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

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Message-Id: <201102012240.38100.jeremy.kerr@canonical.com>
Date:	Tue, 1 Feb 2011 22:40:37 +0800
From:	Jeremy Kerr <jeremy.kerr@...onical.com>
To:	"Uwe Kleine-König" 
	<u.kleine-koenig@...gutronix.de>
Cc:	"Russell King - ARM Linux" <linux@....linux.org.uk>,
	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

Hi Uwe,

Thanks for the feedback, I'm not sure I like the more complex approach though:

> 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
> 		 */
> 		ret = -EBUSY;
> 		goto out_unlock;

Why is this an error? Two separate drivers may be clk_prepare()-ing at the 
same time, which should be acceptable. Both calls should block until the 
prepare is complete.

> 	}
> 	first = clk->prepare_count++ == 0;
> 	if (first)
> 		clk->flags |= CLK_BUSY;
> 	spin_unlock_irqrestore(&clk->enable_lock, flags);
> 
> 	if (!first)
> 		return 0;
> 
> 	if (clk->ops->prepare) {
> 		might_sleep();
> 		ret = clk->ops->prepare(clk);
> 	}
> 
> 	spin_lock_irqsave(&clk->enable_lock, flags);
> 	clk->flags &= ~CLK_BUSY;
> 	if (ret)
> 		clk->prepare_count--;
> out_unlock:
> 	spin_unlock_irqrestore(&clk->enable_lock, flags);
> 
> 	return ret;
> }

Cheers,


Jeremy

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