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

Benjamin Herrenschmidt 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

> 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

Freescale MPC 7450 manual says the same, for example.

> indeed considered as an "edge" event that gets cleared as soon as
> delivered.

> Ben.

WBR, Sergei
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