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

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Message-ID: <20110201151846.GD1147@pengutronix.de>
Date:	Tue, 1 Feb 2011 16:18:46 +0100
From:	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Jeremy Kerr <jeremy.kerr@...onical.com>,
	Dima Zavin <dmitriyz@...gle.com>,
	Saravana Kannan <skannan@...eaurora.org>,
	Lorenzo Pieralisi <Lorenzo.Pieralisi@....com>,
	linux-sh@...r.kernel.org,
	Ben Herrenschmidt <benh@...nel.crashing.org>,
	Sascha Hauer <s.hauer@...gutronix.de>,
	linux-kernel@...r.kernel.org, Paul Mundt <lethal@...ux-sh.org>,
	Ben Dooks <ben-linux@...ff.org>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	linux-arm-kernel@...ts.infradead.org,
	Nicolas Pitre <nicolas.pitre@...aro.org>
Subject: Re: Locking in the clk API, part 2: clk_prepare/clk_unprepare

On Tue, Feb 01, 2011 at 02:39:32PM +0000, Russell King - ARM Linux wrote:
> On Tue, Feb 01, 2011 at 03:18:37PM +0100, Uwe Kleine-König wrote:
> > On Tue, Feb 01, 2011 at 01:15:12PM +0000, Russell King - ARM Linux wrote:
> > > On Tue, Feb 01, 2011 at 11:54:49AM +0100, Uwe Kleine-König wrote:
> > > > Alternatively don't force the sleep in clk_prepare (e.g. by protecting
> > > > prepare_count by a spinlock (probably enable_lock)) and call clk_prepare
> > > > before calling clk->ops->enable?
> > > 
> > > That's a completely bad idea.  I assume you haven't thought about this
> > > very much.
> > Right, but I thought it a bit further than you did.  Like the following:
> >  
> > int clk_prepare(struct clk *clk)
> > {
> > 	int ret = 0, first;
> > 	unsigned long flags;
> > 
> > 	spin_lock_irqsave(&clk->enable_lock, flags);
> > 	if (clk->flags & CLK_BUSY) {
> > 		/* 
> > 		 * this must not happen, please serialize calls to
> > 		 * clk_prepare/clk_enable
> > 		 */
> 
> How do different drivers serialize calls to clk_prepare?  Are you
> really suggesting that we should have a global mutex somewhere to
> prevent this?
yeah, didn't thought about multiple consumers, so (as Jeremy suggested)
the right thing is to sleep until CLK_BUSY is cleared.

Best regards
Uwe

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