linux-kernel - Re: clockevent questions

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Message-ID: <38b2ab8a0705121313k37be467cy55402b490101ac9c@mail.gmail.com>
Date:	Sat, 12 May 2007 22:13:43 +0200
From:	"Francis Moreau" <francis.moro@...il.com>
To:	tglx@...utronix.de
Cc:	linux-kernel@...r.kernel.org
Subject: Re: clockevent questions

Hi Thomas,

Thanks for answering so quickly !

On 5/12/07, Thomas Gleixner <tglx@...utronix.de> wrote:
> Francis,
>
> On Sat, 2007-05-12 at 16:54 +0200, Francis Moreau wrote:
> > I'm trying to use clocksource and clockevent new subsystem. My
> > platform has a timer that I'd like to use both as a clocksource and a
> > clockevent devices.
>
> See arch/i386/kernel/hpet.c
>

thanks for the pointer

> > This timer is continueous in sense that it can run
> > without any interruption
>
> -ENOPARSE
>
> Has your timer a free running counter and a match register based event
> mechanism ?
>

yes

> > so I assume I can flag the clocksource device
> > with "CLOCK_SOURCE_IS_CONTINUOUS". However I noticed that clockevent
> > device can be stopped by using "set_mode()" method. Are these two
> > behaviours compatible ?
>
> the clock event stop only stops the event mechanism, it does not stop
> the counter.
>
> See arch/i386/kernel/hpet.c
>

OK. I got it now, In my initial plan, I was thinking to stop the
counter to stop the event. But that's not the right thing to do. It
seems that I should disable event interrupt instead.

> > Another question is that I have another embedded 16 bits timer that I
> > would like to use. Since the timer is only 16 bits, the maximum
> > interval is tiny, My question if a user ask for a clockevent device
> > using an interval is bigger that 2^16, the clockevent system doesn't
> > return an error. Instead it silently reduce the interval to 2^16. Is
> > this correct ? if so why ?
>
> Yes, it is correct. The generic timer code requests an event in the
> future. It does not care, whether the hardware device can handle that or
> not. So the clock event code limits the delta to the maximum delta the
> device can handle. The interrupt fires and the generic timer code
> reschedules the event with the remaining delta time.
>

Thanks again for explanations. Could you give me a pointer of this reschedules ?

-- 
Francis
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