rust: drm: gem: Simplify use of generics

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>
Reviewed-by: Daniel Almeida <daniel.almeida@collabora.com>
Acked-by: Danilo Krummrich <dakr@kernel.org>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20250908185239.135849-2-lyude@redhat.com
Signed-off-by: Alice Ryhl <aliceryhl@google.com>

2 files changed, 37 insertions, 43 deletions

diff --git a/rust/kernel/drm/driver.rs b/rust/kernel/drm/driver.rs
index fe7e8d06961a..dae0f4d1bbe3 100644
--- a/rust/kernel/drm/driver.rs
+++ b/rust/kernel/drm/driver.rs
@@ -86,6 +86,9 @@ pub struct AllocOps {
 
 /// Trait for memory manager implementations. Implemented internally.
 pub trait AllocImpl: super::private::Sealed + drm::gem::IntoGEMObject {
+    /// The [`Driver`] implementation for this [`AllocImpl`].
+    type Driver: drm::Driver;
+
     /// The C callback operations for this memory manager.
     const ALLOC_OPS: AllocOps;
 }
diff --git a/rust/kernel/drm/gem/mod.rs b/rust/kernel/drm/gem/mod.rs
index b71821cfb5ea..f5b19865d745 100644
--- a/rust/kernel/drm/gem/mod.rs
+++ b/rust/kernel/drm/gem/mod.rs
@@ -15,31 +15,31 @@ use crate::{
 use core::{mem, ops::Deref, ptr::NonNull};
 
 /// GEM object functions, which must be implemented by drivers.
-pub trait BaseDriverObject<T: BaseObject>: Sync + Send + Sized {
+pub trait DriverObject: Sync + Send + Sized {
+    /// Parent `Driver` for this object.
+    type Driver: drm::Driver;
+
     /// Create a new driver data object for a GEM object of a given size.
-    fn new(dev: &drm::Device<T::Driver>, size: usize) -> impl PinInit<Self, Error>;
+    fn new(dev: &drm::Device<Self::Driver>, size: usize) -> impl PinInit<Self, Error>;
 
     /// Open a new handle to an existing object, associated with a File.
     fn open(
-        _obj: &<<T as IntoGEMObject>::Driver as drm::Driver>::Object,
-        _file: &drm::File<<<T as IntoGEMObject>::Driver as drm::Driver>::File>,
+        _obj: &<Self::Driver as drm::Driver>::Object,
+        _file: &drm::File<<Self::Driver as drm::Driver>::File>,
     ) -> Result {
         Ok(())
     }
 
     /// Close a handle to an existing object, associated with a File.
     fn close(
-        _obj: &<<T as IntoGEMObject>::Driver as drm::Driver>::Object,
-        _file: &drm::File<<<T as IntoGEMObject>::Driver as drm::Driver>::File>,
+        _obj: &<Self::Driver as drm::Driver>::Object,
+        _file: &drm::File<<Self::Driver as drm::Driver>::File>,
     ) {
     }
 }
 
 /// Trait that represents a GEM object subtype
 pub trait IntoGEMObject: Sized + super::private::Sealed + AlwaysRefCounted {
-    /// Owning driver for this type
-    type Driver: drm::Driver;
-
     /// Returns a reference to the raw `drm_gem_object` structure, which must be valid as long as
     /// this owning object is valid.
     fn as_raw(&self) -> *mut bindings::drm_gem_object;
@@ -74,25 +74,15 @@ unsafe impl<T: IntoGEMObject> AlwaysRefCounted for T {
     }
 }
 
-/// Trait which must be implemented by drivers using base GEM objects.
-pub trait DriverObject: BaseDriverObject<Object<Self>> {
-    /// Parent `Driver` for this object.
-    type Driver: drm::Driver;
-}
-
-extern "C" fn open_callback<T: BaseDriverObject<U>, U: BaseObject>(
+extern "C" fn open_callback<T: DriverObject>(
     raw_obj: *mut bindings::drm_gem_object,
     raw_file: *mut bindings::drm_file,
 ) -> core::ffi::c_int {
     // SAFETY: `open_callback` is only ever called with a valid pointer to a `struct drm_file`.
-    let file = unsafe {
-        drm::File::<<<U as IntoGEMObject>::Driver as drm::Driver>::File>::from_raw(raw_file)
-    };
-    // SAFETY: `open_callback` is specified in the AllocOps structure for `Object<T>`, ensuring that
-    // `raw_obj` is indeed contained within a `Object<T>`.
-    let obj = unsafe {
-        <<<U as IntoGEMObject>::Driver as drm::Driver>::Object as IntoGEMObject>::from_raw(raw_obj)
-    };
+    let file = unsafe { drm::File::<<T::Driver as drm::Driver>::File>::from_raw(raw_file) };
+    // SAFETY: `open_callback` is specified in the AllocOps structure for `DriverObject<T>`,
+    // ensuring that `raw_obj` is contained within a `DriverObject<T>`
+    let obj = unsafe { <<T::Driver as drm::Driver>::Object as IntoGEMObject>::from_raw(raw_obj) };
 
     match T::open(obj, file) {
         Err(e) => e.to_errno(),
@@ -100,26 +90,21 @@ extern "C" fn open_callback<T: BaseDriverObject<U>, U: BaseObject>(
     }
 }
 
-extern "C" fn close_callback<T: BaseDriverObject<U>, U: BaseObject>(
+extern "C" fn close_callback<T: DriverObject>(
     raw_obj: *mut bindings::drm_gem_object,
     raw_file: *mut bindings::drm_file,
 ) {
     // SAFETY: `open_callback` is only ever called with a valid pointer to a `struct drm_file`.
-    let file = unsafe {
-        drm::File::<<<U as IntoGEMObject>::Driver as drm::Driver>::File>::from_raw(raw_file)
-    };
+    let file = unsafe { drm::File::<<T::Driver as drm::Driver>::File>::from_raw(raw_file) };
+
     // SAFETY: `close_callback` is specified in the AllocOps structure for `Object<T>`, ensuring
     // that `raw_obj` is indeed contained within a `Object<T>`.
-    let obj = unsafe {
-        <<<U as IntoGEMObject>::Driver as drm::Driver>::Object as IntoGEMObject>::from_raw(raw_obj)
-    };
+    let obj = unsafe { <<T::Driver as drm::Driver>::Object as IntoGEMObject>::from_raw(raw_obj) };
 
     T::close(obj, file);
 }
 
 impl<T: DriverObject> IntoGEMObject for Object<T> {
-    type Driver = T::Driver;
-
     fn as_raw(&self) -> *mut bindings::drm_gem_object {
         self.obj.get()
     }
@@ -141,10 +126,12 @@ pub trait BaseObject: IntoGEMObject {
 
     /// Creates a new handle for the object associated with a given `File`
     /// (or returns an existing one).
-    fn create_handle(
-        &self,
-        file: &drm::File<<<Self as IntoGEMObject>::Driver as drm::Driver>::File>,
-    ) -> Result<u32> {
+    fn create_handle<D, F>(&self, file: &drm::File<F>) -> Result<u32>
+    where
+        Self: AllocImpl<Driver = D>,
+        D: drm::Driver<Object = Self, File = F>,
+        F: drm::file::DriverFile<Driver = D>,
+    {
         let mut handle: u32 = 0;
         // SAFETY: The arguments are all valid per the type invariants.
         to_result(unsafe {
@@ -154,10 +141,12 @@ pub trait BaseObject: IntoGEMObject {
     }
 
     /// Looks up an object by its handle for a given `File`.
-    fn lookup_handle(
-        file: &drm::File<<<Self as IntoGEMObject>::Driver as drm::Driver>::File>,
-        handle: u32,
-    ) -> Result<ARef<Self>> {
+    fn lookup_handle<D, F>(file: &drm::File<F>, handle: u32) -> Result<ARef<Self>>
+    where
+        Self: AllocImpl<Driver = D>,
+        D: drm::Driver<Object = Self, File = F>,
+        F: drm::file::DriverFile<Driver = D>,
+    {
         // SAFETY: The arguments are all valid per the type invariants.
         let ptr = unsafe { bindings::drm_gem_object_lookup(file.as_raw().cast(), handle) };
         if ptr.is_null() {
@@ -212,8 +201,8 @@ impl<T: DriverObject> Object<T> {
 
     const OBJECT_FUNCS: bindings::drm_gem_object_funcs = bindings::drm_gem_object_funcs {
         free: Some(Self::free_callback),
-        open: Some(open_callback::<T, Object<T>>),
-        close: Some(close_callback::<T, Object<T>>),
+        open: Some(open_callback::<T>),
+        close: Some(close_callback::<T>),
         print_info: None,
         export: None,
         pin: None,
@@ -296,6 +285,8 @@ impl<T: DriverObject> Deref for Object<T> {
 }
 
 impl<T: DriverObject> AllocImpl for Object<T> {
+    type Driver = T::Driver;
+
     const ALLOC_OPS: AllocOps = AllocOps {
         gem_create_object: None,
         prime_handle_to_fd: None,