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

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Date:	Wed, 03 Jun 2015 10:50:04 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>
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>,
	Len Brown <lenb@...nel.org>
Subject: Re: [PATCH RFC] x86, tsc: Allow for high latency in quick_pit_calibrate()

On 06/03/2015 10:04 AM, Linus Torvalds wrote:
> On Wed, Jun 3, 2015 at 9:47 AM, Thomas Gleixner <tglx@...utronix.de> wrote:
>>
>>> Does anybody know what the base oscillator for HPET tends to be? Also,
>>
>> The most common frequency is 14.318180 MHz.
> 
> I was more thinking along the lines of "which actually crystal is
> driving it" than the frequency.
> 
> That said, that particular frequency does give a clue. That's exactly
> 12x the PIT frequency. So they presumably share the same basic
> crystal, just different PLL's.

Here are the gory details as far as I know them:

This is the default time keeping crystal -- it is 4x the NTSC color
burst frequency, which has been used since the original PC.  It is
actually supposed to be 157.5/11 MHz = 14.31818... MHz ±40 Hz, there is
no zero even though it is often seen in docs.

However, the HPET isn't guaranteed to run at this frequency.  The HPET
frequency is advertised via a control register in HPET space (at offset
0x04, 32-bit COUNTER_CLK_PERIOD in fs).  This register is documented as
readonly

The RTC is probably the most reliable reference clock, in part because
32 kHz crystals are generally calibrated and extremely stable.  However,
to get more than 1 Hz frequency out of it you have to enable interrupts
(which gets you to 8192 Hz).  I have heard claims that this is actually
what Windows uses for calibration, but I don't know.

The ACPI PM_TMR is also fed from the 14.31818 MHz clock () with a
divisor of 4 (putting it exactly at the NTSC color burst frequency of
3.579545... MHz ± 10 Hz).  The PM_TMR, if present, is required to run at
this frequency -- there are no provisions in ACPI for specifying the
frequency.

> Which is a good indication that we should get very similar clock
> stability with the HPET as with the PIC.
> 
> We might even use that kind of knowledge to decide "ok, let's use the
> HPET as the reference clock". IOW, only use the HPET if we find it
> listed, _and_ we get the expected frequency from the HPET_PERIOD
> register.
> 
> Anybody who gives odd HPET_PERIOD values is likely doing somehing
> different and possibly questionable.

It would definitely be fishy if PM_TMR is also supported, since PM_TMR
has to run at a fixed frequency.

The HPET frequency register would be 0x429b176 if run off the 14 MHz clock.

However, if someone does something different the PIC/PM_TMR frequency
might very well be synthesized (or worse, approximated!) and the HPET
might be run off the the real BCLK.  This is a legitimate design.
PIC/PM_TMR being synthesized is fine (might introduce some jitter, but
shouldn't affect calibration), approximated is not...

	-hpa

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