linux-kernel - Re: Locking in the clk API, part 2: clk_prepare/clk_unprepare

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Date:	Fri, 4 Feb 2011 17:33:32 +0800
From:	Richard Zhao <linuxzsc@...il.com>
To:	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>
Cc:	Saravana Kannan <skannan@...eaurora.org>,
	Nicolas Pitre <nicolas.pitre@...aro.org>,
	Lorenzo Pieralisi <Lorenzo.Pieralisi@....com>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	linux-sh@...r.kernel.org,
	Ben Herrenschmidt <benh@...nel.crashing.org>,
	Sascha Hauer <s.hauer@...gutronix.de>,
	Paul Mundt <lethal@...ux-sh.org>, linux-kernel@...r.kernel.org,
	Dima Zavin <dmitriyz@...gle.com>,
	Ben Dooks <ben-linux@...ff.org>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Jeremy Kerr <jeremy.kerr@...onical.com>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: Locking in the clk API, part 2: clk_prepare/clk_unprepare

On Tue, Feb 01, 2011 at 09:43:31PM +0100, Uwe Kleine-König wrote:
> Hello,
> 
> On Tue, Feb 01, 2011 at 12:21:45PM -0800, Saravana Kannan wrote:
> > If the driver is calling clk_prepare() right next to clk_enable()
> > knowing it's been already prepared and will hence be "atomic" (this
> > is actually not true), then by your description, it's pointless to
> > call clk_prepare().
> Well not completely, as it increases the reference count.  The advantage
> would be that clk_enable counts addionally as prepare, so it would be
> impossible to unprepare an enabled clock.  And the other way round an
> unprepared clock would never be enabled.
>  
> > If you want the driver to call clk_prepare() in atomic context
> > because it will be atomic in most cases -- well, that's wrong. It's
> > either atomic or is NOT atomic. There is no in between. If a call is
> > NOT atomic, it can't be called in atomic context. Long story short,
> > if you expect clk_prepare() to be atomic under any circumstance, it
> > beats the point of introducing clk_prepare().
> Well, with my suggestion it's atomic when certain precondions are given.
> IMHO that's better than "atomic in most cases" because the caller can
> assert that everything goes smooth.
> These preconditions are asserted when the driver writer is careful
> enough to stick to the API.
IMHO, clk_prepare is always called in non-atomic context, so it doesn't matter
whether it's really atomic or not. We don't have to make it as atomic as
possible.

Thanks
Richard
> 
> Either my idea is bad or I'm unable to sell it appropriately.  Be it as
> it is, I will stop to make a case for it.
> 
> Best regards and thanks for your try,
> Uwe
> 
> -- 
> Pengutronix e.K.                           | Uwe Kleine-König            |
> Industrial Linux Solutions                 | http://www.pengutronix.de/  |
> 
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