drm/dp_mst: Fix drm RAD print

[Why]
The RAD of sideband message printed today is incorrect.
For RAD stored within MST branch
- If MST branch LCT is 1, it's RAD array is untouched and remained as 0.
- If MST branch LCT is larger than 1, use nibble to store the up facing
  port number in cascaded sequence as illustrated below:

  u8 RAD[0] = (LCT_2_UFP << 4) | LCT_3_UFP
     RAD[1] = (LCT_4_UFP << 4) | LCT_5_UFP
     ...

In drm_dp_mst_rad_to_str(), it wrongly to use BIT_MASK(4) to fetch the port
number of one nibble.

[How]
Adjust the code by:
- RAD array items are valuable only for LCT >= 1.
- Use 0xF as the mask to replace BIT_MASK(4)

V2:
- Document how RAD is constructed (Imre)

V3:
- Adjust the comment for rad[] so kdoc formats it properly (Lyude)

Fixes: 2f015ec6eab6 ("drm/dp_mst: Add sideband down request tracing + selftests")
Cc: Imre Deak <imre.deak@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Harry Wentland <hwentlan@amd.com>
Cc: Lyude Paul <lyude@redhat.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Wayne Lin <Wayne.Lin@amd.com>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250113091100.3314533-2-Wayne.Lin@amd.com

1 files changed, 7 insertions, 0 deletions

diff --git a/include/drm/display/drm_dp_mst_helper.h b/include/drm/display/drm_dp_mst_helper.h
index a80ba457a858..6398a6b50bd1 100644
--- a/include/drm/display/drm_dp_mst_helper.h
+++ b/include/drm/display/drm_dp_mst_helper.h
@@ -222,6 +222,13 @@ struct drm_dp_mst_branch {
 	 */
 	struct list_head destroy_next;
 
+	/**
+	 * @rad: Relative Address of the MST branch.
+	 * For &drm_dp_mst_topology_mgr.mst_primary, it's rad[8] are all 0,
+	 * unset and unused. For MST branches connected after mst_primary,
+	 * in each element of rad[] the nibbles are ordered by the most
+	 * signifcant 4 bits first and the least significant 4 bits second.
+	 */
 	u8 rad[8];
 	u8 lct;
 	int num_ports;