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Date:   Wed, 23 Nov 2022 13:29:56 -0400
From:   Jason Gunthorpe <jgg@ziepe.ca>
To:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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Subject: Re: [PATCH 3/5] driver core: make struct device_type.uevent() take a
 const *
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On Wed, Nov 23, 2022 at 06:01:47PM +0100, Greg Kroah-Hartman wrote:
> On Wed, Nov 23, 2022 at 12:25:32PM -0400, Jason Gunthorpe wrote:
> > On Wed, Nov 23, 2022 at 04:37:59PM +0100, Greg Kroah-Hartman wrote:
> > > 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?
> > 
> > It would be nice to have a shared macro code gen all of the above
> > instead of copy and pasting it. Then maybe other cases beyond struct
> > device could adopt const too..
> 
> I think I tried to create such a beast, but failed, so ended up
> open-coding it in a few places in the USB headers already.  I can try it
> again, but the redirection gets tricky (defines creating defines...)

This seems OK:

#define generic_container_of(in_type, in, out_type, out_member) \
	_Generic(in,                                        \
                  const in_type *: ((const out_type *)container_of(in, out_type, out_member)),   \
                  in_type *: ((out_type *)container_of(in, out_type, out_member)) \
		  )

#define kobj_to_dev(__kobj) \
	generic_container_of(struct kobject, __kobj, struct device, kobj)

Jason