linux-kernel - Re: [RFC,PATCH 1/3] Add a common struct clk

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Message-Id: <201102151526.54280.jeremy.kerr@canonical.com>
Date:	Tue, 15 Feb 2011 15:26:53 +0800
From:	Jeremy Kerr <jeremy.kerr@...onical.com>
To:	Saravana Kannan <skannan@...eaurora.org>
Cc:	linux-arm-kernel@...ts.infradead.org,
	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,
	Russell King <linux@....linux.org.uk>,
	Dima Zavin <dmitriyz@...gle.com>,
	Ben Dooks <ben-linux@...ff.org>,
	"Uwe Kleine-König" 
	<u.kleine-koenig@...gutronix.de>,
	Vincent Guittot <vincent.guittot@...aro.org>
Subject: Re: [RFC,PATCH 1/3] Add a common struct clk

Hi Saravana,

> Sure, one could argue that in some archs for a certain set of clocks the
> slow stuff in prepare/unprepare won't need to be done during set rate --
> say, a simple clock that always runs off the same PLL but just has a
> integer divider to change the rate.
> 
> In those cases, not grabbing the prepare_lock would make the code less
> "locky".
> 
> > We
> > may even want to disallow set_rate (and set_parent) when prepare_count is
> > non- zero.
> 
> This is definitely not right.

Why is that? Consider two devices using one clock; one does some 
initialisation based on the return value of clk_get_rate(), the other calls 
clk_set_rate() some time later. Now the first device is incorrectly 
initialised.

Regardless, this is definitely something to flag for a later discussion. I'm 
happy to return to that, but we should focus on one issue at a time here.

> Changing the rate of a clock when it's
> already enabled/prepared is a very reasonable thing to do. It's only
> doing a set rate at the "same time" as a prepare/unprepare that's wrong
> for some clocks. We could have the specific implementation deal with the
> locking internally.

Yes, hence leaving the locking here to the clock implementation.

> > I'd prefer to enforce the 'sleepability' with might_sleep instead.
> 
> Yeah, I realized this option after sending out my previous email. Please
> do add a might_sleep(). It will actually point out errors (per the new
> clarification) in some serial drivers.

Yep, will do.

> >>> +	.enable_lock	= __SPIN_LOCK_UNLOCKED(name.enable_lock),	\
> >>> +	.prepare_lock	= __MUTEX_INITIALIZER(name.prepare_lock),	\
> >> 
> >> After a long day, I'm not able to wrap my head around this. Probably a
> >> stupid question, but will this name.xxx thing prevent using this
> >> INIT_CLK macro to initialize an array of clocks? More specifically,
> >> prevent the sub class macro (like INIT_CLK_FIXED) from being used to
> >> initialize an array of clocks?
> > 
> > That's correct. For an array of clocks, you'll have to use a different
> > initialiser. We can add helpers for that that when (and if) the need
> > arises.
> 
> Would it even be possible to get this to work for an array? You don't
> have to change this in the patch, but I'm curious to know how to get
> this to work for an array without doing a run time init of the lock.

I'd assume that you'd have to do this at run time, as with any other array of 
structs that contain a mutex or spinlock.

Cheers,


Jeremy
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