[RFC PATCH v2 18/18] rust: drm: kms: add plane damage-clip accessors

[Mike Lothian <mike@fireburn.co.uk>]

A driver whose "scanout" is an expensive encode-and-send (like a USB
display) wants to repaint only the region a client marked dirty rather
than the whole framebuffer every flip. Add two RawPlaneState accessors
over the client's FB_DAMAGE_CLIPS (relative to the old plane state), both
intersected with the visible source area:

  - damage_merged(), wrapping drm_atomic_helper_damage_merged(): merges the
    clips into one rectangle and returns it as a new Rect type (with
    width()/height() helpers). None means nothing to repaint; if the client
    supplied no clips, the full plane rectangle is returned, so a driver can
    always treat Some as "repaint this box".

  - for_each_damage_clip(), driving drm_atomic_helper_damage_iter: invokes a
    closure once per individual clip (the rectangles damage_merged() collapses
    into one), so a driver that forwards each changed region separately -- e.g.
    over USB -- can repaint just those rectangles instead of their bounding box.

drm_atomic_helper_damage_{merged,iter_*} live in <drm/drm_damage_helper.h>,
so add that to bindings_helper.h.

Signed-off-by: Mike Lothian <mike@fireburn.co.uk>
Assisted-by: Claude:claude-opus-4-8 [Claude-Code]
---
 rust/bindings/bindings_helper.h |   1 +
 rust/kernel/drm/kms/plane.rs    | 105 ++++++++++++++++++++++++++++++++
 2 files changed, 106 insertions(+)

diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h
index 7d04abec1215..b8675642c1a8 100644
--- a/rust/bindings/bindings_helper.h
+++ b/rust/bindings/bindings_helper.h
@@ -39,6 +39,7 @@
 #include <drm/drm_atomic_helper.h>
 #include <drm/drm_connector.h>
 #include <drm/drm_crtc.h>
+#include <drm/drm_damage_helper.h>
 #include <drm/drm_blend.h>
 #include <drm/drm_edid.h>
 #include <drm/drm_encoder.h>
diff --git a/rust/kernel/drm/kms/plane.rs b/rust/kernel/drm/kms/plane.rs
index ead875316829..4ae11cec88a4 100644
--- a/rust/kernel/drm/kms/plane.rs
+++ b/rust/kernel/drm/kms/plane.rs
@@ -605,6 +605,49 @@ pub trait FromRawPlaneState: AsRawPlaneState {
     unsafe fn from_raw_mut<'a>(ptr: *mut bindings::drm_plane_state) -> &'a mut Self;
 }
 
+/// A rectangle in a plane's source (pixel) space, as produced by
+/// [`RawPlaneState::damage_merged`].
+///
+/// The box is inclusive on the top-left and exclusive on the bottom-right (`[x1, x2)` by
+/// `[y1, y2)`), matching [`struct drm_rect`].
+///
+/// [`struct drm_rect`]: srctree/include/drm/drm_rect.h
+#[derive(Copy, Clone, Debug, PartialEq, Eq)]
+pub struct Rect {
+    /// Left edge, inclusive.
+    pub x1: i32,
+    /// Top edge, inclusive.
+    pub y1: i32,
+    /// Right edge, exclusive.
+    pub x2: i32,
+    /// Bottom edge, exclusive.
+    pub y2: i32,
+}
+
+impl Rect {
+    #[inline]
+    fn from_raw(r: &bindings::drm_rect) -> Self {
+        Self {
+            x1: r.x1,
+            y1: r.y1,
+            x2: r.x2,
+            y2: r.y2,
+        }
+    }
+
+    /// The width of the rectangle in pixels.
+    #[inline]
+    pub fn width(&self) -> i32 {
+        self.x2 - self.x1
+    }
+
+    /// The height of the rectangle in pixels.
+    #[inline]
+    pub fn height(&self) -> i32 {
+        self.y2 - self.y1
+    }
+}
+
 /// Common methods available on any type which implements [`AsRawPlane`].
 ///
 /// This is implemented internally by DRM, and provides many of the basic methods for working with
@@ -691,6 +734,68 @@ fn atomic_helper_check<S, D>(
         })
     }
 
+    /// Merge all frame-damage clips on this (new) plane state -- relative to `old` -- into a
+    /// single bounding rectangle, intersected with the plane's visible source area.
+    ///
+    /// Returns [`None`] when the plane is not visible or there is nothing to update. If the client
+    /// supplied no explicit damage clips, the full plane rectangle is returned, so a driver can
+    /// always treat [`Some`] as "repaint this rectangle" and fall back to a full-frame update.
+    /// Coordinates are integer pixels in the plane's source space.
+    ///
+    /// [`drm_atomic_helper_damage_merged`]: srctree/include/drm/drm_damage_helper.h
+    fn damage_merged(&self, old: &impl AsRawPlaneState) -> Option<Rect> {
+        let mut rect = bindings::drm_rect {
+            x1: 0,
+            y1: 0,
+            x2: 0,
+            y2: 0,
+        };
+
+        // SAFETY:
+        // - `old` and `self` are valid initialized `drm_plane_state`s via their type invariants.
+        // - `drm_atomic_helper_damage_merged` only reads the two states (to gather the damage
+        //   clips and the source rectangle) and writes the merged result into `rect`; it does not
+        //   mutate the plane state, so deriving a `*mut` from our shared reference is sound.
+        let visible = unsafe {
+            bindings::drm_atomic_helper_damage_merged(
+                core::ptr::from_ref(old.as_raw()),
+                core::ptr::from_ref(self.as_raw()).cast_mut(),
+                &mut rect,
+            )
+        };
+
+        visible.then(|| Rect::from_raw(&rect))
+    }
+
+    /// Invoke `f` once per frame-damage clip on this (new) plane state relative to `old`, each
+    /// intersected with the plane's visible source area -- i.e. the individual rectangles that
+    /// [`Self::damage_merged`] collapses into one. If the client supplied no explicit damage clips,
+    /// `f` is called once with the full plane rectangle. Coordinates are integer pixels in the
+    /// plane's source space.
+    ///
+    /// This lets a driver forward each changed region separately (e.g. to a remote display) instead
+    /// of the bounding box of them all.
+    ///
+    /// [`drm_atomic_helper_damage_iter`]: srctree/include/drm/drm_damage_helper.h
+    fn for_each_damage_clip(&self, old: &impl AsRawPlaneState, mut f: impl FnMut(Rect)) {
+        let mut iter = bindings::drm_atomic_helper_damage_iter::default();
+        let mut clip = bindings::drm_rect::default();
+        // SAFETY:
+        // - `old` and `self` are valid initialized `drm_plane_state`s via their type invariants.
+        // - `drm_atomic_helper_damage_iter_init` only reads the two states to set up `iter`, and
+        //   `_next` only reads `iter` and writes `clip`; neither escapes a pointer.
+        unsafe {
+            bindings::drm_atomic_helper_damage_iter_init(
+                &mut iter,
+                core::ptr::from_ref(old.as_raw()),
+                core::ptr::from_ref(self.as_raw()),
+            );
+            while bindings::drm_atomic_helper_damage_iter_next(&mut iter, &mut clip) {
+                f(Rect::from_raw(&clip));
+            }
+        }
+    }
+
     /// Return the framebuffer currently set for this plane state
     #[inline]
     fn framebuffer<D>(&self) -> Option<&Framebuffer<D>>
-- 
2.55.0