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

- From: Sergei Shtylyov \<hidden\>
- Date: 2007-05-18 13:47:13 
- List: linuxppc-dev
- Page: https://ratatoskr.run/linuxppc-dev/2007/05/957488
- Thread: https://ratatoskr.run/lkml/2007/05/957088/t.md

```text
Hello, I wrote:

>>>>>Yes, on some implementations there can be other conditions that
>>>>>make a decrementer exception go away; there is no contradiction
>>>>>here (thankfully).  My wording was sloppy.

>>>>Some CPUs have the DEC exceptions basically edge triggered (yeah I know

>>>for example?

>>>>it sucks). That's why, among others, the IRQ soft-disable code has code
>>>>to re-trigger DEC exceptions ASAP (by setting it to 1.. note that we
>>>>could probably use 0 here, we've been a bit conservative).

>     Yeah, the classic decrementer is programmed off-by-one.

>>I'm not 100% certain... Paulus thinks all the old 6xx are like that, and
>>maybe POWER4. If I look at the oldest BookIV I can find (the 601), it

>     From the "PowerPC Operating Environment Architecture" that I've already quoated t follows that POWER4-compatible decremented exception *must* be edge triggered.

   ... and cleared when delivered.

>>says that an exception is generated when the MSB transitions from 0 to
>>1. It's not clear wether the exception sticks while that bit is 1 or is
>>indeed considered as an "edge" event that gets cleared as soon as
>>delivered.

WBR, Sergei
```