drm/dp_mst: Fix DSC decompression detection in Synaptics branch devices

mainline inclusion
from mainline-v6.12-rc2
category: bugfix

Atm it's assumed that all Synaptics MST branch devices support DSC,
which is not exposed via a DP-to-DP peer device, rather a control flag
in the branch device's UFP DPCD applying to all the streams going
through it. This isn't true for all adapters with this branch device
though (for instance the Cakitte USBC->2xHDMI adapter reported in the
Closes link below doesn't support DSC).

Fix the above by advertising the DSC decompression support only if the
capability flag for this in the UFP DPCD is actually set.

Cc: Lyude Paul <lyude@redhat.com>
Closes: https://gitlab.freedesktop.org/drm/i915/kernel/-/issues/12047
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240909144650.2931258-1-imre.deak@intel.com
(cherry picked from commit 70a6587
 https://anongit.freedesktop.org/git/drm-misc.git drm-misc-fixes)

Signed-off-by: Ap, Kamal <kamal.ap@intel.corp-partner.google.com>
Reviewed-by: Shyam Sundar Radjacoumar <ssradjacoumar@google.com>
Reviewed-by: Sean Paul <sean@poorly.run>
Reviewed-by: Drew Davenport <ddavenport@chromium.org>
(cherry picked from commit 1b9ebf7a6d8daa6bbd30449a28cc60dd11d12d02)
Signed-off-by: Wentao Guan <guanwentao@uniontech.com>

Author: ideak

drivers/gpu/drm/display/drm_dp_mst_topology.c

@@ -6109,6 +6109,7 @@ struct drm_dp_aux *drm_dp_mst_dsc_aux_for_port(struct drm_dp_mst_port *port)
 	struct drm_dp_aux *immediate_upstream_aux;
 	struct drm_dp_mst_port *fec_port;
 	struct drm_dp_desc desc = {};
+	u8 upstream_dsc;
 	u8 endpoint_fec;
 	u8 endpoint_dsc;
 
@@ -6135,8 +6136,6 @@ struct drm_dp_aux *drm_dp_mst_dsc_aux_for_port(struct drm_dp_mst_port *port)
 
 	/* DP-to-DP peer device */
 	if (drm_dp_mst_is_virtual_dpcd(immediate_upstream_port)) {
-		u8 upstream_dsc;
-
 		if (drm_dp_dpcd_read(&port->aux,
 				     DP_DSC_SUPPORT, &endpoint_dsc, 1) != 1)
 			return NULL;
@@ -6182,6 +6181,13 @@ struct drm_dp_aux *drm_dp_mst_dsc_aux_for_port(struct drm_dp_mst_port *port)
 	if (drm_dp_has_quirk(&desc, DP_DPCD_QUIRK_DSC_WITHOUT_VIRTUAL_DPCD)) {
 		u8 dpcd_ext[DP_RECEIVER_CAP_SIZE];
 
+		if (drm_dp_dpcd_read(immediate_upstream_aux,
+				     DP_DSC_SUPPORT, &upstream_dsc, 1) != 1)
+			return NULL;
+
+		if (!(upstream_dsc & DP_DSC_DECOMPRESSION_IS_SUPPORTED))
+			return NULL;
+
 		if (drm_dp_read_dpcd_caps(immediate_upstream_aux, dpcd_ext) < 0)
 			return NULL;