drm/dp_mst: add passthrough_aux to struct drm_dp_mst_port

Currently, there is no way to identify if DSC pass-through can be
enabled and what aux DSC pass-through requests ought to be sent to. So,
add a variable to struct drm_dp_mst_port that keeps track of the
aforementioned information.
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/display/drm_dp_mst_topology.c

@@ -5908,8 +5908,10 @@ struct drm_dp_aux *drm_dp_mst_dsc_aux_for_port(struct drm_dp_mst_port *port)
 		/* Enpoint decompression with DP-to-DP peer device */
 		if ((endpoint_dsc & DP_DSC_DECOMPRESSION_IS_SUPPORTED) &&
 		    (endpoint_fec & DP_FEC_CAPABLE) &&
-		    (upstream_dsc & 0x2) /* DSC passthrough */)
+		    (upstream_dsc & DP_DSC_PASSTHROUGH_IS_SUPPORTED)) {
+			port->passthrough_aux = &immediate_upstream_port->aux;
 			return &port->aux;
+		}
 
 		/* Virtual DPCD decompression with DP-to-DP peer device */
 		return &immediate_upstream_port->aux;

include/drm/display/drm_dp.h

@@ -239,6 +239,7 @@
 
 #define DP_DSC_SUPPORT                      0x060   /* DP 1.4 */
 # define DP_DSC_DECOMPRESSION_IS_SUPPORTED  (1 << 0)
+# define DP_DSC_PASSTHROUGH_IS_SUPPORTED    (1 << 1)
 
 #define DP_DSC_REV                          0x061
 # define DP_DSC_MAJOR_MASK                  (0xf << 0)

include/drm/display/drm_dp_mst_helper.h

@@ -86,6 +86,8 @@ struct drm_dp_vcpi {
  * @next: link to next port on this branch device
  * @aux: i2c aux transport to talk to device connected to this port, protected
  * by &drm_dp_mst_topology_mgr.base.lock.
+ * @passthrough_aux: parent aux to which DSC pass-through requests should be
+ * sent, only set if DSC pass-through is possible.
  * @parent: branch device parent of this port
  * @vcpi: Virtual Channel Payload info for this port.
  * @connector: DRM connector this port is connected to. Protected by
@@ -140,6 +142,7 @@ struct drm_dp_mst_port {
 	 */
 	struct drm_dp_mst_branch *mstb;
 	struct drm_dp_aux aux; /* i2c bus for this port? */
+	struct drm_dp_aux *passthrough_aux;
 	struct drm_dp_mst_branch *parent;
 
 	struct drm_dp_vcpi vcpi;