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

[Marc Gonzalez <marc.w.gonzalez@free.fr>]

On 12/12/2019 15:17, Russell King - ARM Linux admin wrote:

> 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.

I don't think so ;-)

If devm_add() returns NULL, then we have called __clk_put(dev, &clk);

Regards.