linux-kernel - Re: Locking in the clk API

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Message-Id: <201101111744.59712.jeremy.kerr@canonical.com>
Date:	Tue, 11 Jan 2011 17:44:59 +0800
From:	Jeremy Kerr <jeremy.kerr@...onical.com>
To:	"Russell King - ARM Linux" <linux@....linux.org.uk>
Cc:	linux-sh@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org,
	Ben Herrenschmidt <benh@...nel.crashing.org>,
	Sascha Hauer <s.hauer@...gutronix.de>,
	Lorenzo Pieralisi <Lorenzo.Pieralisi@....com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	"Uwe Kleine-König" 
	<u.kleine-koenig@...gutronix.de>
Subject: Re: Locking in the clk API

Hi Russell,

> I object to this as one of the purposes behind the clk API is to allow
> power savings to be made, and unless we can perform clk enable/disable
> from atomic contexts, the best you can do is enable the clock when the
> device is probed and disable it when it's released.
> 
> [...]
>
> Sometimes the only point that you know you need the clock enabled is when
> your driver has already been called in an atomic context.

.. provided that the enable (and subsequent things that depend on the clock 
signal to be valid) can't be deferred; I'm not sure how often this will be 
possible.

So, it sounds like the best approach is to provide an atomic clk_enable. I 
agree with Sascha that the clk_enable and clk_enable_atomic polarity makes the 
most sense, so how about:

int clk_enable(struct clk *clk)
{
	might_sleep();

	[...]
}

int clk_enable_atomic(struct clk *clk)
{
	BUG_ON(!(clk->flags & CLK_ATOMIC));

	[...]
}

Paul: even though you mention that the atomic clocks are the usual case, I 
think that this way around illustrates the atomic 'restriction' at the call 
site more clearly. When the drivers don't care about the atomicity, 
clk_enable() is fine. When drivers do need an atomic clock, 
clk_enable_atomic() shows this requirement.

Cheers,

Jeremy
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