linux-kernel - Re: [PATCH 2/5] driver core: Introduce an API constify_device_find_child

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Message-ID: <96268791-4c32-4390-97bd-758e6525bd11@quicinc.com>
Date: Tue, 13 Aug 2024 19:15:30 +0800
From: quic_zijuhu <quic_zijuhu@...cinc.com>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
CC: Zijun Hu <zijun_hu@...oud.com>, "Rafael J. Wysocki" <rafael@...nel.org>,
        Davidlohr Bueso <dave@...olabs.net>,
        Jonathan Cameron
	<jonathan.cameron@...wei.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Alison
 Schofield <alison.schofield@...el.com>,
        Vishal Verma
	<vishal.l.verma@...el.com>,
        Ira Weiny <ira.weiny@...el.com>,
        Dan Williams
	<dan.j.williams@...el.com>,
        Takashi Sakamoto <o-takashi@...amocchi.jp>,
        Timur
 Tabi <timur@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Eric
 Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni
	<pabeni@...hat.com>, <linux-kernel@...r.kernel.org>,
        <linux-cxl@...r.kernel.org>, <linux1394-devel@...ts.sourceforge.net>,
        <netdev@...r.kernel.org>
Subject: Re: [PATCH 2/5] driver core: Introduce an API
 constify_device_find_child_helper()

On 8/13/2024 6:57 PM, Greg Kroah-Hartman wrote:
> On Tue, Aug 13, 2024 at 06:50:04PM +0800, quic_zijuhu wrote:
>> On 8/13/2024 5:45 PM, Greg Kroah-Hartman wrote:
>>> On Sun, Aug 11, 2024 at 08:18:08AM +0800, Zijun Hu wrote:
>>>> From: Zijun Hu <quic_zijuhu@...cinc.com>
>>>>
>>>> Introduce constify_device_find_child_helper() to replace existing
>>>> device_find_child()'s usages whose match functions will modify
>>>> caller's match data.
>>>
>>> Ick, that's not a good name, it should be "noun_verb" with the subsystem being on the prefix always.
>>>
>> okay, got it.
>>
>> is it okay to use device_find_child_mut() suggested by Przemek Kitszel ?
> 
> No, just switch all callers over to be const and keep the same name.
> 
>>> But why is this even needed?  Device pointers are NOT const for the
>>> obvious reason that they can be changed by loads of different things.
>>> Trying to force them to be const is going to be hard, if not impossible.
>>>
>>
>> [PATCH 3/5] have more discussion about these questions with below link:
>> https://lore.kernel.org/all/8b8ce122-f16b-4207-b03b-f74b15756ae7@icloud.com/
>>
>>
>> The ultimate goal is to make device_find_child() have below prototype:
>>
>> struct device *device_find_child(struct device *dev, const void *data,
>> 		int (*match)(struct device *dev, const void *data));
>>
>> Why ?
>>
>> (1) It does not make sense, also does not need to, for such device
>> finding operation to modify caller's match data which is mainly
>> used for comparison.
>>
>> (2) It will make the API's match function parameter have the same
>> signature as all other APIs (bus|class|driver)_find_device().
>>
>>
>> My idea is that:
>> use device_find_child() for READ only accessing caller's match data.
>>
>> use below API if need to Modify caller's data as
>> constify_device_find_child_helper() does.
>> int device_for_each_child(struct device *dev, void *data,
>>                     int (*fn)(struct device *dev, void *data));
>>
>> So the The ultimate goal is to protect caller's *match data* @*data  NOT
>> device @*dev.
> 
> Ok, sorry, I was confused.
> 

Current prototype of the API:
struct device *device_find_child(struct device *dev, void *data,
                                 int (*match)(struct device *dev, void
*data));								

prototype we want:

struct device *device_find_child(struct device *dev, const void *data,
                                 int (*match)(struct device *dev, const
void *data));

The only differences are shown below:
void *data -> const void *data  // as argument of paramter @data of
(*match)().
int (*match)(struct device *dev, void *data) -> int (*match)(struct
device *dev, const void *data).

We don't change type of @dev. we just make above two parameters have the
same types as below existing finding APIs.

struct device *bus_find_device(const struct bus_type *bus, struct device
*start,
                               const void *data,
                               int (*match)(struct device *dev, const
void *data));
struct device *driver_find_device(const struct device_driver *drv,
                                  struct device *start, const void *data,
                                  int (*match)(struct device *dev, const
void *data));
struct device *class_find_device(const struct class *class, const struct
device *start,
                                 const void *data, int (*match)(struct
device *, const void *));
> thanks,
> 
> greg k-h