linux-kernel - Re: Locking in the clk API

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Message-Id: <201101111211.30172.jeremy.kerr@canonical.com>
Date:	Tue, 11 Jan 2011 12:11:29 +0800
From:	Jeremy Kerr <jeremy.kerr@...onical.com>
To:	Paul Mundt <lethal@...ux-sh.org>
Cc:	linux-sh@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org,
	Ben Herrenschmidt <benh@...nel.crashing.org>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	"Uwe Kleine-K??nig" <u.kleine-koenig@...gutronix.de>,
	Sascha Hauer <s.hauer@...gutronix.de>,
	Lorenzo Pieralisi <Lorenzo.Pieralisi@....com>
Subject: Re: Locking in the clk API

Hi Paul,

> This looks like a complete disaster, and is also completely inconsistent
> with how the API is being used by the vast majority of users today.

I've been basing this on the mxc clock code, which acquires a mutex for all 
clk_enable()s. This may not be representative of the majority of clock usage.

>From a quick search there are a few other cases of non-atomic clock usage:

tcc:		clk_enable() acquires a global clocks_mutex
tegra:	has a clk_enable_cansleep()
davinci: clk_set_parent() aquires a global clocks_mutex

Excluding the davinci code (we won't worry about set_parent for now...), if we 
can port mxc and tcc to a sleepable clk_enable, perhaps we could just go with 
purely atomic operations.

We'd still need some method of using sleeping clocks though. How about making 
clk_enable() BUG if the clock is not atomic, and add clk_enable_cansleep() for 
the cases where clk->ops.enable may sleep.

Do we need something similar for other parts of the API? (clk_set_rate?)

Cheers,

Jeremy
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