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

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Date:	Fri, 4 Feb 2011 20:51:15 +0900
From:	Jassi Brar <jassisinghbrar@...il.com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
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 4, 2011 at 8:18 PM, Russell King - ARM Linux
<linux@....linux.org.uk> wrote:
> 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.
Then, how does that function catch a driver that, say, doesn't do clk_prepare
but share the clk with another already active driver?
Because you said - "We're after detecting drivers missing calls to
clk_prepare()"

The point is, there is difference between detecting drivers that miss
the clk_prepare
and ensuring clk_prepare has been called before any call to
clk_enable. And making
that clear helps get rid of lots of confusion/misunderstanding. Uwe
seems to have
had similar confusions.


> 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.
I am not asking what you think.
In my second last post, I am rather asking the other way around - that
let us not worry
about drivers missing the clk_prepare and not try to catch those by the new API.


> 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.
Exactly.
All we need is to ensure clk_prepare has been called atleast once before
any call to clk_enable.
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