Add disable-direct-scanout debug flag

niri-config/src/lib.rs

@@ -977,6 +977,8 @@ pub struct DebugConfig {
     pub enable_overlay_planes: bool,
     #[knuffel(child)]
     pub disable_cursor_plane: bool,
+    #[knuffel(child)]
+    pub disable_direct_scanout: bool,
     #[knuffel(child, unwrap(argument))]
     pub render_drm_device: Option<PathBuf>,
     #[knuffel(child)]

src/backend/mod.rs

@@ -144,6 +144,13 @@ impl Backend {
         }
     }
 
+    pub fn on_debug_config_changed(&mut self) {
+        match self {
+            Backend::Tty(tty) => tty.on_debug_config_changed(),
+            Backend::Winit(_) => (),
+        }
+    }
+
     pub fn tty(&mut self) -> &mut Tty {
         if let Self::Tty(v) = self {
             v

src/backend/tty.rs

@@ -900,6 +900,7 @@ impl Tty {
         if self.debug_tint {
             compositor.set_debug_flags(DebugFlags::TINT);
         }
+        compositor.use_direct_scanout(!config.debug.disable_direct_scanout);
 
         let mut dmabuf_feedback = None;
         if let Ok(primary_renderer) = self.gpu_manager.single_renderer(&self.primary_render_node) {
@@ -1717,6 +1718,19 @@ impl Tty {
         self.refresh_ipc_outputs(niri);
     }
 
+    pub fn on_debug_config_changed(&mut self) {
+        let config = self.config.borrow();
+        let debug = &config.debug;
+        let use_direct_scanout = !debug.disable_direct_scanout;
+
+        // FIXME: reload other flags if possible?
+        for device in self.devices.values_mut() {
+            for surface in device.surfaces.values_mut() {
+                surface.compositor.use_direct_scanout(use_direct_scanout);
+            }
+        }
+    }
+
     pub fn get_device_from_node(&mut self, node: DrmNode) -> Option<&mut OutputDevice> {
         self.devices.get_mut(&node)
     }

src/niri.rs

@@ -852,6 +852,7 @@ impl State {
         let mut libinput_config_changed = false;
         let mut output_config_changed = false;
         let mut window_rules_changed = false;
+        let mut debug_config_changed = false;
         let mut old_config = self.niri.config.borrow_mut();
 
         // Reload the cursor.
@@ -910,6 +911,10 @@ impl State {
             });
         }
 
+        if config.debug != old_config.debug {
+            debug_config_changed = true;
+        }
+
         *old_config = config;
 
         // Release the borrow.
@@ -978,6 +983,10 @@ impl State {
             }
         }
 
+        if debug_config_changed {
+            self.backend.on_debug_config_changed();
+        }
+
         if window_rules_changed {
             let _span = tracy_client::span!("recompute window rules");
 

wiki/Configuration:-Debug-Options.md

@@ -15,6 +15,7 @@ debug {
     // preview-render "screen-capture"
     enable-overlay-planes
     disable-cursor-plane
+    disable-direct-scanout
     render-drm-device "/dev/dri/renderD129"
     dbus-interfaces-in-non-session-instances
     wait-for-frame-completion-before-queueing
@@ -62,6 +63,16 @@ debug {
 }
 ```
 
+### `disable-direct-scanout`
+
+Disable direct scanout to both the primary plane and the overlay planes.
+
+```
+debug {
+    disable-direct-scanout
+}
+```
+
 ### `render-drm-device`
 
 Override the DRM device that niri will use for all rendering.