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

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Message-ID: <CA+55aFwpqi70kHA-HJY8YoHtnT74PuFgQixrJhZjWDbhqYpzsQ@mail.gmail.com>
Date:	Wed, 3 Jun 2015 10:04:12 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	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>,
	"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, 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.

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.

            Linus
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