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

Asahi Lina
2023-04-06T05:10:04Z

Same as the prior email, this was supposed to go to Daniel...

On 06/04/2023 14.02, 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.
>
> 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.
>
> ~~ Lina
>

~~ Lina