Message-ID: <4D486E08.9000709@codeaurora.org>
Date: Tue, 01 Feb 2011 12:33:12 -0800
From: Saravana Kannan <skannan@codeaurora.org>
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To: Russell King - ARM Linux <linux@arm.linux.org.uk>
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	<u.kleine-koenig@pengutronix.de>,
        Nicolas Pitre <nicolas.pitre@linaro.org>,
        Lorenzo Pieralisi <Lorenzo.Pieralisi@arm.com>,
        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
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>
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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

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