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

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Message-ID: <20110201151418.GN31216@n2100.arm.linux.org.uk>
Date:	Tue, 1 Feb 2011 15:14:18 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>
Cc:	Jassi Brar <jassisinghbrar@...il.com>,
	Jeremy Kerr <jeremy.kerr@...onical.com>,
	Nicolas Pitre <nicolas.pitre@...aro.org>,
	Lorenzo Pieralisi <Lorenzo.Pieralisi@....com>,
	linux-sh@...r.kernel.org,
	Ben Herrenschmidt <benh@...nel.crashing.org>,
	Sascha Hauer <s.hauer@...gutronix.de>,
	Paul Mundt <lethal@...ux-sh.org>, linux-kernel@...r.kernel.org,
	Dima Zavin <dmitriyz@...gle.com>,
	Saravana Kannan <skannan@...eaurora.org>,
	Ben Dooks <ben-linux@...ff.org>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: Locking in the clk API, part 2: clk_prepare/clk_unprepare

On Tue, Feb 01, 2011 at 03:00:24PM +0100, Uwe Kleine-König wrote:
> On Tue, Feb 01, 2011 at 10:05:56PM +0900, Jassi Brar wrote:
> > 2011/2/1 Uwe Kleine-König <u.kleine-koenig@...gutronix.de>:
> > 
> > .....
> > 
> > > Do you plan to handle the case that clk_enable is called while prepare
> > > isn't completed (considering the special case "not called at all")?
> > > Maybe BUG_ON(clk->ops->prepare && !clk->prepare_count)?
> > Sounds better than the second option.
> > 
> > > Alternatively don't force the sleep in clk_prepare (e.g. by protecting
> > > prepare_count by a spinlock (probably enable_lock)) and call clk_prepare
> > > before calling clk->ops->enable?
> > That might result in a driver working on some platforms(those have
> > atomic clk_prepare)
> > and not on others(those have sleeping).
> The first option has the same result.  E.g. on some platforms
> clk->ops->prepare might be NULL, on others it's not.

If clk->ops->prepare is NULL, then clk_prepare() better return success
as it should mean "no preparation necessary", not "someone didn't
implement it so its an error".

Calling clk->ops->enable() with a spinlock held will ensure that no one
tries to make that method sleep, so if people want sleeping stuff they
have to use the clk_prepare() stuff.  It's a self-enforcing API which
ensures that we don't get sleeping stuff inside clk_enable().

And with a check in clk_enable() for a preparation, it helps to ensure
that drivers do call clk_prepare() before clk_enable() - though it can't
guarantee it in every case.
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