linux-kernel - Re: [PATCH 3/3] extcon: Simplify extcon_dev_register() prototype by removing unnecessary parameter

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Date:	Wed, 04 Sep 2013 14:18:30 +0900
From:	Chanwoo Choi <cw00.choi@...sung.com>
To:	Greg KH <gregkh@...uxfoundation.org>
Cc:	linux-kernel@...r.kernel.org, myungjoo.ham@...sung.com,
	kyungmin.park@...sung.com, Graeme Gregory <gg@...mlogic.co.uk>,
	Kishon Vijay Abraham I <kishon@...com>,
	Charles Keepax <ckeepax@...nsource.wolfsonmicro.com>,
	Mark Brown <broonie@...nel.org>
Subject: Re: [PATCH 3/3] extcon: Simplify extcon_dev_register() prototype by
 removing unnecessary parameter

On 09/04/2013 10:16 AM, Greg KH wrote:
> On Wed, Sep 04, 2013 at 09:17:02AM +0900, Chanwoo Choi wrote:
>> On 09/04/2013 12:57 AM, Greg KH wrote:
>>> On Mon, Sep 02, 2013 at 10:13:44AM +0900, Chanwoo Choi wrote:
>>>> Hi Greg,
>>>>
>>>> On 09/02/2013 09:40 AM, Greg KH wrote:
>>>>> On Mon, Sep 02, 2013 at 09:20:08AM +0900, Chanwoo Choi wrote:
>>>>>> This patch remove extcon_dev_register()'s second parameter which means
>>>>>> the pointer of parent device to simplify prototype of this function.
>>>>>
>>>>> No, please don't.  You want the parent to be passed in, as the core
>>>>> needs it when it is registered with the system, otherwise it will not
>>>>> show up in sysfs properly (i.e. you can't set it afterwards.)
>>>>
>>>> Currently, each extcon driver have allocated memory for extcon device
>>>> by using devm_kzalloc() in each extcon device driver.
>>>
>>> That seems backwards, the extcon core should be the one doing the
>>> allocation, and ownership of the device, like all other subsystem cores
>>> do.  That makes the driver logic much simpler, and the lifetime
>>> ownership correct (i.e. what happens when a device is unbound from a
>>> driver by userspace?  The driver can't control the device memory
>>> anymore...)
>>>
>>
>> OK,
>> The extcon core will control memory allocation instead of extcon device driver
>> as following.
>> - devm_extcon_allocate_device(struct device *dev)
> 
> Huh?  Why do you need a devm allocator?  This is a "real" child device,
> just create it with a "extcon_create_device()" or some such call, like
> all other busses do?

I refer Input/IIO subsystem to check the process of device registration.
Input subsystem has following functions for memory allocation of input device
and input device registration.

Input subsystem
- struct input_dev *devm_input_allocate_device(struct device *device)
- struct input_dev *input_allocate_device(void)
- int input_register_device(struct input_dev *dev)
drivers/input/input.c

devm_input_allocate_device()/input_allocate_device() can allocate
memory for input device in input core. And then created input device
pass input_register_device() as parameter.

So, input device driver haven't executed kmalloc() or devm_kmalloc()
to allocate memory of input device by using input_allocate_device()/
devm_input_allocate_device().

Also,IIO subsystem has separate iio_device_alloc() function
to allocate memory for iio device. But IIO subsystem hasn't
"devm_" allocator. So, If iio device driver fail initialization
in *_probe, should execute iio_device_free() to free allocated memory.

IIO Subsystem
- iio_device_alloc(int sizeof_priv)
- iio_device_register(struct iio_dev *indio_dev);
drivers/iio/industrialio-core.c

So, I think extcon subsystem need extcon_allocate_device()/devm_extcon_allocate_device()
to allocate for memory extcon device. To implement devm_extcon_allocate_device() function
, extcon subsystem need extcon_allocate_device() because devm_extcon_allocate_device()
must call extcon_allocate_device for memory allocation.

If extcon_allocate_device()/devm_extcon_allocate_device() is implemented to extcon core,
I think extcon core can control the memory operation of extcon device.

EXTCON Subsystem
- devm_extcon_allocate_device(struct device *dev)
- extcon_allocate_device(void)
- extcon_dev_register(struct extcon_dev *edev)

If I'm wrong, please correct me. Thanks.

Best Regards,
Chanwoo Choi

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