drm: Add connector property to limit max bpc

At times 12bpc HDMI cannot be driven due to faulty cables, dongles
level shifters etc. To workaround them we may need to drive the output
at a lower bpc. Currently the user space does not have a way to limit
the bpc. The default bpc to be programmed is decided by the driver and
is run against connector limitations.

Creating a new connector property "max bpc" in order to limit the bpc.
xrandr can make use of this connector property to make sure that bpc does
not exceed the configured value. This property can be used by userspace to
set the bpc.

V2: Initialize max_bpc to satisfy kms_properties
V3: Move the property to drm_connector
V4: Split drm and i915 components(Ville)
V5: Make the property per connector(Ville)
V6: Compare the requested bpc to connector bpc(Daniel)
    Move the attach_property function to core(Ville)
V7: Fix checkpatch warnings
V8: Simplify the connector check code(Ville)
V9: Const display_info(Ville)
V10,V11: Fix CI issues.
V12: Add the Kernel documentation(Daniel)
V14: Crossreference the function name in the doc(Daniel)

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Kishore Kadiyala <kishore.kadiyala@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Manasi Navare <manasi.d.navare@intel.com>
Cc: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Cc: Sunpeng Li <sunpeng.li@amd.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Radhakrishna Sripada <radhakrishna.sripada@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181012184233.29250-1-radhakrishna.sripada@intel.com

drivers/gpu/drm/drm_atomic.c

@@ -390,6 +390,11 @@ static int drm_atomic_connector_check(struct drm_connector *connector,
 {
 	struct drm_crtc_state *crtc_state;
 	struct drm_writeback_job *writeback_job = state->writeback_job;
+	const struct drm_display_info *info = &connector->display_info;
+
+	state->max_bpc = info->bpc ? info->bpc : 8;
+	if (connector->max_bpc_property)
+		state->max_bpc = min(state->max_bpc, state->max_requested_bpc);
 
 	if ((connector->connector_type != DRM_MODE_CONNECTOR_WRITEBACK) || !writeback_job)
 		return 0;

drivers/gpu/drm/drm_atomic_helper.c

@@ -658,6 +658,10 @@ drm_atomic_helper_check_modeset(struct drm_device *dev,
 			if (old_connector_state->link_status !=
 			    new_connector_state->link_status)
 				new_crtc_state->connectors_changed = true;
+
+			if (old_connector_state->max_requested_bpc !=
+			    new_connector_state->max_requested_bpc)
+				new_crtc_state->connectors_changed = true;
 		}
 
 		if (funcs->atomic_check)

drivers/gpu/drm/drm_atomic_uapi.c

@@ -740,6 +740,8 @@ static int drm_atomic_connector_set_property(struct drm_connector *connector,
 
 		return set_out_fence_for_connector(state->state, connector,
 						   fence_ptr);
+	} else if (property == connector->max_bpc_property) {
+		state->max_requested_bpc = val;
 	} else if (connector->funcs->atomic_set_property) {
 		return connector->funcs->atomic_set_property(connector,
 				state, property, val);
@@ -804,6 +806,8 @@ drm_atomic_connector_get_property(struct drm_connector *connector,
 		*val = 0;
 	} else if (property == config->writeback_out_fence_ptr_property) {
 		*val = 0;
+	} else if (property == connector->max_bpc_property) {
+		*val = state->max_requested_bpc;
 	} else if (connector->funcs->atomic_get_property) {
 		return connector->funcs->atomic_get_property(connector,
 				state, property, val);

drivers/gpu/drm/drm_connector.c

@@ -916,6 +916,13 @@ DRM_ENUM_NAME_FN(drm_get_content_protection_name, drm_cp_enum_list)
  *	  is no longer protected and userspace should take appropriate action
  *	  (whatever that might be).
  *
+ * max bpc:
+ *	This range property is used by userspace to limit the bit depth. When
+ *	used the driver would limit the bpc in accordance with the valid range
+ *	supported by the hardware and sink. Drivers to use the function
+ *	drm_connector_attach_max_bpc_property() to create and attach the
+ *	property to the connector during initialization.
+ *
  * Connectors also have one standardized atomic property:
  *
  * CRTC_ID:
@@ -1583,6 +1590,40 @@ void drm_connector_set_link_status_property(struct drm_connector *connector,
 }
 EXPORT_SYMBOL(drm_connector_set_link_status_property);
 
+/**
+ * drm_connector_attach_max_bpc_property - attach "max bpc" property
+ * @connector: connector to attach max bpc property on.
+ * @min: The minimum bit depth supported by the connector.
+ * @max: The maximum bit depth supported by the connector.
+ *
+ * This is used to add support for limiting the bit depth on a connector.
+ *
+ * Returns:
+ * Zero on success, negative errno on failure.
+ */
+int drm_connector_attach_max_bpc_property(struct drm_connector *connector,
+					  int min, int max)
+{
+	struct drm_device *dev = connector->dev;
+	struct drm_property *prop;
+
+	prop = connector->max_bpc_property;
+	if (!prop) {
+		prop = drm_property_create_range(dev, 0, "max bpc", min, max);
+		if (!prop)
+			return -ENOMEM;
+
+		connector->max_bpc_property = prop;
+	}
+
+	drm_object_attach_property(&connector->base, prop, max);
+	connector->state->max_requested_bpc = max;
+	connector->state->max_bpc = max;
+
+	return 0;
+}
+EXPORT_SYMBOL(drm_connector_attach_max_bpc_property);
+
 /**
  * drm_connector_init_panel_orientation_property -
  *	initialize the connecters panel_orientation property

include/drm/drm_connector.h

@@ -508,6 +508,18 @@ struct drm_connector_state {
 	 * drm_writeback_signal_completion()
 	 */
 	struct drm_writeback_job *writeback_job;
+
+	/**
+	 * @max_requested_bpc: Connector property to limit the maximum bit
+	 * depth of the pixels.
+	 */
+	u8 max_requested_bpc;
+
+	/**
+	 * @max_bpc: Connector max_bpc based on the requested max_bpc property
+	 * and the connector bpc limitations obtained from edid.
+	 */
+	u8 max_bpc;
 };
 
 /**
@@ -973,6 +985,12 @@ struct drm_connector {
 	 */
 	struct drm_property_blob *path_blob_ptr;
 
+	/**
+	 * @max_bpc_property: Default connector property for the max bpc to be
+	 * driven out of the connector.
+	 */
+	struct drm_property *max_bpc_property;
+
 #define DRM_CONNECTOR_POLL_HPD (1 << 0)
 #define DRM_CONNECTOR_POLL_CONNECT (1 << 1)
 #define DRM_CONNECTOR_POLL_DISCONNECT (1 << 2)
@@ -1268,6 +1286,8 @@ void drm_connector_set_link_status_property(struct drm_connector *connector,
 					    uint64_t link_status);
 int drm_connector_init_panel_orientation_property(
 	struct drm_connector *connector, int width, int height);
+int drm_connector_attach_max_bpc_property(struct drm_connector *connector,
+					  int min, int max);
 
 /**
  * struct drm_tile_group - Tile group metadata