Messages in this thread | | | Date | Wed, 23 Nov 2022 16:48:41 +0100 | Subject | Re: [PATCH 3/5] driver core: make struct device_type.uevent() take a const * | From | Maximilian Luz <> |
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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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