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

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Message-ID: <20110204111841.GG14627@n2100.arm.linux.org.uk>
Date:	Fri, 4 Feb 2011 11:18:41 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Jassi Brar <jassisinghbrar@...il.com>
Cc:	Richard Zhao <linuxzsc@...il.com>,
	Nicolas Pitre <nicolas.pitre@...aro.org>,
	Lorenzo Pieralisi <Lorenzo.Pieralisi@....com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	linux-sh@...r.kernel.org,
	Ben Herrenschmidt <benh@...nel.crashing.org>,
	Sascha Hauer <s.hauer@...gutronix.de>,
	Paul Mundt <lethal@...ux-sh.org>,
	Stephen Boyd <sboyd@...eaurora.org>,
	linux-kernel@...r.kernel.org, Dima Zavin <dmitriyz@...gle.com>,
	Saravana Kannan <skannan@...eaurora.org>,
	Ben Dooks <ben-linux@...ff.org>,
	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>,
	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 Fri, Feb 04, 2011 at 08:04:03PM +0900, Jassi Brar wrote:
> On Fri, Feb 4, 2011 at 7:48 PM, Russell King - ARM Linux
> <linux@....linux.org.uk> wrote:
> 
> > int clk_enable(struct clk *clk)
> > {
> >        unsigned long flags;
> >        int ret = 0;
> >
> >        if (clk) {
> >                if (WARN_ON(!clk->prepare_count))
> >                        return -EINVAL;
> >
> >                spin_lock_irqsave(&clk->lock, flags);
> >                if (clk->enable_count++ == 0)
> >                        ret = clk->ops->enable(clk);
> >                spin_unlock_irqrestore(&clk->lock, flags);
> >        }
> >        return ret;
> > }
> >
> > is entirely sufficient to catch the case of a single-use clock not being
> > prepared before clk_enable() is called.
> >
> > We're after detecting drivers missing calls to clk_prepare(), we're not
> > after detecting concurrent calls to clk_prepare()/clk_unprepare().
> 
> I hope you mean 'making sure the clock is prepared before it's enabled
> ' rather than
> 'catching a driver that doesn't do clk_prepare before clk_enable'.
> Because, the above implementation still doesn't catch a driver that
> doesn't call clk_prepare
> but simply uses a clock that happens to have been already prepare'd by
> some other
> driver or the platform.

No, I mean what I said.

The only way to do what you're asking is to attach a list of identifiers
which have prepared a clock to the struct clk, where each identifier is
unique to each driver instance.

So what that becomes is:

struct prepared_instance {
	struct list_head node;
	void *driver_id;
};

int clk_prepare(struct clk *clk, void *driver_id)
{
	struct prepared_instance *inst;
	int ret = 0;

	if (clk) {
		inst = kmalloc(sizeof(*inst), GFP_KERNEL);
		if (!inst)
			return -ENOMEM;

		inst->driver_id = driver_id;

		mutex_lock(&clk->mutex);
		if (clk->prepare_count++ == 0)
			ret = clk->ops->prepare(clk);

		if (ret == 0) {
			spin_lock_irqsave(&clk->lock, flags);
			list_add(&inst->node, &clk->prepare_list);
			spin_unlock_irqrestore(&clk->lock, flags);
		} else
			clk->prepare_count--;
		mutex_unlock(&clk->mutex);
	}
	return ret;
}

int clk_enable(struct clk *clk, void *driver_id)
{
	unsigned long flags;
	int ret = 0;

	if (clk) {
		struct prepare_instance *inst;

		spin_lock_irqsave(&clk->lock, flags);
		list_for_each_entry(inst, &clk->prepare_list, node)
			if (inst == driver_id)
				ret = -EINVAL;

		if (ret == 0 && clk->enable_count++ == 0) {
			ret = clk->ops->enable(clk);
			if (ret)
				clk->enable_count--;
		}
		spin_unlock_irqrestore(&clk->lock, flags);
	}
	return ret;
}

I think that's going completely over the top, and adds needless complexity
to drivers, which now have to pass an instance specific cookie into every
clk API call.
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