linux-kernel - Re: [PATCH v3 3/5] clk: introduce the common clock framework

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Date:	Thu, 1 Dec 2011 08:45:47 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Paul Walmsley <paul@...an.com>
Cc:	Mike Turquette <mturquette@...com>, linux-kernel@...r.kernel.org,
	linux-omap@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	jeremy.kerr@...onical.com, broonie@...nsource.wolfsonmicro.com,
	tglx@...utronix.de, linus.walleij@...ricsson.com,
	amit.kucheria@...aro.org, dsaxena@...aro.org, patches@...aro.org,
	linaro-dev@...ts.linaro.org, grant.likely@...retlab.ca,
	sboyd@...cinc.com, shawn.guo@...escale.com, skannan@...cinc.com,
	magnus.damm@...il.com, arnd.bergmann@...aro.org,
	eric.miao@...aro.org, richard.zhao@...aro.org,
	Mike Turquette <mturquette@...aro.org>
Subject: Re: [PATCH v3 3/5] clk: introduce the common clock framework

On Wed, Nov 30, 2011 at 06:20:50PM -0700, Paul Walmsley wrote:
> 1. When a clock user calls clk_enable() on a clock, the clock framework 
> should prevent other users of the clock from changing the clock's rate.  
> This should persist until the clock user calls clk_disable() (but see also 
> #2 below).  This will ensure that clock users can rely on the rate 
> returned by clk_get_rate(), as long as it's called between clk_enable() 
> and clk_disable().  And since the clock's rate is guaranteed to remain the 
> same during this time, code that cannot tolerate clock rate changes 
> without special handling (such as driver code for external I/O devices) 
> will work safely without further modification.

So, if you have a PLL whose parent clock is not used by anything else.
You want to program it to a certain rate.

You call clk_disable() on the PLL clock.  This walks up the tree and
disables the parent.  You then try to set the rate using clk_set_rate().
clk_set_rate() in this circumstance can't wait for the PLL to lock
because it can't - there's no reference clock for it.

You then call clk_enable().  The PLL now takes its time to lock.  You
can't sleep in clk_enable() because it might be called from atomic
contexts, so you have to spin waiting for this.

Overloading clk_disable/clk_enable in this way is a bad solution to
this problem.
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