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

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Date:	Wed, 09 Feb 2011 20:26:44 -0800
From:	Saravana Kannan <skannan@...eaurora.org>
To:	Nicolas Pitre <nicolas.pitre@...aro.org>
CC:	Jeremy Kerr <jeremy.kerr@...onical.com>,
	Dima Zavin <dmitriyz@...gle.com>,
	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>,
	linux-kernel@...r.kernel.org, Paul Mundt <lethal@...ux-sh.org>,
	Ben Dooks <ben-linux@...ff.org>,
	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>,
	Russell King <linux@....linux.org.uk>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [RFC, PATCH 3/3] clk: add warnings for incorrect enable/prepare
 semantics

On 02/07/2011 06:24 AM, Nicolas Pitre wrote:
> 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).
>

Agree with Nicholas and Uwe. +1 for this request.

-Saravana

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