linux-kernel - Re: [RFC, PATCH 3/3] clk: add warnings for incorrect enable/prepare semantics

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Message-ID: <alpine.LFD.2.00.1102070908120.14920@xanadu.home>
Date:	Mon, 7 Feb 2011 09:24:58 -0500 (EST)
From:	Nicolas Pitre <nicolas.pitre@...aro.org>
To:	Jeremy Kerr <jeremy.kerr@...onical.com>
cc:	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.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>,
	Dima Zavin <dmitriyz@...gle.com>,
	Saravana Kannan <skannan@...eaurora.org>,
	Ben Dooks <ben-linux@...ff.org>,
	Russell King <linux@....linux.org.uk>
Subject: Re: [RFC, PATCH 3/3] clk: add warnings for incorrect enable/prepare
 semantics

On Mon, 7 Feb 2011, Jeremy Kerr wrote:

> Hi Uwe,
> 
> > This implies the warning is only issued on clocks that have a prepare
> > callback.  If we want to enforce the new API the warning here shouldn't
> > depend on clk->ops->prepare.  (clk_prepare and clk_unprepare need to
> > be changed then to adapt the prepare_count even in the absence of
> > clk->ops->prepare.)
> 
> Yeah, it's a decision about either adding a small cost to all clk_prepare()s 
> (ie, adding cost when there is no prepare callback), or checking for the 
> correct prepare/enable semantics for all clocks (even when it doesn't matter 
> for that particular clock). I chose the first as more important, but happy to 
> go either way here.

The prepare method being called from non-atomic context cannot be 
considered to be in a speed critical path.  Most of the time, this is 
going to be called on driver initialization or the like, and that's a 
relatively rare event. Therefore this really small cost to clk_prepare() 
is definitively worth it to help proper usage of the API.  If this ever 
becomes a problem then this could be confined to some CONFIG_CLK_DEBUG 
or the like.  But when introducing a new API it is best to be more 
strict to help people get its usage right (without going overboard with 
it of course).


Nicolas
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