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

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Message-ID: <alpine.DEB.2.11.1506022130330.20347@nanos>
Date:	Tue, 2 Jun 2015 21:33:25 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Adrian Hunter <adrian.hunter@...el.com>
cc:	LKML <linux-kernel@...r.kernel.org>,
	Andy Lutomirski <luto@...capital.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andi Kleen <ak@...ux.intel.com>, 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 Thu, 21 May 2015, Adrian Hunter wrote:

> If it takes longer than 12us to read the PIT counter lsb/msb,
> then the error margin will never fall below 500ppm within 50ms,
> and Fast TSC calibration will always fail.

So finally the legacy PIT emulation takes longer than the 30 years old
hardware implementation. Progress!
 
> This patch detects when that will happen and switches to using
> a slightly different algorithm that takes advantage of the PIT's
> latch comand.

Is there really no smarter way to figure out the TSC frequency on
modern systems?

Thanks,

	tglx
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