linux-kernel - Re: [PATCH v13 3/6] clk: Make clk API return per-user struct clk instances

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Date:	Fri, 6 Feb 2015 13:39:20 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Stephen Boyd <sboyd@...eaurora.org>
Cc:	Sylwester Nawrocki <s.nawrocki@...sung.com>,
	Tomeu Vizoso <tomeu.vizoso@...labora.com>,
	Paul Walmsley <paul@...an.com>,
	Tony Lindgren <tony@...mide.com>, linux-kernel@...r.kernel.org,
	Mike Turquette <mturquette@...aro.org>,
	linux-omap@...r.kernel.org,
	Javier Martinez Canillas <javier.martinez@...labora.co.uk>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v13 3/6] clk: Make clk API return per-user struct clk
 instances

On Thu, Feb 05, 2015 at 05:35:28PM -0800, Stephen Boyd wrote:
> On 02/05/15 16:42, Russell King - ARM Linux wrote:
> > On Thu, Feb 05, 2015 at 02:14:01PM -0800, Stephen Boyd wrote:
> >> Actually we can bury the __clk_create_clk() inside
> >> __of_clk_get_from_provider(). We should also move __clk_get() into there
> >> because right now we have a hole where whoever calls
> >> of_clk_get_from_provider() never calls __clk_get() on the clk, leading
> >> to possible badness. v2 coming soon.
> > There's some other issues here too...
> >
> > sound/soc/kirkwood/kirkwood-i2s.c:
> >
> >         priv->clk = devm_clk_get(&pdev->dev, np ? "internal" : NULL);
> > ...
> >         priv->extclk = devm_clk_get(&pdev->dev, "extclk");
> >         if (IS_ERR(priv->extclk)) {
> > ...
> > 	} else {
> >                 if (priv->extclk == priv->clk) {
> >                         devm_clk_put(&pdev->dev, priv->extclk);
> >                         priv->extclk = ERR_PTR(-EINVAL);
> >                 } else {
> >                         dev_info(&pdev->dev, "found external clock\n");
> >                         clk_prepare_enable(priv->extclk);
> >                         soc_dai = kirkwood_i2s_dai_extclk;
> >                 }
> >
> > It should be fine provided your "trick" is only done for DT clocks,
> > but not for legacy - with legacy, a NULL in the clkdev tables will
> > match both these requests, hence the need to compare the clk_get()
> > return value to tell whether we get the same clock.
> >
> 
> Are we still talking about of_clk_get_from_provider()? Or are we talking
> about comparing struct clk pointers?

Comparing struct clk pointers, and the implications of the patch changing
the clk_get() et.al. to be unique struct clk pointers.

> From what I can tell this code is
> now broken because we made all clk getting functions (there's quite a
> few...) return unique pointers every time they're called. It seems that
> the driver wants to know if extclk and clk are the same so it can do
> something differently in kirkwood_set_rate(). Do we need some sort of
> clk_equal(struct clk *a, struct clk *b) function for drivers like this?

Well, the clocks in question are the SoC internal clock (which is more or
less fixed, but has a programmable divider) and an externally supplied
clock, and the IP has a multiplexer on its input which allows us to select
between those two sources.

If it were possible to bind both to the same clock, it wouldn't be a
useful configuration - nothing would be gained from doing so in terms of
available rates.

What the comparison is there for is to catch the case with legacy lookups
where a clkdev lookup entry with a NULL connection ID results in matching
any connection ID passed to clk_get().  If the patch changes this, then
we will have a regression - and this is something which needs fixing
_before_ we do this "return unique clocks".

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