Don't change the max_bpc connector prop if mode=current.

As things are, even when mode=current is specified on the .ini file,
a full modeset is needed (and done), which causes a very noticeable
screen blinking. That is because setting the max_bpc on a connector
needs full modesetting.
The idea here is that if mode=current on the .ini, no modesetting
should be done, so the current max_bpc is programmed into the
connector.
But if a custom max-bpc=... is specified, that will be used instead,
even if mode=current on the .ini

Fixes: https://gitlab.freedesktop.org/wayland/weston/-/issues/660

Signed-off-by: vanfanel <redwindwanderer@gmail.com>
(cherry picked from commit 3240ccc69d1488003c1cfc36d23750145d4f13f7)

compositor/main.c

@@ -2051,7 +2051,8 @@ drm_backend_output_configure(struct weston_output *output,
 	enum weston_drm_backend_output_mode mode =
 		WESTON_DRM_BACKEND_OUTPUT_PREFERRED;
 	uint32_t transform = WL_OUTPUT_TRANSFORM_NORMAL;
-	uint32_t max_bpc;
+	uint32_t max_bpc = 0;
+	bool max_bpc_specified = false;
 	char *s;
 	char *modeline = NULL;
 	char *gbm_format = NULL;
@@ -2063,16 +2064,19 @@ drm_backend_output_configure(struct weston_output *output,
 		return -1;
 	}
 
-	weston_config_section_get_uint(section, "max-bpc", &max_bpc, 16);
-	api->set_max_bpc(output, max_bpc);
-
 	weston_config_section_get_string(section, "mode", &s, "preferred");
+	if (weston_config_section_get_uint(section, "max-bpc", &max_bpc, 16) == 0)
+		max_bpc_specified = true;
 
 	if (strcmp(s, "off") == 0) {
 		assert(0 && "off was supposed to be pruned");
 		return -1;
 	} else if (wet->drm_use_current_mode || strcmp(s, "current") == 0) {
 		mode = WESTON_DRM_BACKEND_OUTPUT_CURRENT;
+		/* If mode=current and no max-bpc was specfied on the .ini file,
+		   use current max_bpc so full modeset is not done. */
+		if (!max_bpc_specified) 
+			max_bpc = 0;
 	} else if (strcmp(s, "preferred") != 0) {
 		modeline = s;
 		s = NULL;
@@ -2086,6 +2090,8 @@ drm_backend_output_configure(struct weston_output *output,
 	}
 	free(modeline);
 
+	api->set_max_bpc(output, max_bpc);
+
 	if (count_remaining_heads(output, NULL) == 1) {
 		struct weston_head *head = weston_output_get_first_head(output);
 		transform = weston_head_get_transform(head);

include/libweston/backend-drm.h

@@ -84,7 +84,8 @@ struct weston_drm_output_api {
 	 * The property is used for working around faulty sink hardware like
 	 * monitors or media converters that mishandle the kernel driver
 	 * chosen bits-per-channel on the physical link. When having trouble,
-	 * try a lower value like 8.
+	 * try a lower value like 8. A value of 0 means that the current max
+	 * bpc will be reprogrammed.
 	 *
 	 * The value actually used in KMS is silently clamped to the range the
 	 * KMS driver claims to support. The default value is 16.

libweston/backend-drm/drm-internal.h

@@ -517,6 +517,7 @@ struct drm_head {
 	struct backlight *backlight;
 
 	drmModeModeInfo inherited_mode;	/**< Original mode on the connector */
+	uint32_t inherited_max_bpc;	/**< Original max_bpc on the connector */
 	uint32_t inherited_crtc_id;	/**< Original CRTC assignment */
 };
 

libweston/backend-drm/drm.c

@@ -1388,6 +1388,12 @@ drm_head_read_current_setup(struct drm_head *head, struct drm_device *device)
 		drmModeFreeCrtc(crtc);
 	}
 
+	/* Get the current max_bpc that's currently configured to
+	 * this connector. */
+	head->inherited_max_bpc = drm_property_get_value(
+		&head->connector.props[WDRM_CONNECTOR_MAX_BPC],
+		head->connector.props_drm, 0);
+
 	return 0;
 }
 

libweston/backend-drm/kms.c

@@ -912,20 +912,29 @@ drm_connector_set_max_bpc(struct drm_connector *connector,
 			  drmModeAtomicReq *req)
 {
 	const struct drm_property_info *info;
+	struct drm_head *head;
+	struct drm_backend *backend = output->device->backend;
 	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);
+	if (output->max_bpc == 0) {
+		/* A value of 0 means that the current max_bpc must be programmed. */
+		head = drm_head_find_by_connector(backend, connector->connector_id);
+		max_bpc = head->inherited_max_bpc;
+	} else {
+		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);
+
+		max_bpc = MAX(a, MIN(output->max_bpc, b));
+	}
 
-	max_bpc = MAX(a, MIN(output->max_bpc, b));
 	return connector_add_prop(req, connector,
 				  WDRM_CONNECTOR_MAX_BPC, max_bpc);
 }

man/weston-drm.man

@@ -172,7 +172,7 @@ silenty clamped to the hardware driver supported range. This artificially
 limits the driver chosen link bits-per-channel which may be useful for working
 around sink hardware (e.g. monitor) limitations. The default is 16 which is
 practically unlimited. If you need to work around hardware issues, try a lower
-value like 8.
+value like 8. A value of 0 means that the current max bpc will be reprogrammed.
 
 .SS Section remote-output
 .TP