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    SubjectRe: [PATCH] hpet: hpet driver periodic timer setup bug fixes
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    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@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?
    >

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

    Thanks,
    Venki



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