Date:   Wed, 23 Nov 2022 16:48:41 +0100
From:   Maximilian Luz <luzmaximilian@...il.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Matthew Wilcox <willy@...radead.org>
Cc:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        linux-kernel@...r.kernel.org, Jens Axboe <axboe@...nel.dk>,
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        Mark Gross <markgross@...nel.org>,
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Subject: Re: [PATCH 3/5] driver core: make struct device_type.uevent() take a
 const *

On 11/23/22 16:37, Greg Kroah-Hartman wrote:
> On Wed, Nov 23, 2022 at 02:52:59PM +0000, Matthew Wilcox wrote:
>> On Wed, Nov 23, 2022 at 02:59:00PM +0100, Maximilian Luz wrote:
>>> On 11/23/22 14:34, Andy Shevchenko wrote:
>>>> On Wed, Nov 23, 2022 at 02:14:31PM +0100, Maximilian Luz wrote:
>>>>> On 11/23/22 13:25, Greg Kroah-Hartman wrote:
>>>>>> The uevent() callback in struct device_type should not be modifying the
>>>>>> device that is passed into it, so mark it as a const * and propagate the
>>>>>> function signature changes out into all relevant subsystems that use
>>>>>> this callback.
>>>>
>>>> [...]
>>>>
>>>>>> -static inline struct ssam_device *to_ssam_device(struct device *d)
>>>>>> +static inline struct ssam_device *to_ssam_device(const struct device *d)
>>>>>>     {
>>>>>>     	return container_of(d, struct ssam_device, dev);
>>>>>>     }
>>>>>
>>>>> I am slightly conflicted about this change as that now more or less
>>>>> implicitly drops the const. So I'm wondering if it wouldn't be better to
>>>>> either create a function specifically for const pointers or to just
>>>>> open-code it in the instance above.
>>>>>
>>>>> I guess we could also convert this to a macro. Then at least there
>>>>> wouldn't be an explicit and potentially misleading const-conversion
>>>>> indicated in the function signature.
>>>>
>>>> This is an intermediate step as far as I know since moving container_of to
>>>> recognize const is a bit noisy right now. I guess you can find a discussion
>>>> on the topic between Greg and Sakari.
>>>
>>> Thanks! I assume you are referring to the following?
>>>
>>> 	https://lore.kernel.org/lkml/4218173bd72b4f1899d4c41a8e251f0d@AcuMS.aculab.com/T/
>>>
>>> As far as I can tell this is only a warning in documentation, not
>>> compile time (which would probably be impossible?).
>>>
>>> As I've said I'd be fine with converting the function to a macro (and
>>> preferably adding a similar warning like the one proposed in that
>>> thread). The point that irks me up is just that, as proposed, the
>>> function signature would now advertise a conversion that should never be
>>> happening.
>>>
>>> Having two separate functions would create a compile-time guarantee, so
>>> I'd prefer that, but I can understand if that might be considered too
>>> noisy in code. Or if there is a push to make container_of() emit a
>>> compile-time warning I'd also be perfectly happy with converting it to a
>>> macro now as that'd alleviate the need for functions in the future.
>>
>> Can't we do:
>>
>> static inline const struct ssam_device *to_ssam_device(const struct device *d)
>> {
>> 	return container_of(d, const struct ssam_device, dev);
>> }
>>
> 
> You could, if you can always handle a const pointer coming out of this
> function, but I don't think you can.
> 
> What you might want to do instead, and I'll be glad to do it for all of
> the functions like this I change, is to do what we have for struct
> device now:
> 
> static inline struct device *__kobj_to_dev(struct kobject *kobj)
> {
>          return container_of(kobj, struct device, kobj);
> }
> 
> static inline const struct device *__kobj_to_dev_const(const struct kobject *kobj)
> {
>          return container_of(kobj, const struct device, kobj);
> }
> 
> /*
>   * container_of() will happily take a const * and spit back a non-const * as it
>   * is just doing pointer math.  But we want to be a bit more careful in the
>   * driver code, so manually force any const * of a kobject to also be a const *
>   * to a device.
>   */
> #define kobj_to_dev(kobj)                                       \
>          _Generic((kobj),                                        \
>                   const struct kobject *: __kobj_to_dev_const,   \
>                   struct kobject *: __kobj_to_dev)(kobj)
> 
> 
> Want me to do the same thing here as well?

That looks great! Thanks!

I would very much prefer that.

Regards,
Max

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