linux-kernel - Re: [PATCH 01/36] clk: Introduce clk_try

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Message-ID: <20150121150511.GB22884@ulmo.nvidia.com>
Date:	Wed, 21 Jan 2015 16:05:12 +0100
From:	Thierry Reding <thierry.reding@...il.com>
To:	Stephen Boyd <sboyd@...eaurora.org>
Cc:	dri-devel@...ts.freedesktop.org, linux-tegra@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Russell King <linux@....linux.org.uk>,
	Mike Turquette <mturquette@...aro.org>
Subject: Re: [PATCH 01/36] clk: Introduce clk_try_parent()

On Tue, Jan 20, 2015 at 11:21:24AM -0800, Stephen Boyd wrote:
> On 01/20/2015 02:48 AM, Thierry Reding wrote:
> >  
> >  /**
> > + * clk_try_parent - check if a clock can be the parent clock source of another
> > + * @clk: clock source
> > + * @parent: parent clock source
> > + *
> > + * This is like clk_set_parent(), except that it only checks that parent can
> > + * be the parent clock source for clock.
> > + *
> > + * Returns success (0) or negative errno.
> > + */
> > +int clk_try_parent(struct clk *clk, struct clk *parent)
> > +{
> > +	int err = 0;
> > +
> > +	if (!clk || !parent)
> > +		return -EINVAL;
> 
> NULL clock should be a nop, so return success in either case.

Okay.

> > +
> > +	if ((clk->num_parents > 1) && !clk->ops->set_parent)
> > +		return -ENOSYS;
> 
> This suffers from the same problem as discussed in another thread where
> the mux is read-only and the parent is the current parent. That case
> shouldn't fail.

Okay, if I do a lookup on the parent names array as you suggested below
I don't need to consider this anyway.

> > +
> > +	clk_prepare_lock();
> > +
> > +	if (clk->parent == parent)
> > +		goto unlock;
> > +
> > +	err = clk_fetch_parent_index(clk, parent);
> > +	if (err > 0)
> > +		err = 0;
> > +
> 
> Given that we just throw away the index, perhaps we should just loop
> over the parent_names array searching for a name match on the parent's
> name. If we did that this entire function would be lockless too.

Done.

> > +unlock:
> > +	clk_prepare_unlock();
> > +
> > +	return err;
> > +}
> > +EXPORT_SYMBOL_GPL(clk_try_parent);
> > +
> > +/**
> >   * clk_set_parent - switch the parent of a mux clk
> >   * @clk: the mux clk whose input we are switching
> >   * @parent: the new input to clk
> > diff --git a/include/linux/clk.h b/include/linux/clk.h
> > index fb1ac65f127c..94da8c68a515 100644
> > --- a/include/linux/clk.h
> > +++ b/include/linux/clk.h
> > @@ -328,6 +328,15 @@ long clk_round_rate(struct clk *clk, unsigned long rate);
> >  int clk_set_rate(struct clk *clk, unsigned long rate);
> >  
> >  /**
> > + * clk_try_parent - check if a clock can be the parent clock source of another
> > + * @clk: clock source
> > + * @parent: parent clock source
> > + *
> > + * Returns success (0) or negative errno.
> 
> Why not a bool? Do we really care why we can't set the parent in the
> error case?

A bool should do fine. I guess a negative error code would've been
easier to propagate, but we can probably just return an -EINVAL if
clk_has_parent() fails.

> > + */
> > +int clk_try_parent(struct clk *clk, struct clk *parent);
> 
> The name makes me think of mutex_trylock(), so I immediately think this
> tries to set the parent. Perhaps a better name would be
> clk_can_have_parent() or clk_has_parent()?

clk_has_parent() sounds good to me.

Thierry

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