drm/dp_mst: Fix GUID DPCD write to non-root MST branch devices

The return value on success of drm_dp_send_dpcd_write() called for
non-root MST branch devices from drm_dp_check_mstb_guid() is the number
of bytes transferred. Atm this return value (in case of a complete read)
will be regarded incorrectly as an error by the caller of
drm_dp_check_mstb_guid(). Fix this by converting the return value for a
complete read to the expected success code (0) and for a partial read to
a failure code (-EPROTO).

Fixes: 2554da0de3e8 ("drm/display: dp-mst-topology: use new DCPD access helpers")
Cc: Dmitry Baryshkov <[email protected]>
Cc: Lyude Paul <[email protected]>
Reviewed-by: Jani Nikula <[email protected]>
Reviewed-by: Lyude Paul <[email protected]>
Signed-off-by: Imre Deak <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

1 files changed, 2 insertions, 0 deletions

diff --git a/drivers/gpu/drm/display/drm_dp_mst_topology.c b/drivers/gpu/drm/display/drm_dp_mst_topology.c
index e8716e73480b..8c312c1f1724 100644
--- a/drivers/gpu/drm/display/drm_dp_mst_topology.c
+++ b/drivers/gpu/drm/display/drm_dp_mst_topology.c
@@ -2200,6 +2200,8 @@ static int drm_dp_check_mstb_guid(struct drm_dp_mst_branch *mstb, guid_t *guid)
 			ret = drm_dp_send_dpcd_write(mstb->mgr,
 						     mstb->port_parent,
 						     DP_GUID, sizeof(buf), buf);
+			if (ret >= 0)
+				ret = ret == sizeof(buf) ? 0 : -EPROTO;
 		} else {
 			ret = drm_dp_dpcd_write_data(mstb->mgr->aux,
 						     DP_GUID, buf, sizeof(buf));