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

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Message-ID: <4D486E08.9000709@codeaurora.org>
Date:	Tue, 01 Feb 2011 12:33:12 -0800
From:	Saravana Kannan <skannan@...eaurora.org>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
CC:	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>,
	Nicolas Pitre <nicolas.pitre@...aro.org>,
	Lorenzo Pieralisi <Lorenzo.Pieralisi@....com>,
	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 02/01/2011 07:24 AM, Russell King - ARM Linux wrote:
> 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;
> }
>
> I think we want to take a common mutex not only for clk_prepare(), but
> also for clk_set_rate().  If prepare() is waiting for a PLL to lock,
> we don't want a set_rate() interfering with that.

Looks like this is the best acknowledgment/response I can expect to get 
from Russell on this point that I raised.

Jeremy,

When you update the comments/doc to indicate clk_prepare/unprepare is 
not atomic, can you also update the comment for set_rate() and mark it 
as non-atomic?

Thanks for starting this thread. My efforts to reignite the other thread 
didn't go anywhere. Glad to see it's moving forward.

> I'd also be tempted at this stage to build-in a no-op dummy clock,
> that being the NULL clk:
>[snip]
> as we have various platforms defining a dummy struct clk as a way of
> satisfying various driver requirements.  These dummy clocks are exactly
> that - they're complete no-ops.

Unrelated to this thread, but I Ack this request too.

-Saravana

-- 
Sent by an employee of the Qualcomm Innovation Center, Inc.
The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum.
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