WIP backend-drm: set connector max bpc

When enabling HDR or WCG modes, 8 bits per channel is not enough. Try to
push the cable as high as it can, up to 16.

Theoretically, this should avoid seeing banding when HDR mode is
correctly programmed, but I'm not that far yet.

This also makes SDR mode to explicitly program max bpc to 8. That's what
one would expect to use, so might as well make that, rather than rely on
luck.

TODO: wayland/weston#612



Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>

libweston/backend-drm/drm-internal.h

@@ -187,6 +187,7 @@ enum wdrm_connector_property {
 	WDRM_CONNECTOR_HDCP_CONTENT_TYPE,
 	WDRM_CONNECTOR_PANEL_ORIENTATION,
 	WDRM_CONNECTOR_HDR_OUTPUT_METADATA,
+	WDRM_CONNECTOR_MAX_BPC,
 	WDRM_CONNECTOR__COUNT
 };
 

libweston/backend-drm/kms.c

@@ -146,6 +146,7 @@ const struct drm_property_info connector_props[] = {
 	[WDRM_CONNECTOR_HDR_OUTPUT_METADATA] = {
 		.name = "HDR_OUTPUT_METADATA",
 	},
+	[WDRM_CONNECTOR_MAX_BPC] = { .name = "max bpc", },
 };
 
 const struct drm_property_info crtc_props[] = {
@@ -924,6 +925,35 @@ drm_connector_set_hdcp_property(struct drm_connector *connector,
 	assert(ret == 0);
 }
 
+static int
+drm_connector_set_max_bpc(struct drm_connector *connector,
+			  struct drm_output *output,
+			  drmModeAtomicReq *req)
+{
+	const struct drm_property_info *info;
+	uint64_t max_bpc;
+	uint64_t a, b;
+
+	if (!drm_connector_has_prop(connector, WDRM_CONNECTOR_MAX_BPC))
+		return 0;
+
+	info = &connector->props[WDRM_CONNECTOR_MAX_BPC];
+	assert(info->flags & DRM_MODE_PROP_RANGE);
+	assert(info->num_range_values == 2);
+	a = info->range_values[0];
+	b = info->range_values[1];
+	assert(a <= b);
+
+	/* HDR modes need more bits, SDR mode is happy with 8 bits */
+	if (output->base.eotf_mode == WESTON_EOTF_MODE_SDR)
+		max_bpc = MIN(MAX(8, a), b);
+	else
+		max_bpc = MIN(MAX(16, a), b);
+
+	return connector_add_prop(req, connector,
+				  WDRM_CONNECTOR_MAX_BPC, max_bpc);
+}
+
 static int
 drm_output_apply_state_atomic(struct drm_output_state *state,
 			      drmModeAtomicReq *req,
@@ -983,6 +1013,8 @@ drm_output_apply_state_atomic(struct drm_output_state *state,
 						  WDRM_CONNECTOR_HDR_OUTPUT_METADATA,
 						  output->hdr_output_metadata_blob_id);
 		}
+
+		ret |= drm_connector_set_max_bpc(&head->connector, output, req);
 	}
 
 	if (ret != 0) {