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

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Date:	Fri, 18 May 2007 17:41:26 +0400
From:	Sergei Shtylyov <sshtylyov@...mvista.com>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	Dave Liu <r63238@...escale.com>, linuxppc-dev@...abs.org,
	tglx@...utronix.de, mingo@...e.hu, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2.6.21-rt2] PowerPC: decrementer clockevent driver

Hello.

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 
triggered.

> 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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