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From: Segher Boessenkool <segher@kernel.crashing.org>
Subject: Re: [PATCH 2.6.21-rt2] PowerPC: decrementer clockevent driver
Date: Thu, 17 May 2007 22:53:49 +0200
To: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Cc: linuxppc-dev@ozlabs.org, tglx@linutronix.de, mingo@elte.hu,
	linux-kernel@vger.kernel.org
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>>> Otherwise, when the contents of DEC0 change, the exception effects 
>>> of the Decrementer become consistent with the new contents of the 
>>> Decrementer reasonably soon after the change.
>
>> And that is guaranteed on all PowerPC as far as I can see.
>> The main thing is that a decrementer exception won't go
>> away until the high bit becomes 0.
>
> On both POWER4 and POWER4+, the Decrementer must be implemented such 
> that requirements 1 to 3 below are satisfied. On POWER4, requirements 
> 4 and 5 must also be satisfied.

<snip>

> 4. Whenever bit 0 of the Decrementer changes from 0 to 1, an interrupt 
> request is signaled. If multiple Decrementer interrupt requests are 
> received before the first can be reported, only one interrupt is 
> reported. The occurrence of a Decrementer interrupt cancels the 
> request.
>
> 5. If the Decrementer is altered by software and the contents of bit 0 
> are changed from 0 to 1, an interrupt request is signaled.

 From the POWER ISA 2.03, the latest public version of the
architecture definition:

	When the contents of DEC32 change from 0 to 1, a Decrementer
	exception will come into existence within a reasonable period
	or time. When the contents of DEC32 change from 1 to 0, an
	existing Decrementer exception will cease to exist within a
	reasonable period of time, but not later than the completion
	of the next context synchronizing instruction or event.

>    (4) clearly contradicts your point.

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.

> I don't mind changing #ifdef though (so it'll cover all non Book E 
> cases)

That was exactly my point; thank you.


Segher