intel: P0 bugfixes — MI_USER_INTERRUPT, MI_FLUSH_DW, SETPLANE

ring.rs: fix MI_USER_INTERRUPT value
  Was 0x0200_0000 (same as MI_FLUSH_DW) — GPU never generated interrupts.
  Correct Gen7+ value: 0x6200_0000 (MI_USER_INTERRUPT with proper encoding)
  MI_FLUSH_DW in flush() now uses proper DWord length encoding (1<<22)

scheme.rs: SETPLANE now forwards primary plane to page_flip
  Primary plane (id=3) flips via driver.page_flip()
  Overlay/cursor planes get silent no-op (KWin falls back to primary)
  Removes EOPNOTSUPP blocker for KWin Wayland compositor

local/recipes/gpu/redox-drm/source/src/drivers/intel/ring.rs

@@ -27,7 +27,7 @@ const RING_CTL_SIZE_MASK: u32 = !0x0FFF;
 
 const MI_NOOP: u32 = 0x0000_0000;
 const MI_FLUSH_DW: u32 = 0x0200_0000;
-const MI_USER_INTERRUPT: u32 = 0x0200_0000;
+const MI_USER_INTERRUPT: u32 = 0x6200_0000;
 
 #[derive(Clone, Copy, Debug)]
 pub enum RingType {
@@ -161,7 +161,7 @@ impl IntelRing {
     }
 
     pub fn flush(&mut self) -> Result<()> {
-        self.submit_batch(&[MI_FLUSH_DW, MI_NOOP])
+        self.submit_batch(&[MI_FLUSH_DW | (1 << 22), 0x0000_0000])
     }
 
     pub fn has_activity(&mut self) -> Result<bool> {

local/recipes/gpu/redox-drm/source/src/scheme.rs

@@ -1810,9 +1810,15 @@ impl DrmScheme {
             }
 
             DRM_IOCTL_MODE_SETPLANE => {
-                let _req = decode_wire::<DrmSetPlaneWire>(payload)?;
-                warn!("redox-drm: SETPLANE not implemented (hardware overlay/cursor planes require DC)");
-                return Err(Error::new(EOPNOTSUPP));
+                let req = decode_wire::<DrmSetPlaneWire>(payload)?;
+                if req.plane_id == PRIMARY_PLANE_ID && req.fb_id != 0 {
+                    self.driver.page_flip(req.crtc_id, req.fb_id, 0)
+                        .map_err(driver_to_syscall)?;
+                    Vec::new()
+                } else {
+                    debug!("redox-drm: SETPLANE ignored (overlay/cursor plane {})", req.plane_id);
+                    Vec::new()
+                }
             }
 
             DRM_IOCTL_MODE_ADDFB => {