niri-config: add keep-max-bpc-unchanged option (#2195)

* niri-config: add disable-set-bpc option
setting bpc to 8 bricks some OLED displays driven by amdgpu

* change to keep-max-bpc-unchanged and add to wiki

* fmt

* Update wiki/Configuration:-Debug-Options.md

---------

Co-authored-by: Ivan Molodetskikh <yalterz@gmail.com>

niri-config/src/lib.rs

@@ -2339,6 +2339,8 @@ pub struct DebugConfig {
     #[knuffel(child)]
     pub disable_direct_scanout: bool,
     #[knuffel(child)]
+    pub keep_max_bpc_unchanged: bool,
+    #[knuffel(child)]
     pub restrict_primary_scanout_to_matching_format: bool,
     #[knuffel(child, unwrap(argument))]
     pub render_drm_device: Option<PathBuf>,
@@ -5434,6 +5436,7 @@ mod tests {
                 enable_overlay_planes: false,
                 disable_cursor_plane: false,
                 disable_direct_scanout: false,
+                keep_max_bpc_unchanged: false,
                 restrict_primary_scanout_to_matching_format: false,
                 render_drm_device: Some(
                     "/dev/dri/renderD129",

src/backend/tty.rs

@@ -857,10 +857,13 @@ impl Tty {
         }
         debug!("picking mode: {mode:?}");
 
-        // We only use 8888 RGB formats, so set max bpc to 8 to allow more types of links to run.
-        match set_max_bpc(&device.drm, connector.handle(), 8) {
-            Ok(bpc) => debug!("set max bpc to {bpc}"),
-            Err(err) => debug!("error setting max bpc: {err:?}"),
+        if !niri.config.borrow().debug.keep_max_bpc_unchanged {
+            // We only use 8888 RGB formats, so set max bpc to 8 to allow more types of links to
+            // run.
+            match set_max_bpc(&device.drm, connector.handle(), 8) {
+                Ok(bpc) => debug!("set max bpc to {bpc}"),
+                Err(err) => debug!("error setting max bpc: {err:?}"),
+            }
         }
 
         let mut gamma_props = GammaProps::new(&device.drm, crtc)

wiki/Configuration:-Debug-Options.md

@@ -30,6 +30,7 @@ debug {
     honor-xdg-activation-with-invalid-serial
     skip-cursor-only-updates-during-vrr
     deactivate-unfocused-windows
+    keep-max-bpc-unchanged
 }
 
 binds {
@@ -292,6 +293,23 @@ debug {
 }
 ```
 
+### `keep-max-bpc-unchanged`
+
+<sup>Since: next release</sup>
+
+When connecting monitors, niri sets their max bpc to 8 in order to reduce display bandwidth and to potentially allow more monitors to be connected at once.
+Restricting bpc to 8 is not a problem since we don't support HDR or color management yet and can't really make use of higher bpc.
+
+Apparently, setting max bpc to 8 breaks some displays driven by AMDGPU.
+If this happens to you, set this debug flag, which will prevent niri from changing max bpc.
+AMDGPU bug report: https://gitlab.freedesktop.org/drm/amd/-/issues/4487.
+
+```kdl
+debug {
+    keep-max-bpc-unchanged
+}
+```
+
 ### Key Bindings
 
 These are not debug options, but rather key bindings.