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

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Message-ID: <464DADFC.3080802@ru.mvista.com>
Date:	Fri, 18 May 2007 17:45:32 +0400
From:	Sergei Shtylyov <sshtylyov@...mvista.com>
To:	Matt Sealey <matt@...esi-usa.com>
Cc:	Kumar Gala <galak@...nel.crashing.org>, 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.

Matt Sealey 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.

> Wouldn't it be possible to use SoC timer functionality, too, and not limit
> this to what is only present on the CPU die?

    If you have some SoC timers, you're welcome to add support for it. :-)

> I guess the real question is, how high resolution does a high resolution
> timer need to be,

    In the order of microseconds.

 > and how often do you want to be handling interrupts?

    That depends on a kind of functionality you need: HRT and/or dynamic tick...

> The 52XX has 8 general purpose timers and 2 slice timers. Both will
> generate an interrupt once the clock hits a preset value, the timing
> of which is determined by prescalers and the IPB clock..

WBR, Sergei
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