[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...@canonical.com>
---
 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