Date: Tue, 1 Feb 2011 20:32:01 +0100
From: Uwe =?iso-8859-1?Q?Kleine-K=F6nig?= 
	<u.kleine-koenig@pengutronix.de>
To: Russell King - ARM Linux <linux@arm.linux.org.uk>
Cc: Nicolas Pitre <nicolas.pitre@linaro.org>,
        Lorenzo Pieralisi <Lorenzo.Pieralisi@arm.com>,
        Saravana Kannan <skannan@codeaurora.org>, linux-sh@vger.kernel.org,
        Ben Herrenschmidt <benh@kernel.crashing.org>,
        Sascha Hauer <s.hauer@pengutronix.de>,
        Paul Mundt <lethal@linux-sh.org>, linux-kernel@vger.kernel.org,
        Dima Zavin <dmitriyz@google.com>, Ben Dooks <ben-linux@fluff.org>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Jeremy Kerr <jeremy.kerr@canonical.com>,
        linux-arm-kernel@lists.infradead.org
Subject: Re: Locking in the clk API, part 2: clk_prepare/clk_unprepare
Message-ID: <20110201193201.GH1147@pengutronix.de>
References: <201102011711.31258.jeremy.kerr@canonical.com>
 <20110201105449.GY1147@pengutronix.de>
 <20110201131512.GH31216@n2100.arm.linux.org.uk>
 <20110201141837.GA1147@pengutronix.de>
 <20110201143932.GK31216@n2100.arm.linux.org.uk>
 <20110201151846.GD1147@pengutronix.de>
 <20110201152458.GP31216@n2100.arm.linux.org.uk>
 <20110201155344.GF1147@pengutronix.de>
 <20110201170637.GR31216@n2100.arm.linux.org.uk>
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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-K?nig 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-K?nig 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-K?nig            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |
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