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

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Date:	Wed, 30 Nov 2011 19:13:53 -0700 (MST)
From:	Paul Walmsley <paul@...an.com>
To:	Mike Turquette <mturquette@...com>
cc:	linux@....linux.org.uk, 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, aul@...an.com,
	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

Hi

a few initial comments on clk_get_parent().

On Mon, 21 Nov 2011, Mike Turquette wrote:

> +/**
> + * clk_get_parent - return the parent of a clk
> + * @clk: the clk whose parent gets returned
> + *
> + * Simply returns clk->parent.  It is up to the caller to hold the
> + * prepare_lock, if desired.  Returns NULL if clk is NULL.
> + */
> +struct clk *clk_get_parent(struct clk *clk)
> +{
> +	if (!clk)
> +		return NULL;
> +
> +	return clk->parent;
> +}
> +EXPORT_SYMBOL_GPL(clk_get_parent);

This implementation of clk_get_parent() has similar problems to the 
clk_get_rate() implementation:

    http://lkml.org/lkml/2011/11/30/403

clk_get_parent() should return an error if some other entity can change 
the clock's parent between the time that clk_get_parent() returns, and the 
time that the returned struct clk * is used.

For this to work, we need to define when clk_get_parent() should succeed. 
If we follow the protocol outlined in the above URL about clk_get_rate(), 
and we stipulate that when we block rate changes, we also block parent 
changes, then this should be fairly straightforward.  clk_get_parent() can 
succeed:

1. between a clk_enable() and a clk_disable() or clk_allow_rate_changes(),

2. between a clk_block_rate_changes() and a clk_enable(), clk_disable(), 
   or clk_allow_rate_changes()

As with clk_get_rate(), include/linux/clk.h is missing documentation of 
what the error return value should be.  This should be a little easier to 
define than with clk_get_rate(), but I don't think the error value should 
be NULL.  This is because NULL is a valid return value when 
clk_get_parent() is called on root clocks.  Better to use 
ERR_PTR(-EINVAL/-EBUSY/etc.).

- Paul
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