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

[Thomas Gleixner]

On Wed, 15 Sep 2010, Borislav Petkov wrote:

> From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> 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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