[PATCH v2 1/2] Partially revert "rust: drm: gem: Implement AlwaysRefCounted for all gem objects automatically"

[Lyude Paul <lyude@redhat.com>]

Currently in order to implement AlwaysRefCounted for gem objects, we use a
blanket implementation:

  unsafe impl<T: IntoGEMObject> AlwaysRefCounted for T { … }

While this technically works, it comes with the rather unfortunate downside
that attempting to create a similar blanket implementation in any other
kernel crate will now fail in a rather confusing way.

Using an example from the (not yet upstream) rust DRM KMS bindings, if we
were to add:

  unsafe impl<T: RcModeObject> AlwaysRefCounted for T { … }

Then the moment that both blanket implementations are present in the same
kernel tree, compilation fails with the following:

   error[E0119]: conflicting implementations of trait `types::AlwaysRefCounted`
      --> rust/kernel/drm/kms.rs:504:1
       |
   504 | unsafe impl<T: RcModeObject> AlwaysRefCounted for T {
       | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation
       |
      ::: rust/kernel/drm/gem/mod.rs:97:1
       |
   97  | unsafe impl<T: IntoGEMObject> AlwaysRefCounted for T {
       | ---------------------------------------------------- first implementation here

So, revert these changes for now. The proper fix for this is to introduce a
macro for copy/pasting the same implementation of AlwaysRefCounted around.

This reverts commit 38cb08c3fcd3f3b1d0225dcec8ae50fab5751549.

Signed-off-by: Lyude Paul <lyude@redhat.com>

---
V2:
* Rewrite the commit message to explain a bit more why we don't want a
  blanket implementation for this.

---
 rust/kernel/drm/gem/mod.rs | 36 ++++++++++++++++--------------------
 1 file changed, 16 insertions(+), 20 deletions(-)

diff --git a/rust/kernel/drm/gem/mod.rs b/rust/kernel/drm/gem/mod.rs
index 6ccbb25628a1e..daa92f0eac68e 100644
--- a/rust/kernel/drm/gem/mod.rs
+++ b/rust/kernel/drm/gem/mod.rs
@@ -55,26 +55,6 @@ pub trait IntoGEMObject: Sized + super::private::Sealed + AlwaysRefCounted {
     unsafe fn from_raw<'a>(self_ptr: *mut bindings::drm_gem_object) -> &'a Self;
 }
 
-// SAFETY: All gem objects are refcounted.
-unsafe impl<T: IntoGEMObject> AlwaysRefCounted for 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: NonNull<Self>) {
-        // SAFETY: We either hold the only refcount on `obj`, or one of many - meaning that no one
-        // else could possibly hold a mutable reference to `obj` and thus this immutable reference
-        // is safe.
-        let obj = unsafe { obj.as_ref() }.as_raw();
-
-        // SAFETY:
-        // - The safety requirements guarantee that the refcount is non-zero.
-        // - We hold no references to `obj` now, making it safe for us to potentially deallocate it.
-        unsafe { bindings::drm_gem_object_put(obj) };
-    }
-}
-
 extern "C" fn open_callback<T: DriverObject>(
     raw_obj: *mut bindings::drm_gem_object,
     raw_file: *mut bindings::drm_file,
@@ -273,6 +253,22 @@ extern "C" fn free_callback(obj: *mut bindings::drm_gem_object) {
     }
 }
 
+// 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: 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()) }
+    }
+}
+
 impl<T: DriverObject> super::private::Sealed for Object<T> {}
 
 impl<T: DriverObject> Deref for Object<T> {

base-commit: cf4fd52e323604ccfa8390917593e1fb965653ee
-- 
2.51.0