Re: [PATCH v1] clk: Convert managed get functions to devm_add_action API

[Russell King - ARM Linux admin]

On Thu, Dec 12, 2019 at 02:53:40PM +0100, Marc Gonzalez wrote:
> On 11/12/2019 23:28, Dmitry Torokhov wrote:
> 
> > On Wed, Dec 11, 2019 at 05:17:28PM +0100, Marc Gonzalez wrote:
> >
> >> What is the rationale for the devm_add_action API?
> > 
> > For one-off and maybe complex unwind actions in drivers that wish to use
> > devm API (as mixing devm and manual release is verboten). Also is often
> > used when some core subsystem does not provide enough devm APIs.
> 
> Thanks for the insight, Dmitry. Thanks to Robin too.
> 
> This is what I understand so far:
> 
> devm_add_action() is nice because it hides/factorizes the complexity
> of the devres API, but it incurs a small storage overhead of one
> pointer per call, which makes it unfit for frequently used actions,
> such as clk_get.
> 
> Is that correct?
> 
> My question is: why not design the API without the small overhead?
> 
> Proof of concept below:
> 
> 
> diff --git a/drivers/base/devres.c b/drivers/base/devres.c
> index 0bbb328bd17f..76392dd6273b 100644
> --- a/drivers/base/devres.c
> +++ b/drivers/base/devres.c
> @@ -685,6 +685,20 @@ int devres_release_group(struct device *dev, void *id)
>  }
>  EXPORT_SYMBOL_GPL(devres_release_group);
>  
> +void *devm_add(struct device *dev, dr_release_t func, void *arg, size_t size)
> +{
> +	void *data = devres_alloc(func, size, GFP_KERNEL);
> +
> +	if (data) {
> +		memcpy(data, arg, size);
> +		devres_add(dev, data);
> +	} else
> +		func(dev, arg);
> +
> +	return data;
> +}
> +EXPORT_SYMBOL_GPL(devm_add);
> +
>  /*
>   * Custom devres actions allow inserting a simple function call
>   * into the teadown sequence.
> diff --git a/drivers/clk/clk-devres.c b/drivers/clk/clk-devres.c
> index be160764911b..8db671823126 100644
> --- a/drivers/clk/clk-devres.c
> +++ b/drivers/clk/clk-devres.c
> @@ -4,6 +4,11 @@
>  #include <linux/export.h>
>  #include <linux/gfp.h>
>  
> +static void __clk_put(struct device *dev, void *data)
> +{
> +	clk_put(*(struct clk **)data);
> +}
> +
>  static void devm_clk_release(struct device *dev, void *res)
>  {
>  	clk_put(*(struct clk **)res);
> @@ -11,19 +16,11 @@ static void devm_clk_release(struct device *dev, void *res)
>  
>  struct clk *devm_clk_get(struct device *dev, const char *id)
>  {
> -	struct clk **ptr, *clk;
> -
> -	ptr = devres_alloc(devm_clk_release, sizeof(*ptr), GFP_KERNEL);
> -	if (!ptr)
> -		return ERR_PTR(-ENOMEM);
> +	struct clk *clk = clk_get(dev, id);
>  
> -	clk = clk_get(dev, id);
> -	if (!IS_ERR(clk)) {
> -		*ptr = clk;
> -		devres_add(dev, ptr);
> -	} else {
> -		devres_free(ptr);
> -	}
> +	if (!IS_ERR(clk))
> +		if (!devm_add(dev, __clk_put, &clk, sizeof(clk)))
> +			clk = ERR_PTR(-ENOMEM);

You leak clk here.

-- 
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