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Date: Mon, 10 Aug 2009 21:03:12 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: "Pallipadi, Venkatesh" <venkatesh.pallipadi@intel.com>
Cc: linux-kernel@vger.kernel.org, Nils Carlson <nils.carlson@ericsson.com>,
       David Brownell <david-b@pacbell.net>
Subject: Re: [PATCH] hpet: hpet driver periodic timer setup bug fixes
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On Mon, 10 Aug 2009 15:05:04 -0700 "Pallipadi, Venkatesh" <venkatesh.pallipadi@intel.com> wrote:

> From: Nils Carlson <nils.carlson@ericsson.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?

Thanks.