[PATCH] drm/dp_mst: Fix drm_dp_send_dpcd_write() return code

[Lyude Paul]

drm_dp_mst_wait_tx_reply() returns > 1 if time elapsed in
wait_event_timeout() before check_txmsg_state(mgr, txmsg) evaluated to
true. However, we make the mistake of returning this time from
drm_dp_send_dpcd_write() on success instead of returning the number of
bytes written - causing spontaneous failures during link probing:

[drm:drm_dp_send_link_address [drm_kms_helper]] *ERROR* GUID check on
10:01 failed: 3975

Yikes! So, fix this by returning the number of bytes written on success
instead.

Signed-off-by: Lyude Paul <lyude at redhat.com>
Fixes: cb897542c6d2 ("drm/dp_mst: Fix W=1 warnings")
Cc: Benjamin Gaignard <benjamin.gaignard at st.com>
Cc: Sean Paul <sean at poorly.run>
---
 drivers/gpu/drm/drm_dp_mst_topology.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/drm_dp_mst_topology.c b/drivers/gpu/drm/drm_dp_mst_topology.c
index 03a1496f6120..21dc78cb4ba6 100644
--- a/drivers/gpu/drm/drm_dp_mst_topology.c
+++ b/drivers/gpu/drm/drm_dp_mst_topology.c
@@ -3436,8 +3436,12 @@ static int drm_dp_send_dpcd_write(struct drm_dp_mst_topology_mgr *mgr,
 	drm_dp_queue_down_tx(mgr, txmsg);
 
 	ret = drm_dp_mst_wait_tx_reply(mstb, txmsg);
-	if (ret > 0 && txmsg->reply.reply_type == DP_SIDEBAND_REPLY_NAK)
-		ret = -EIO;
+	if (ret > 0) {
+		if (txmsg->reply.reply_type == DP_SIDEBAND_REPLY_NAK)
+			ret = -EIO;
+		else
+			ret = size;
+	}
 
 	kfree(txmsg);
 fail_put:
-- 
2.25.3