linux-kernel - Re: [PATCH] hpet: hpet driver periodic timer setup bug fixes

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Date:	Tue, 11 Aug 2009 11:07:42 -0700
From:	"Pallipadi, Venkatesh" <venkatesh.pallipadi@...el.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Nils Carlson <nils.carlson@...csson.com>,
	David Brownell <david-b@...bell.net>
Subject: Re: [PATCH] hpet: hpet driver periodic timer setup bug fixes

On Mon, 2009-08-10 at 21:03 -0700, Andrew Morton wrote:
> On Mon, 10 Aug 2009 15:05:04 -0700 "Pallipadi, Venkatesh" <venkatesh.pallipadi@...el.com> wrote:
> 
> > From: Nils Carlson <nils.carlson@...csson.com>
> > 
> > The periodic interrupt from drivers/char/hpet.c does not work correctly,
> > both when using the periodic capability of the hardware and while
> > emulating the periodic interrupt (when hardware does not support
> > periodic mode).
> > 
> > With timers capable of periodic interrupts, the comparator field is first
> > set with the period value followed by set of hidden accumulator,
> > which has the side effect of overwriting the comparator value. This
> > results in wrong periodicity for the interrupts. For,
> > periodic interrupts to work, following steps are necessary, in that order.
> > * Set config with Tn_VAL_SET_CNF bit
> > * Write to hidden accumulator, the value written is the time when the
> >   first interrupt should be generated
> > * Write compartor with period interval for subsequent interrupts
> > (http://www.intel.com/hardwaredesign/hpetspec_1.pdf )
> > 
> > When emulating periodic timer with timers not capable of periodic
> > interrupt, driver is adding the period to counter value instead of
> > comparator value, which causes slow drift when using this emulation.
> > 
> > Also, driver seems to add hpetp->hp_delta both while setting up
> > periodic interrupt and while emulating periodic interrupts with timers
> > not capable of doing periodic interrupts. This hp_delta will result in
> > slower than expected interrupt rate and should not be used while setting
> > the interval.
> > 
> 
> Do you have any thoughts on which kernel versions this fix should be
> merged into, and when?
> 

Should be OK let this bake for a while and merge this in after .31.

Thanks,
Venki

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