Date: Tue, 1 Feb 2011 15:24:58 +0000
From: Russell King - ARM Linux <linux@arm.linux.org.uk>
To: Uwe =?iso-8859-1?Q?Kleine-K=F6nig?= 
	<u.kleine-koenig@pengutronix.de>
Cc: Jeremy Kerr <jeremy.kerr@canonical.com>, Dima Zavin <dmitriyz@google.com>,
        Saravana Kannan <skannan@codeaurora.org>,
        Lorenzo Pieralisi <Lorenzo.Pieralisi@arm.com>,
        linux-sh@vger.kernel.org, Ben Herrenschmidt <benh@kernel.crashing.org>,
        Sascha Hauer <s.hauer@pengutronix.de>, linux-kernel@vger.kernel.org,
        Paul Mundt <lethal@linux-sh.org>, Ben Dooks <ben-linux@fluff.org>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        linux-arm-kernel@lists.infradead.org,
        Nicolas Pitre <nicolas.pitre@linaro.org>
Subject: Re: Locking in the clk API, part 2: clk_prepare/clk_unprepare
Message-ID: <20110201152458.GP31216@n2100.arm.linux.org.uk>
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>
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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;
}

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.

I'd also be tempted at this stage to build-in a no-op dummy clock,
that being the NULL clk:

int clk_prepare(struct clk *clk)
{
	int ret = 0;

	if (clk) {
		mutex_lock(&clk->mutex);
		if (clk->prepared == 0)
			ret = clk->ops->prepare(clk);
		if (ret == 0)
			clk->prepared++;
		mutex_unlock(&clk->mutex);
	}

	return ret;
}

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.
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