clk: readd refcounting for struct clk instances

On 09/15, Heiko Stübner wrote:
> With the split into struct clk and struct clk_core, clocks lost the
> ability for nested __clk_get clkdev calls. While it stays possible to
> call __clk_get, the first call to (__)clk_put will clear the struct clk,
> making subsequent clk_put calls run into a NULL pointer dereference.
> 
> One prime example of this sits in the generic power domain code, where it
> is possible to add the clocks both by name and by passing in a struct clk
> via pm_clk_add_clk(). __pm_clk_add() in turn then calls __clk_get to
> increase the refcount, so that the original code can put the clock again.
> 
> A possible call-path looks like
> clk = of_clk_get();
> pm_clk_add_clk(dev, clk);
> clk_put(clk);
> 
> with pm_clk_add_clk() => __pm_clk_add() then calling __clk_get on the clk
> and later clk_put when the pm clock list gets destroyed, thus creating
> a NULL pointer deref, as the struct clk doesn't exist anymore.
> 
> So add a separate refcounting for struct clk instances and only clean up
> once the refcount reaches zero.
> 
> Signed-off-by: Heiko Stuebner <heiko@sntech.de>
> ---
> I stumbled upon this while applying the new Rockchip power-domain driver,
> but I guess the underlying issue is universal and probably present since
> the original clk/clk_core split, so this probably counts as fix.

Ok. The fix makes sense, but I wonder why we do this. Perhaps we
should stop exporting __clk_get() and __clk_put() to everything
that uses clkdev in the kernel. They're called per-user clks for
a reason. There's one user. Now we have two users of the same
struct clk instance, so we have to refcount it too? I hope the
shared clk instance isn't being used to set rates in two pieces
of code.

And this only affects pm_clk_add_clk() callers right? So the only
caller in the kernel (drivers/clk/shmobile/clk-mstp.c) doesn't
seem to have this problem right now because it never calls
clk_put() on the pointer it passes to pm_clk_add_clk().

So what if we did the patch below? This would rely on callers not
calling clk_put() before calling pm_clk_remove() or
pm_clk_destroy(), and the life cycle would be clear, but the
sharing is still there. Or we could say that after a clk is given
to pm_clk_add_clk() that the caller shouldn't touch it anymore,
like shmobile is doing right now. Then there's nothing to do
besides remove the extra __clk_get() call in the pm layer.

> 
> @@ -2606,6 +2607,18 @@ static void __clk_release(struct kref *ref)
>  	kfree(core);
>  }
>  
> +static void __clk_release(struct kref *ref)
> +{
> +	struct clk *clk = container_of(ref, struct clk, ref);
> +
> +	hlist_del(&clk->clks_node);
> +	if ((clk->min_rate > clk->core->req_rate ||
> +	     clk->max_rate < clk->core->req_rate))

Why did we grow a pair of parenthesis?

> +		clk_core_set_rate_nolock(clk->core, clk->core->req_rate);
> +
> +	kfree(clk);
> +}
> +
>  /*
>   * Empty clk_ops for unregistered clocks. These are used temporarily
>   * after clk_unregister() was called on a clock and until last clock

clk: readd refcounting for struct clk instances

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