linux-kernel - Re: [PATCH -v1.1] HPET: Fix HPET readout for small deltas

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Date:	Tue, 30 Nov 2010 06:32:01 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Borislav Petkov <bp@...64.org>,
	"a.p.zijlstra@...llo.nl" <a.p.zijlstra@...llo.nl>,
	john stultz <johnstul@...ibm.com>,
	"Herrmann3, Andreas" <Andreas.Herrmann3@....com>,
	"mtosatti@...hat.com" <mtosatti@...hat.com>,
	"heiko.carstens@...ibm.com" <heiko.carstens@...ibm.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"avi@...hat.com" <avi@...hat.com>, Ingo Molnar <mingo@...e.hu>,
	"hpa@...ux.intel.com" <hpa@...ux.intel.com>,
	Markus Trippelsdorf <markus@...ppelsdorf.de>
Subject: Re: [PATCH -v1.1] HPET: Fix HPET readout for small deltas

On Mon, Nov 29, 2010 at 11:36:05PM +0100, Thomas Gleixner wrote:
> On Mon, 29 Nov 2010, Borislav Petkov wrote:
> >  
> > +	/* Accept only sensible values written by BIOS */
> > +	if (hpet_tbl->minimum_tick < hpet_min_tick)
> > +		hpet_min_tick = hpet_tbl->minimum_tick;
> 
> I ran this through everything I could get hold of. And as I feared
> when ACPI was mentioned in the first place, I found at least three
> machines which have hpet_tbl->minimum_tick < 4. Two of those failed to
> boot.

Damn, that's just f*cked up.

> This is all doomed to fail. 
> 
> 1) ACPI's trustworthiness aproaches ZERO
> 
> 2) The chipset manufactures who implement the "HPET spec" are even
>    worse. They could tell us the exact number of cycles which are
>    necessary to make these "specificatin compliant" trainwrecks
>    functional, but all we get is a reference to #1.

... and also, there's the notion of different HPET implementations from
the different chipset vendors hiding under the compliancy blanket. I
could venture a wild guess which shouldn't be far from the truth that
the "exact number of cycles" is different for each implementation.

> Crap. If you don't come up with some real facts,

Sorry, this is all I got ;-( - I'm sure you can imagine why...

> I'm simply going to commit
> 
> diff --git a/arch/x86/kernel/hpet.c b/arch/x86/kernel/hpet.c
> index ae03cab..0388a70 100644
> --- a/arch/x86/kernel/hpet.c
> +++ b/arch/x86/kernel/hpet.c
> @@ -408,7 +408,7 @@ static int hpet_next_event(unsigned long delta,
>  	 */
>  	res = (s32)(cnt - hpet_readl(HPET_COUNTER));
>  
> -	return res < 8 ? -ETIME : 0;
> +	return res < 128 ? -ETIME : 0;
>  }
>  
>  static void hpet_legacy_set_mode(enum clock_event_mode mode,
> 
> along with the corresponding fix for the min_delta_ns value.

I'm afraid this is our only option right now.

-- 
Regards/Gruss,
    Boris.
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