:	Wed, 3 Jun 2015 18:47:37 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
cc:	Ingo Molnar <mingo@...nel.org>,
	Adrian Hunter <adrian.hunter@...el.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Andy Lutomirski <luto@...capital.net>,
	Andi Kleen <ak@...ux.intel.com>,
	the arch/x86 maintainers <x86@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>, Len Brown <lenb@...nel.org>
Subject: Re: [PATCH RFC] x86, tsc: Allow for high latency in
 quick_pit_calibrate()

On Wed, 3 Jun 2015, Linus Torvalds wrote:
> On Tue, Jun 2, 2015 at 11:20 PM, Ingo Molnar <mingo@...nel.org> wrote:
> > Given that Windows relies on the
> > HPET for timekeeping, it might get more attention than the PIT?
> 
> Does Windows actually do that? Becuase if so, that's just about the
> strongest argument for HPET there is - it's likely to work, and the
> frequency is likely to be correct.

At least it used to. Not sure if it still does. 
 
> We've had issues with HPET, but for calibration it might very well be
> the right thing to do.

Right. The issues we had were on the clock events side caused by the
match register delayed writes. I've never seen a bug report about the
frequency being completely wrong, except for crap values which we
filter out. Though, HPET period can be off by more than 500ppm, but I
don't think that matters anymore for timekeeping as we switch to TSC
only after the long term calibration. The quick calibration is just
for getting the TSC frequency roughly correct for udelay.

> Does anybody know what the base oscillator for HPET tends to be? Also,

The most common frequency is 14.318180 MHz.

> some googling shows a vmware paper that is not that impressed with the
> HPET.

Yeah, they complain about period being off by 600ppm and therefor
being useless for timekeeping, but that's not what we want to use it
for.
 
> The good thing about the PIT is that it's just *specified* to be
> driven off a real crystal running at a very fixed frequency. There's
> no gray areas there. Sure, virtualization can screw it up (but will
> likely screw up other higher-resolution clocks even more), and hw
> designers can cause problems, but it's been pretty reliable.

The other known frequency clock is pmtimer. We use hpet and pmtimer in
the slow calibration fallback already.
 
> (Yeah, the CMOS RTC clock should be too, as George Spelvin points out.
> That might be worth looking at too).

Indeed.

Thanks,

	tglx
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