Default to unrestricted primary plane scanout

niri-config/src/lib.rs

@@ -1641,6 +1641,8 @@ pub struct DebugConfig {
     pub disable_cursor_plane: bool,
     #[knuffel(child)]
     pub disable_direct_scanout: bool,
+    #[knuffel(child)]
+    pub restrict_primary_scanout_to_matching_format: bool,
     #[knuffel(child, unwrap(argument))]
     pub render_drm_device: Option<PathBuf>,
     #[knuffel(child)]

src/backend/tty.rs

@@ -1336,21 +1336,29 @@ impl Tty {
 
         // Overlay planes are disabled by default as they cause weird performance issues on my
         // system.
-        let mut flags =
-            FrameFlags::ALLOW_PRIMARY_PLANE_SCANOUT | FrameFlags::ALLOW_CURSOR_PLANE_SCANOUT;
-        {
+        let flags = {
             let debug = &self.config.borrow().debug;
+
+            let primary_scanout_flag = if debug.restrict_primary_scanout_to_matching_format {
+                FrameFlags::ALLOW_PRIMARY_PLANE_SCANOUT
+            } else {
+                FrameFlags::ALLOW_PRIMARY_PLANE_SCANOUT_ANY
+            };
+            let mut flags = primary_scanout_flag | FrameFlags::ALLOW_CURSOR_PLANE_SCANOUT;
+
             if debug.enable_overlay_planes {
                 flags.insert(FrameFlags::ALLOW_OVERLAY_PLANE_SCANOUT);
             }
             if debug.disable_direct_scanout {
-                flags.remove(FrameFlags::ALLOW_PRIMARY_PLANE_SCANOUT);
+                flags.remove(primary_scanout_flag);
                 flags.remove(FrameFlags::ALLOW_OVERLAY_PLANE_SCANOUT);
             }
             if debug.disable_cursor_plane {
                 flags.remove(FrameFlags::ALLOW_CURSOR_PLANE_SCANOUT);
             }
-        }
+
+            flags
+        };
 
         // Hand them over to the DRM.
         let drm_compositor = &mut surface.compositor;

wiki/Configuration:-Debug-Options.md

@@ -16,6 +16,7 @@ debug {
     enable-overlay-planes
     disable-cursor-plane
     disable-direct-scanout
+    restrict-primary-scanout-to-matching-format
     render-drm-device "/dev/dri/renderD129"
     force-pipewire-invalid-modifier
     dbus-interfaces-in-non-session-instances
@@ -84,6 +85,20 @@ debug {
 }
 ```
 
+### `restrict-primary-scanout-to-matching-format`
+
+Restricts direct scanout to the primary plane to when the window buffer exactly matches the composition swapchain format.
+
+This flag may prevent unexpected bandwidth changes when going between composition and scanout.
+The plan is to make it default in the future, when we implement a way to tell the clients the composition swapchain format.
+As is, it may prevent some clients (mpv on my machine) from scanning out to the primary plane.
+
+```kdl
+debug {
+    restrict-primary-scanout-to-matching-format
+}
+```
+
 ### `render-drm-device`
 
 Override the DRM device that niri will use for all rendering.