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

[Sergei Shtylyov]

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
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