Subject: Re: [PATCH v2 1/8] fwctl: Add basic structure for a class subsystem with a cdev

From: Jonathan Cameron <[email-masked]>
Date: Tue, 30 Jul 2024 18:15:00 +0100

> 
> > > +/**
> > > + * fwctl_alloc_device - Allocate a fwctl
> > > + * @parent: Physical device that provides the FW interface
> > > + * @ops: Driver ops to register
> > > + * @drv_struct: 'struct driver_fwctl' that holds the struct fwctl_device
> > > + * @member: Name of the struct fwctl_device in @drv_struct
> > > + *
> > > + * This allocates and initializes the fwctl_device embedded in the drv_struct.
> > > + * Upon success the pointer must be freed via fwctl_put(). Returns NULL on
> > > + * failure. Returns a 'drv_struct *' on success, NULL on error.
> > > + */
> > > +#define fwctl_alloc_device(parent, ops, drv_struct, member)                  \
> > > +	container_of(_fwctl_alloc_device(                                    \
> > > +			     parent, ops,                                    \
> > > +			     sizeof(drv_struct) +                            \
> > > +				     BUILD_BUG_ON_ZERO(                      \
> > > +					     offsetof(drv_struct, member))), \  
> > Doesn't that fire a build_bug when the member is at the start of drv_struct?
> > Or do I have that backwards?  
> 
> BUILD_BUG_ON(true) == failure, evaluates to void
> BUILD_BUG_ON_ZERO(true) == fails, evaluates to 0
> BUILD_BUG_ON_ZERO(false) == false, evaluates to 0
> 
> It is a bit confusing name, it is not ON_ZERO it is BUG_ON return ZERO

Ah.  That indeed got me.  ouch.

> 
> > Does container_of() safely handle a NULL?  
> 
> Generally no, nor does it handle ERR_PTR, but it does work for both if
> the offset is 0.

Ah.  Good point, I'd neglected the zero offset meaning it is really
just a fancy pointer type cast.

> 
> The BUILD_BUG guarentees the 0 offset both so that the casting inside
> _fwctl_alloc_device() works and we can use safely use container_of()
> to enforce the type check.
> 
> What do you think about writing it like this instead:
> 
> #define fwctl_alloc_device(parent, ops, drv_struct, member)               \
> 	({                                                                \
> 		static_assert(__same_type(struct fwctl_device,            \
> 					  ((drv_struct *)NULL)->member)); \
> 		static_assert(offsetof(drv_struct, member) == 0);         \
> 		(drv_struct *)_fwctl_alloc_device(parent, ops,            \
> 						  sizeof(drv_struct));    \
> 	})
> 
> ?
> 
> In some ways I like it better..
Seems more readable to me and avoids entertaining corners of the previous approach.

Jonathan

> Thanks,
> Jason