From 4f32a0b76f5a71bda761fb509c663a3ae9b8b90b Mon Sep 17 00:00:00 2001
From: Simon Ser <contact@emersion.fr>
Date: Fri, 29 May 2026 16:56:16 +0200
Subject: [PATCH] backend/drm: add support for LUMINANCE KMS property

References: https://lore.kernel.org/dri-devel/20260528054911.1513208-1-superm1@kernel.org/
---
 backend/drm/atomic.c             |  9 +++++++++
 backend/drm/drm.c                | 18 +++++++++++++++++-
 backend/drm/properties.c         |  1 +
 include/backend/drm/drm.h        |  2 ++
 include/backend/drm/properties.h |  1 +
 5 files changed, 30 insertions(+), 1 deletion(-)

diff --git a/backend/drm/atomic.c b/backend/drm/atomic.c
index daa8ba9bf..7a2ff5e37 100644
--- a/backend/drm/atomic.c
+++ b/backend/drm/atomic.c
@@ -394,6 +394,11 @@ bool drm_atomic_connector_prepare(struct wlr_drm_connector_state *state, bool mo
 		return false;
 	}
 
+	double brightness = (state->base->committed & WLR_OUTPUT_STATE_BRIGHTNESS) ?
+		state->base->brightness : output->brightness;
+	uint64_t luminance = conn->luminance_bounds[0] +
+		round(brightness * (conn->luminance_bounds[1] - conn->luminance_bounds[0]));
+
 	state->mode_id = mode_id;
 	state->gamma_lut = gamma_lut;
 	state->fb_damage_clips = fb_damage_clips;
@@ -401,6 +406,7 @@ bool drm_atomic_connector_prepare(struct wlr_drm_connector_state *state, bool mo
 	state->vrr_enabled = vrr_enabled;
 	state->colorspace = colorspace;
 	state->hdr_output_metadata = hdr_output_metadata;
+	state->luminance = luminance;
 	return true;
 }
 
@@ -573,6 +579,9 @@ static void atomic_connector_add(struct atomic *atom,
 	if (conn->props.hdr_output_metadata != 0) {
 		atomic_add(atom, conn->id, conn->props.hdr_output_metadata, state->hdr_output_metadata);
 	}
+	if (conn->props.luminance != 0) {
+		atomic_add(atom, conn->id, conn->props.luminance, state->luminance);
+	}
 	atomic_add(atom, crtc->id, crtc->props.mode_id, state->mode_id);
 	atomic_add(atom, crtc->id, crtc->props.active, active);
 	if (active) {
diff --git a/backend/drm/drm.c b/backend/drm/drm.c
index d2f75f71f..6f66d23e8 100644
--- a/backend/drm/drm.c
+++ b/backend/drm/drm.c
@@ -44,7 +44,8 @@ static const uint32_t COMMIT_OUTPUT_STATE =
 	WLR_OUTPUT_STATE_SIGNAL_TIMELINE |
 	WLR_OUTPUT_STATE_COLOR_TRANSFORM |
 	WLR_OUTPUT_STATE_IMAGE_DESCRIPTION |
-	WLR_OUTPUT_STATE_COLOR_REPRESENTATION;
+	WLR_OUTPUT_STATE_COLOR_REPRESENTATION |
+	WLR_OUTPUT_STATE_BRIGHTNESS;
 
 static const uint32_t SUPPORTED_OUTPUT_STATE =
 	WLR_OUTPUT_STATE_BACKEND_OPTIONAL | COMMIT_OUTPUT_STATE;
@@ -118,6 +119,12 @@ bool check_drm_features(struct wlr_drm_backend *drm) {
 		wlr_log(WLR_INFO, "DRM_CLIENT_CAP_CURSOR_PLANE_HOTSPOT supported");
 	}
 #endif
+#define DRM_CLIENT_CAP_LUMINANCE 8
+#ifdef DRM_CLIENT_CAP_LUMINANCE
+	if (drm->iface == &atomic_iface && drmSetClientCap(drm->fd, DRM_CLIENT_CAP_LUMINANCE, 1) == 0) {
+		wlr_log(WLR_INFO, "DRM_CLIENT_CAP_LUMINANCE supported");
+	}
+#endif
 
 	if (drm->iface == &legacy_iface) {
 		drm->supports_tearing_page_flips = drmGetCap(drm->fd, DRM_CAP_ASYNC_PAGE_FLIP, &cap) == 0 && cap == 1;
@@ -1736,6 +1743,15 @@ static bool connect_drm_connector(struct wlr_drm_connector *wlr_conn,
 	}
 	output->adaptive_sync_supported = vrr_capable;
 
+	if (wlr_conn->props.luminance != 0) {
+		if (introspect_drm_prop_range(drm->fd, wlr_conn->props.luminance,
+				&wlr_conn->luminance_bounds[0], &wlr_conn->luminance_bounds[1])) {
+			output->brightness_supported = true;
+		} else {
+			wlr_log(WLR_ERROR, "Failed to introspect 'LUMINANCE' property");
+		}
+	}
+
 	size_t edid_len = 0;
 	uint8_t *edid = get_drm_prop_blob(drm->fd,
 		wlr_conn->id, wlr_conn->props.edid, &edid_len);
diff --git a/backend/drm/properties.c b/backend/drm/properties.c
index 4c6bdcb36..9336209d4 100644
--- a/backend/drm/properties.c
+++ b/backend/drm/properties.c
@@ -26,6 +26,7 @@ static const struct prop_info connector_info[] = {
 	{ "DPMS", INDEX(dpms) },
 	{ "EDID", INDEX(edid) },
 	{ "HDR_OUTPUT_METADATA", INDEX(hdr_output_metadata) },
+	{ "LUMINANCE", INDEX(luminance) },
 	{ "PATH", INDEX(path) },
 	{ "content type", INDEX(content_type) },
 	{ "link-status", INDEX(link_status) },
diff --git a/include/backend/drm/drm.h b/include/backend/drm/drm.h
index a8c5e077a..ef294e1c5 100644
--- a/include/backend/drm/drm.h
+++ b/include/backend/drm/drm.h
@@ -164,6 +164,7 @@ struct wlr_drm_connector_state {
 	bool vrr_enabled;
 	uint32_t colorspace;
 	uint32_t hdr_output_metadata;
+	uint64_t luminance;
 };
 
 /**
@@ -199,6 +200,7 @@ struct wlr_drm_connector {
 	drmModeConnection status;
 	uint32_t id;
 	uint64_t max_bpc_bounds[2];
+	uint64_t luminance_bounds[2];
 	struct wlr_drm_lease *lease;
 
 	struct wlr_drm_crtc *crtc;
diff --git a/include/backend/drm/properties.h b/include/backend/drm/properties.h
index 20e0a5bff..bd6bed641 100644
--- a/include/backend/drm/properties.h
+++ b/include/backend/drm/properties.h
@@ -28,6 +28,7 @@ struct wlr_drm_connector_props {
 	uint32_t crtc_id;
 	uint32_t colorspace;
 	uint32_t hdr_output_metadata;
+	uint32_t luminance;
 };
 
 struct wlr_drm_crtc_props {