Segher Boessenkool wrote:

quoted

quoted

quoted

+     * 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...

quoted

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>

quoted

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