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SubjectRe: [PATCH 2.6.21-rt2] PowerPC: decrementer clockevent driver
Segher Boessenkool wrote:
>>>> + * 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.

On both POWER4 and POWER4+, the Decrementer must be implemented such that
requirements 1 to 3 below are satisfied. On POWER4, requirements 4 and 5 must
also be satisfied.

1. The operation of the Time Base and the Decrementer is coherent, i.e., the
counters are driven by the same fundamental time base.
2. Loading a GPR from the Decrementer shall have no effect on the accuracy of
the Decrementer.

3. Storing a GPR to the Decrementer shall replace the value in the Decrementer
with the value in the GPR.

4. Whenever bit 0 of the Decrementer changes from 0 to 1, an interrupt request
is signaled. If multiple Decrementer interrupt requests are received before
the first can be reported, only one interrupt is reported. The occurrence of a
Decrementer interrupt cancels the request.

5. If the Decrementer is altered by software and the contents of bit 0 are
changed from 0 to 1, an interrupt request is signaled.

(4) clearly contradicts your point. I don't mind changing #ifdef though
(so it'll cover all non Book E cases)

> Segher

WBR, Sergei
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