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

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Date:	Tue, 1 Feb 2011 20:32:01 +0100
From:	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Nicolas Pitre <nicolas.pitre@...aro.org>,
	Lorenzo Pieralisi <Lorenzo.Pieralisi@....com>,
	Saravana Kannan <skannan@...eaurora.org>,
	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 05:06:37PM +0000, Russell King - ARM Linux wrote:
> On Tue, Feb 01, 2011 at 04:53:44PM +0100, Uwe Kleine-König wrote:
> > On Tue, Feb 01, 2011 at 03:24:58PM +0000, Russell King - ARM Linux wrote:
> > > On Tue, Feb 01, 2011 at 04:18:46PM +0100, Uwe Kleine-König wrote:
> > > > yeah, didn't thought about multiple consumers, so (as Jeremy suggested)
> > > > the right thing is to sleep until CLK_BUSY is cleared.
> > > 
> > > A simpler way to write this is:
> > > 
> > > int clk_prepare(struct clk *clk)
> > > {
> > > 	int ret = 0;
> > > 
> > > 	mutex_lock(&clk->mutex);
> > > 	if (clk->prepared == 0)
> > > 		ret = clk->ops->prepare(clk);
> > > 	if (ret == 0)
> > > 		clk->prepared++;
> > > 	mutex_unlock(&clk->mutex);
> > > 
> > > 	return ret;
> > > }
> > But you cannot call this in atomic context when you know the clock is
> > already prepared.
> 
> So?  You're not _supposed_ to call it from any atomic context ever.
My motivation for a more complicated clk_prepare was to make clk_prepare
atomic when that's possible (i.e. when the clk is already prepared) and
call it before the enable callback in clk_enable.  Then everything
behaves nicely even if clk_enable is called from atomic context provided
that the clock was prepared before (or doesn't need to).

If a driver writer doesn't know that a certain clock might need to sleep
at some point he runs into an atomic might_sleep with your approach and
with mine.

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |
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