Re: [PATCH v3 01/14] rust: drm: gem: Simplify use of generics

[Lyude Paul <lyude@redhat.com>]

On Mon, 2025-09-01 at 12:37 -0300, Daniel Almeida wrote:
> Hi Lyude, thanks a lot for working on this! :)
> 
> > On 29 Aug 2025, at 19:35, Lyude Paul <lyude@redhat.com> wrote:
> > 
> > Now that my rust skills have been honed, I noticed that there's a lot of
> > generics in our gem bindings that don't actually need to be here. Currently
> > the hierarchy of traits in our gem bindings looks like this:
> > 
> >  * Drivers implement:
> >    * BaseDriverObject<T: DriverObject> (has the callbacks)
> >    * DriverObject (has the drm::Driver type)
> >  * Crate implements:
> >    * IntoGEMObject for Object<T> where T: DriverObject
> >      Handles conversion to/from raw object pointers
> >    * BaseObject for T where T: IntoGEMObject
> >      Provides methods common to all gem interfaces
> > 
> >  Also of note, this leaves us with two different drm::Driver associated
> >  types:
> >    * DriverObject::Driver
> >    * IntoGEMObject::Driver
> > 
> > I'm not entirely sure of the original intent here unfortunately (if anyone
> > is, please let me know!), but my guess is that the idea would be that some
> > objects can implement IntoGEMObject using a different ::Driver than
> > DriverObject - presumably to enable the usage of gem objects from different
> > drivers. A reasonable usecase of course.
> > 
> > However - if I'm not mistaken, I don't think that this is actually how
> > things would go in practice. Driver implementations are of course
> > implemented by their associated drivers, and generally drivers are not
> > linked to each-other when building the kernel. Which is to say that even in
> > a situation where we would theoretically deal with gem objects from another
> > driver, we still wouldn't have access to its drm::driver::Driver
> > implementation. It's more likely we would simply want a variant of gem
> > objects in such a situation that have no association with a
> > drm::driver::Driver type.
> > 
> > Taking that into consideration, we can assume the following:
> > * Anything that implements BaseDriverObject will implement DriverObject
> >  In other words, all BaseDriverObjects indirectly have an associated
> >  ::Driver type - so the two traits can be combined into one with no
> >  generics.
> > * Not everything that implements IntoGEMObject will have an associated
> >  ::Driver, and that's OK.
> > 
> > And with this, we now can do quite a bit of cleanup with the use of
> > generics here. As such, this commit:
> > 
> > * Removes the generics on BaseDriverObject
> > * Moves DriverObject::Driver into BaseDriverObject
> > * Removes DriverObject
> > * Removes IntoGEMObject::Driver
> > * Add AllocImpl::Driver, which we can use as a binding to figure out the
> >  correct File type for BaseObject
> > 
> > Leaving us with a simpler trait hierarchy that now looks like this:
> > 
> >  * Drivers implement: BaseDriverObject
> >  * Crate implements:
> >    * IntoGEMObject for Object<T> where T: DriverObject
> >    * BaseObject for T where T: IntoGEMObject
> > 
> > Which makes the code a lot easier to understand and build on :).
> > 
> > Signed-off-by: Lyude Paul <lyude@redhat.com>
> > 
> > ---
> > V2:
> > * Don't refer to Object<T> in callbacks, as this would result in drivers
> >  getting the wrong gem object type for shmem gem objects once we add
> >  support for those. Instead, we'll just add a type alias to clean this
> >  part up.
> > V3:
> > * Fix nova compilation
> > * Also, add an associated driver type to AllocImpl - as we still need the
> >  current driver accessible from BaseObject so that we can use the driver's
> >  various associated types, like File
> > V4:
> 
> ?
> 
> This is v3. Can you clarify this before we go further? :)

Whoops! Usually the way that I keep track of my changelogs is by typing them
in squash! commits, and then leaving them below the patch cutline once I
squash everything. It looks like at some point I mistakenly typed V4: on one
of the squash commits instead of V3: and didn't catch it. So, that comment is
definitely for V3 - not V4 which doesn't exist yet

> 
> > * Add missing Object = Self constraint to type bounds for create_handle,
> >  lookup_handle. I forgot that if drivers can have private gem objects with
> >  a different data layout, we can only guarantee gem objects with handles
> >  are of the same gem object type as the main one in use by the driver.
> > 
> > Signed-off-by: Lyude Paul <lyude@redhat.com>
> 
> — Daniel
> 

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

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