[WIP RFC v2 22/35] rust: drm/kms: Add DriverPlane::atomic_update()

[Lyude Paul <lyude@redhat.com>]

A mandatory trait method used for implementing DRM's atomic plane update
callback.

Signed-off-by: Lyude Paul <lyude@redhat.com>
---
 rust/kernel/drm/kms/plane.rs | 39 +++++++++++++++++++++++++++++++++++-
 1 file changed, 38 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/drm/kms/plane.rs b/rust/kernel/drm/kms/plane.rs
index d6e11a65cc101..506ed5ced1270 100644
--- a/rust/kernel/drm/kms/plane.rs
+++ b/rust/kernel/drm/kms/plane.rs
@@ -75,7 +75,7 @@ pub trait DriverPlane: Send + Sync + Sized {
             begin_fb_access: None, // TODO: someday?
             end_fb_access: None, // TODO: someday?
             atomic_check: None,
-            atomic_update: None,
+            atomic_update: if Self::HAS_ATOMIC_UPDATE { Some(atomic_update_callback::<Self>) } else { None },
             atomic_enable: None, // TODO
             atomic_disable: None, // TODO
             atomic_async_check: None, // TODO
@@ -103,6 +103,21 @@ pub trait DriverPlane: Send + Sync + Sized {
     ///
     /// Drivers may use this to instantiate their [`DriverPlane`] object.
     fn new(device: &Device<Self::Driver>, args: Self::Args) -> impl PinInit<Self, Error>;
+
+    /// The optional [`drm_plane_helper_funcs.atomic_update`] hook for this plane.
+    ///
+    /// Drivers may use this to customize the atomic update phase of their [`Plane`] objects. If not
+    /// specified, this function is a no-op.
+    ///
+    /// [`drm_plane_helper_funcs.atomic_update`]: srctree/include/drm/drm_modeset_helper_vtables.h
+    fn atomic_update(
+        plane: &Plane<Self>,
+        new_state: BorrowedPlaneState<'_, PlaneState<Self::State>>,
+        old_state: &PlaneState<Self::State>,
+        state: &AtomicStateMutator<Self::Driver>
+    ) {
+        build_error::build_error("This should not be reachable")
+    }
 }
 
 /// The generated C vtable for a [`DriverPlane`].
@@ -757,3 +772,25 @@ fn deref_mut(&mut self) -> &mut Self::Target {
     // - The cast to `drm_plane_state` is safe via `PlaneState`s type invariants.
     unsafe { bindings::__drm_atomic_helper_plane_reset(plane, Box::into_raw(new).cast()) };
 }
+
+unsafe extern "C" fn atomic_update_callback<T: DriverPlane>(
+    plane: *mut bindings::drm_plane,
+    state: *mut bindings::drm_atomic_state,
+) {
+    // SAFETY:
+    // * We're guaranteed `plane` is of type `Plane<T>` via type invariants.
+    // * We're guaranteed by DRM that `plane` is pointing to a valid initialized state.
+    let plane = unsafe { Plane::from_raw(plane) };
+
+    // SAFETY: DRM guarantees `state` points to a valid `drm_atomic_state`
+    let state = unsafe { AtomicStateMutator::new(NonNull::new_unchecked(state)) };
+
+    // SAFETY: Since we are in the atomic update callback, we're guaranteed by DRM that both the old
+    // and new atomic state are present within `state`
+    let (old_state, new_state) = unsafe {(
+        state.get_old_plane_state(plane).unwrap_unchecked(),
+        state.get_new_plane_state(plane).unwrap_unchecked(),
+    )};
+
+    T::atomic_update(plane, new_state, old_state, &state);
+}
-- 
2.46.1