[PATCHv2 1/2] drm/display/dp: Check for MSTM_CAP before MSTM_CTRL write

[Arun R Murthy <arun.r.murthy@xxxxxxxxx>]

With DP2.1, multistream packetization and the underneth MST protocol
will be required for SST. So check for MSTM_CAP to see if MST is really
required and skip the MSTM_CTRL write so that we ensure that only the
underneth protocol and the multistream packetization will be enabled and
sink will not be confused by a corresponding dpcd write.

Signed-off-by: Arun R Murthy <arun.r.murthy@xxxxxxxxx>
---
 drivers/gpu/drm/display/drm_dp_mst_topology.c | 38 +++++++++++--------
 1 file changed, 22 insertions(+), 16 deletions(-)

diff --git a/drivers/gpu/drm/display/drm_dp_mst_topology.c b/drivers/gpu/drm/display/drm_dp_mst_topology.c
index 8ca01a6bf645..c5b3e51ea0c9 100644
--- a/drivers/gpu/drm/display/drm_dp_mst_topology.c
+++ b/drivers/gpu/drm/display/drm_dp_mst_topology.c
@@ -3666,12 +3666,14 @@ int drm_dp_mst_topology_mgr_set_mst(struct drm_dp_mst_topology_mgr *mgr, bool ms
 		mgr->mst_primary = mstb;
 		drm_dp_mst_topology_get_mstb(mgr->mst_primary);
 
-		ret = drm_dp_dpcd_writeb(mgr->aux, DP_MSTM_CTRL,
-					 DP_MST_EN |
-					 DP_UP_REQ_EN |
-					 DP_UPSTREAM_IS_SRC);
-		if (ret < 0)
-			goto out_unlock;
+		if (drm_dp_read_mst_cap(mgr->aux, mgr->dpcd)) {
+			ret = drm_dp_dpcd_writeb(mgr->aux, DP_MSTM_CTRL,
+						 DP_MST_EN |
+						 DP_UP_REQ_EN |
+						 DP_UPSTREAM_IS_SRC);
+			if (ret < 0)
+				goto out_unlock;
+		}
 
 		/* Write reset payload */
 		drm_dp_dpcd_write_payload(mgr, 0, 0, 0x3f);
@@ -3684,7 +3686,8 @@ int drm_dp_mst_topology_mgr_set_mst(struct drm_dp_mst_topology_mgr *mgr, bool ms
 		mstb = mgr->mst_primary;
 		mgr->mst_primary = NULL;
 		/* this can fail if the device is gone */
-		drm_dp_dpcd_writeb(mgr->aux, DP_MSTM_CTRL, 0);
+		if (drm_dp_read_mst_cap(mgr->aux, mgr->dpcd))
+			drm_dp_dpcd_writeb(mgr->aux, DP_MSTM_CTRL, 0);
 		ret = 0;
 		mgr->payload_id_table_cleared = false;
 
@@ -3724,8 +3727,9 @@ drm_dp_mst_topology_mgr_invalidate_mstb(struct drm_dp_mst_branch *mstb)
 void drm_dp_mst_topology_mgr_suspend(struct drm_dp_mst_topology_mgr *mgr)
 {
 	mutex_lock(&mgr->lock);
-	drm_dp_dpcd_writeb(mgr->aux, DP_MSTM_CTRL,
-			   DP_MST_EN | DP_UPSTREAM_IS_SRC);
+	if (drm_dp_read_mst_cap(mgr->aux, mgr->dpcd))
+		drm_dp_dpcd_writeb(mgr->aux, DP_MSTM_CTRL,
+				   DP_MST_EN | DP_UPSTREAM_IS_SRC);
 	mutex_unlock(&mgr->lock);
 	flush_work(&mgr->up_req_work);
 	flush_work(&mgr->work);
@@ -3773,13 +3777,15 @@ int drm_dp_mst_topology_mgr_resume(struct drm_dp_mst_topology_mgr *mgr,
 		goto out_fail;
 	}
 
-	ret = drm_dp_dpcd_writeb(mgr->aux, DP_MSTM_CTRL,
-				 DP_MST_EN |
-				 DP_UP_REQ_EN |
-				 DP_UPSTREAM_IS_SRC);
-	if (ret < 0) {
-		drm_dbg_kms(mgr->dev, "mst write failed - undocked during suspend?\n");
-		goto out_fail;
+	if (drm_dp_read_mst_cap(mgr->aux, mgr->dpcd)) {
+		ret = drm_dp_dpcd_writeb(mgr->aux, DP_MSTM_CTRL,
+					 DP_MST_EN |
+					 DP_UP_REQ_EN |
+					 DP_UPSTREAM_IS_SRC);
+		if (ret < 0) {
+			drm_dbg_kms(mgr->dev, "mst write failed - undocked during suspend?\n");
+			goto out_fail;
+		}
 	}
 
 	/* Some hubs forget their guids after they resume */
-- 
2.25.1