[PATCH] drm/dp: For MST hub, Get max_link_rate&max_lane from extended rx capability field if EXTENDED_RECEIVER_CAPABILITY_FILED_PRESENT is set.

[Koba Ko]

Currently, DRM get the capability of the mst hub only from DP_DPCD_REV and
get the slower speed even the mst hub can run in the faster speed.

As per DP-1.3, First check DP_EXTENDED_RECEIVER_CAP_FIELD_PRESENT.
If DP_EXTENDED_RECEIVER_CAP_FIELD_PRESENT is 1, read the DP_DP13_DPCD_REV to
get the faster capability.
If DP_EXTENDED_RECEIVER_CAP_FIELD_PRESENT is 0, read DP_DPCD_REV.

Signed-off-by: Koba Ko <koba.ko@xxxxxxxxxxxxx>
---
 drivers/gpu/drm/drm_dp_mst_topology.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/drm_dp_mst_topology.c b/drivers/gpu/drm/drm_dp_mst_topology.c
index 67dd72ea200e..3b84c6801281 100644
--- a/drivers/gpu/drm/drm_dp_mst_topology.c
+++ b/drivers/gpu/drm/drm_dp_mst_topology.c
@@ -3497,6 +3497,8 @@ static int drm_dp_get_vc_payload_bw(u8 dp_link_bw, u8  dp_link_count)
 int drm_dp_mst_topology_mgr_set_mst(struct drm_dp_mst_topology_mgr *mgr, bool mst_state)
 {
 	int ret = 0;
+	u8 dpcd_ext = 0;
+	unsigned int dpcd_offset = 0;
 	struct drm_dp_mst_branch *mstb = NULL;
 
 	mutex_lock(&mgr->payload_lock);
@@ -3510,9 +3512,15 @@ int drm_dp_mst_topology_mgr_set_mst(struct drm_dp_mst_topology_mgr *mgr, bool ms
 		struct drm_dp_payload reset_pay;
 
 		WARN_ON(mgr->mst_primary);
+		drm_dp_dpcd_read(mgr->aux,
+				 DP_TRAINING_AUX_RD_INTERVAL,
+				 &dpcd_ext, sizeof(dpcd_ext));
+
+		dpcd_offset =
+			((dpcd_ext & DP_EXTENDED_RECEIVER_CAP_FIELD_PRESENT) ?  DP_DP13_DPCD_REV : DP_DPCD_REV);
 
 		/* get dpcd info */
-		ret = drm_dp_dpcd_read(mgr->aux, DP_DPCD_REV, mgr->dpcd, DP_RECEIVER_CAP_SIZE);
+		ret = drm_dp_dpcd_read(mgr->aux, dpcd_offset, mgr->dpcd, DP_RECEIVER_CAP_SIZE);
 		if (ret != DP_RECEIVER_CAP_SIZE) {
 			DRM_DEBUG_KMS("failed to read DPCD\n");
 			goto out_unlock;
-- 
2.25.1