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

From: Benjamin Herrenschmidt <[email-masked]>
Date: Fri, 18 May 2007 17:08:15 +1000

On Fri, 2007-05-18 at 13:39 +0800, Dave Liu 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).

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
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.

Ben.