Subject: Re: [PATCH 2.6.21-rt2] PowerPC: decrementer clockevent driver

From: Segher Boessenkool <[email-masked]>
Date: Thu, 17 May 2007 21:50:52 +0200

>>> +     * We must write a positive value to the decrementer to clear
>>> +     * the interrupt on the IBM 970 CPU series.  In periodic mode,
>>> +     * this happens when the decrementer gets reloaded later, but
>>> +     * in one-shot mode, we have to do it here since an event 
>>> handler
>>> +     * may skip loading the new value...
>
>> Nothing special about 970 here -- on *every* PowerPC,
>> a decrementer exception exists as long as the high
>> bit of the decrementer equals 1.  BookE is different
>> of course.  Some other CPUs might deviate from the
>> architecture as well.
>
>    Quoting "PowerPC Operating Environment Architecture":

<snip>

> Otherwise, when the contents of DEC0 change, the exception effects of 
> the Decrementer become consistent with the new contents of the 
> Decrementer reasonably soon after the change.

And that is guaranteed on all PowerPC as far as I can see.
The main thing is that a decrementer exception won't go
away until the high bit becomes 0.

Segher