Re: [Linaro-mm-sig] Re: [PATCH RFC 18/18] drm/asahi: Add the Asahi driver for Apple AGX GPUs

[Daniel Vetter]

On Thu, Apr 06, 2023 at 02:02:55PM +0900, Asahi Lina wrote:
> On 05/04/2023 23.44, Daniel Vetter wrote:
> > On Tue, Mar 07, 2023 at 11:25:43PM +0900, Asahi Lina wrote:
> > > +/// Look up a GEM object handle for a `File` and return an `ObjectRef` for it.
> > > +pub(crate) fn lookup_handle(file: &DrmFile, handle: u32) -> Result<ObjectRef> {
> > > +    Ok(ObjectRef::new(shmem::Object::lookup_handle(file, handle)?))
> > > +}
> > 
> > So maybe my expectations for rust typing is a bit too much, but I kinda
> > expected this to be fully generic:
> > 
> > - trait Driver (drm_driver) knows the driver's object type
> > - a generic create_handle function could ensure that for drm_file (which
> >    is always for a specific drm_device and hence Driver) can ensure at the
> >    type level that you only put the right objects into the drm_file
> > - a generic lookup_handle function on the drm_file knows the Driver trait
> >    and so can give you back the right type right away.
> > 
> > Why the wrapping, what do I miss?
> 
> Sigh, so this is one of the many ways I'm trying to work around the "Rust
> doesn't do subclasses" problem (so we can figure out what the best one is
> ^^).
> 
> The generic shmem::Object::lookup_handle() call *is* fully generic and will
> get you back a driver-specific object. But since Rust doesn't do
> subclassing, what you get back isn't a driver-specific type T, but rather a
> (reference to a) shmem::Object<T>. T represents the inner driver-specific
> data/functionality (only), and the outer shmem::Object<T> includes the
> actual drm_gem_shmem_object plus a T. This is backwards from C, where you
> expect the opposite situation where T contains a shmem object, but that just
> doesn't work with Rust because there's no way to build a safe API around
> that model as far as I know.

Ah I think I just got confused. I did untangle (I think at least) the
Object<T> trick, I guess the only thing that confused me here is why this
is in the shmem module? Or is that the rust problem again?

I'd kinda have expected that we'd have a gem::Object<T> here that the
lookup_handle function returns. So for the shmem case I guess that would
then be gem::Object<shmem::Object<T>> for the driver type T with driver
specific stuff? I guess not very pretty ...

> Now the problem is from the higher layers I want object operations that
> interact with the shmem::Object<T> (that is, they call generic GEM functions
> on the object). Options so far:
> 
> 1. Add an outer wrapper and put that functionality there.
> 2. Just have the functions on T as helpers, so you need to call T::foo(obj)
> instead of obj.foo().
> 3. Use the undocumented method receiver trait thing to make shmem::Object<T>
> a valid `self` type, plus add auto-Deref to shmem::Object. Then obj.foo()
> works.
> 
> #1 is what I use here. #2 is how the driver-specific File ioctl callbacks
> are implemented, and also sched::Job<T>. #3 is used for fence callbacks
> (FenceObject<T>). None of them are great, and I'd love to hear what people
> think of the various options...
> 
> There are other unexplored options, like in this GEM case it could be
> covered with a driver-internal auxiliary trait impl'd on shmem::Object<T>
> buuut that doesn't work when you actually need callbacks on T itself to
> circle back to shmem::Object<T>, as is the case with File/Job/FenceObject.

Ok I think I'm completely lost here. But I also havent' looked at how this
is all really used in the driver, it's really just the shmem:: module in
the lookup_handle function which looked strange to me.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
hvettp://blog.ffwll.ch