linux-kernel - Re: Locking in the clk API

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Message-ID: <20110121220238.GE23151@n2100.arm.linux.org.uk>
Date:	Fri, 21 Jan 2011 22:02:38 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Nicolas Pitre <nicolas.pitre@...aro.org>
Cc:	Dima Zavin <dmitriyz@...gle.com>,
	Saravana Kannan <skannan@...eaurora.org>,
	Lorenzo Pieralisi <Lorenzo.Pieralisi@....com>,
	linux-sh@...r.kernel.org,
	Ben Herrenschmidt <benh@...nel.crashing.org>,
	Sascha Hauer <s.hauer@...gutronix.de>,
	linux-kernel@...r.kernel.org, Paul Mundt <lethal@...ux-sh.org>,
	Ben Dooks <ben-linux@...ff.org>,
	Uwe Kleine-K??nig <u.kleine-koenig@...gutronix.de>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Jeremy Kerr <jeremy.kerr@...onical.com>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: Locking in the clk API

On Fri, Jan 21, 2011 at 04:53:44PM -0500, Nicolas Pitre wrote:
> So I think that the API must be augmented with more methods, such as:
> 
> clk_slow_enable():
>   - may sleep
>   - may be a no-op if the clk_fast_enable() is supported
> 
> clk_fast_enable():
>   - may not sleep, used in atomic context
>   - may be a no-op if controlling the clock takes time, in which case 
>     clk_slow_enable() must have set the clock up entirely
> 
> ... and similar for clk_slow_disable() and clk_fast_disable().

Isn't this along the same lines as my clk_prepare() vs clk_enable()
suggestion?

I suggested that clk_prepare() be callable only from non-atomic contexts,
and do whatever's required to ensure that the clock is available.  That
may end up enabling the clock as a result.

clk_enable() callable from atomic contexts, and turns the clock on if
the hardware supports such an operation.

So, if you have something like:

Xtal--->PLL--->Routing/Masking--->Device

clk = clk_get() returns the clock for the device.

clk_prepare(clk) would walk up the clock tree, selecting the routing and
preparing each clock.  Clocks prior to _and_ including the PLL would need
to be enabled.

clk_enable(clk) would walk up the tree if the clock isn't already enabled,
calling clk_enable() on the parent clock.  As we require prepared clocks
to already be enabled, this automatically stops at the PLL.

To encourage correct usage, we just need to make sure that clk_prepare()
has a might_sleep() thing, and clk_enable() throws a fit if it's used
on a clk without prepare being used first.  The second point is not easy
to do in a foolproof manner though, but doing _something_ is better than
nothing.
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