Re: [PATCH 3/5] driver core: make struct device_type.uevent() take a const *

Maximilian Luz
2022-11-23T14:04:00Z

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.

Regards,
Max