Re: [PATCH v2 7/8] rust: drm: gem: Add GEM object abstraction

[Lyude Paul <lyude@redhat.com>]

On Thu, 2025-04-17 at 22:31 +0200, Danilo Krummrich wrote:
> On Thu, Apr 17, 2025 at 02:42:24PM -0400, Lyude Paul wrote:
> > On Fri, 2025-04-11 at 01:55 +0200, Danilo Krummrich wrote:
> > > +/// A base GEM object.
> > > +///
> > > +/// Invariants
> > > +///
> > > +/// `self.dev` is always a valid pointer to a `struct drm_device`.
> > > +#[repr(C)]
> > > +#[pin_data]
> > > +pub struct Object<T: DriverObject + Send + Sync> {
> > > +    obj: Opaque<bindings::drm_gem_object>,
> > > +    dev: ptr::NonNull<bindings::drm_device>,
> > 
> > Not a huge deal but why don't we just use NonNull<device::Device<T::Driver>>
> > here?
> 
> Yeah, we could indeed also use NonNull<drm::Device<T::Driver>> instead, but I
> think it doesn't really make a difference.
> 
> We only need it in Object::dev(), and the unsafe call would change from
> 
> 	unsafe { drm::Device::as_ref(self.dev.as_ptr()) }
> 
> to
> 	unsafe { &*self.dev.as_ptr() }
> 
> I'm fine either way.

If it doesn't make a difference then yeah, personally I would prefer the rust
type over mixing the C type in. I think my preference just comes from the fact
it feels more natural to use the rust-defined wrapper type wherever possible
especially since it will give a bit more of a helpful documentation blurb for
the type when using rust-analyzer. This can be done in a follow-up patch if
you want as well

> 
> > > +// SAFETY: Instances of `Object<T>` are always reference-counted.
> > > +unsafe impl<T: DriverObject> crate::types::AlwaysRefCounted for Object<T> {
> > > +    fn inc_ref(&self) {
> > > +        // SAFETY: The existence of a shared reference guarantees that the refcount is non-zero.
> > > +        unsafe { bindings::drm_gem_object_get(self.as_raw()) };
> > > +    }
> > > +
> > > +    unsafe fn dec_ref(obj: ptr::NonNull<Self>) {
> > > +        // SAFETY: `obj` is a valid pointer to an `Object<T>`.
> > > +        let obj = unsafe { obj.as_ref() };
> > > +
> > > +        // SAFETY: The safety requirements guarantee that the refcount is non-zero.
> > > +        unsafe { bindings::drm_gem_object_put(obj.as_raw()) }
> > > +    }
> > > +}
> > 
> > So - as far as I can tell pretty much every gem object is going to be using
> > the same object_get/object_put() functions - so instead of implementing
> > AlwaysRefCounted for Object<T> why not handle this the other way around?
> > 
> > unsafe impl<T: IntoGEMObject> AlwaysRefCounted for T {
> >   /* ... */
> > }
> > 
> > That way you can also make IntoGEMObject a super-trait of AlwaysRefCounted, so
> > the AlwaysRefCounted trait bound will be implied instead of having to specify
> > it manually all over the place.
> 
> That is a great idea!
> 
> Since the current implementation should be correct, do you want to implement
> this improvement in a subsequent patch? :)
> 

-- 
Cheers,
 Lyude Paul (she/her)
 Software Engineer at Red Hat

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