linux-kernel - Re: [RFC] Reentrant clock sources

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Message-ID: <20081126035251.GA3707@elte.hu>
Date:	Wed, 26 Nov 2008 04:52:51 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	john stultz <johnstul@...ibm.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Magnus Damm <magnus.damm@...il.com>,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	lethal@...ux-sh.org, mingo@...hat.com
Subject: Re: [RFC] Reentrant clock sources


* john stultz <johnstul@...ibm.com> wrote:

> On Wed, 2008-11-26 at 04:07 +0100, Ingo Molnar wrote:
> > * Thomas Gleixner <tglx@...utronix.de> wrote:
> > 
> > > > +	cycle_t (*vread)(struct clocksource *cs);
> > > 
> > > This is crap. vread can not access the clocksource.
> > 
> > i think 'reentrant' in the sense of creating self-sufficient driver 
> > entities. vread wont (and shouldnt) call ->vread() recursively - but 
> > it might want to access fields on the clocksource.
> 
> I think Thomas' issue is that vread() *cannot* access fields on the 
> clocksource (since vread has to be careful not to access any 
> non-vsyscall mapped memory).

ah, yeah - i was thinking about ->read().

in a more dynamic driver model the clocksource driver could store 
dynamic data like target port/memory address next to the clocksource 
driver, and access that - if the driver pointer is passed in.

> However not all clocksources use vread(), but really I'm not quite 
> clear on what one would want to access in the clocksource structure 
> when making a ->read() call.
> 
> So maybe a further description of what specific need motivates this 
> change would be helpful? The brief description of power management 
> doesn't quite click in my head yet.

yeah, that would be nice.

	Ingo
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