Add emulate-zero-presentation-time debug flag

niri-config/src/lib.rs

@@ -810,6 +810,8 @@ pub struct DebugConfig {
     pub disable_cursor_plane: bool,
     #[knuffel(child, unwrap(argument))]
     pub render_drm_device: Option<PathBuf>,
+    #[knuffel(child)]
+    pub emulate_zero_presentation_time: bool,
 }
 
 impl Config {

resources/default-config.kdl

@@ -557,4 +557,9 @@ debug {
     // Enable the color-transformations capability of the Smithay renderer.
     // May cause a slight decrease in rendering performance.
     // enable-color-transformations-capability
+
+    // Emulate zero (unknown) presentation time returned from DRM.
+    // This is a thing on NVIDIA proprietary drivers, so this flag can be
+    // used to test that we don't break too hard on those systems.
+    // emulate-zero-presentation-time
 }

src/backend/tty.rs

@@ -902,6 +902,11 @@ impl Tty {
                 Duration::ZERO
             }
         };
+        let presentation_time = if niri.config.borrow().debug.emulate_zero_presentation_time {
+            Duration::ZERO
+        } else {
+            presentation_time
+        };
 
         let message = if presentation_time.is_zero() {
             format!("vblank on {name}, presentation time unknown")