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

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Date:	Fri, 18 May 2007 18:52:42 +0400
From:	Sergei Shtylyov <sshtylyov@...mvista.com>
To:	Albert Cahalan <acahalan@...il.com>
Cc:	galak@...nel.crashing.org, tglx@...utronix.de,
	linuxppc-dev@...abs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2.6.21-rt2] PowerPC: decrementer clockevent driver

Hello.

Albert Cahalan wrote:

>>>>> I haven't looked at all the new clock/timer code, is there any
>>>>> utility in having support for more than one clock source?

>>>> Of course, you may register as many as you like.

>>> Sure, but is there any utility in registering more than the
>>> decrementer on PPC?

>> Not yet. I'm not sure I know any other PPC CPU facility fitting
>> for clockevents. In theory, FIT could be used -- but its period
>> is measured in powers of 2, IIRC.

> I'd really like to have that as an option. It would allow oprofile
> to safely use hardware events on the MPC74xx "G4" processors.
> Alternately it would allow thermal events. It is safe to use at
> most one of the three (decrementer,profiling,thermal) interrupts.
> If two were to hit at the same time, badness happens.

    Unfortunately, FIT exists only on Book E CPUs and MPC74xx aren't Book E, IIUC.

> It's possible to wrapper the interrupt in something that divides
> down, calling the normal code only some of the time. I think one
> of the FIT choices is about 4 kHz on my system, which would be OK.

    Erm, are you sure you have FIT (or is your system not MPC74xx based)?

> Full oprofile functionality would be wonderful.

WBR, Sergei
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