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Date: Fri, 18 May 2007 17:48:34 +0400
From: Sergei Shtylyov <sshtylyov@ru.mvista.com>
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To: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Subject: Re: [PATCH 2.6.21-rt2] PowerPC: decrementer clockevent driver
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Cc: linuxppc-dev@ozlabs.org, tglx@linutronix.de,
	Dave Liu <r63238@freescale.com>, mingo@elte.hu,
	linux-kernel@vger.kernel.org
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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