linux-kernel - Re: [PATCH RFC] x86: hpet: Avoid the readback penalty

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Date:	Wed, 15 Sep 2010 16:42:38 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Borislav Petkov <borislav.petkov@....com>
cc:	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	"H. Peter Anvin" <hpa@...or.com>, Nix <nix@...eri.org.uk>,
	Artur Skawina <art.08.09@...il.com>,
	Damien Wyart <damien.wyart@...e.fr>,
	John Drescher <drescherjm@...il.com>,
	Venkatesh Pallipadi <venki@...gle.com>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	"Herrmann3, Andreas" <Andreas.Herrmann3@....com>,
	Suresh Siddha <suresh.b.siddha@...el.com>
Subject: Re: [PATCH RFC] x86: hpet: Avoid the readback penalty

On Wed, 15 Sep 2010, Borislav Petkov wrote:

> From: Thomas Gleixner <tglx@...utronix.de>
> Date: Wed, Sep 15, 2010 at 09:11:57AM -0400
> 
> > On Tue, 14 Sep 2010, tip-bot for Thomas Gleixner wrote:
> > > x86: hpet: Work around hardware stupidity
> > 
> > After my brain recovered from yesterdays exposure with the x86 timer
> > horror, I came up with a different solution for this problem, which
> > avoids the readback of the compare register completely. It works
> > nicely on my affected ATI system, but needs some exposure to the other
> > machines.
> 
> Will run in on a couple of SBx00 machines I got here.
> 
> ...
> 
> > If cmp is less than 8 HPET clock cycles, then we decide that the event
> > has happened already and return -ETIME. That covers the above #1 and
> > #2 problems which would cause a wait for HPET wraparound (~306
> > seconds).
> 
> Make sense. I guess you're choosing a value of 8 just to be on the safe
> side wrt to HPET clock cycles it takes to write the cmp register?

Yes, I do _NOT_ trust those hardware dudes at all. A factor 4 seems to
be an appropriate choice:)

Thanks,

	tglx
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