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

diff --git a/drivers/gpu/drm/display/drm_dp_mst_topology.c b/drivers/gpu/drm/display/drm_dp_mst_topology.c
index f8cd094efa3c0bd6f75b52a0410b0910d8026a76..271a00b81264c9959a394399bf2507abb354da8b 100644 (file)
--- a/drivers/gpu/drm/display/drm_dp_mst_topology.c
+++ b/drivers/gpu/drm/display/drm_dp_mst_topology.c
@@ -175,13 +175,13 @@ static int
 drm_dp_mst_rad_to_str(const u8 rad[8], u8 lct, char *out, size_t len)
 {
        int i;
-       u8 unpacked_rad[16];
+       u8 unpacked_rad[16] = {};
 
-       for (i = 0; i < lct; i++) {
+       for (i = 1; i < lct; i++) {
                if (i % 2)
-                       unpacked_rad[i] = rad[i / 2] >> 4;
+                       unpacked_rad[i] = rad[(i - 1) / 2] >> 4;
                else
-                       unpacked_rad[i] = rad[i / 2] & BIT_MASK(4);
+                       unpacked_rad[i] = rad[(i - 1) / 2] & 0xF;
        }
 
        /* TODO: Eventually add something to printk so we can format the rad

diff --git a/include/drm/display/drm_dp_mst_helper.h b/include/drm/display/drm_dp_mst_helper.h
index a80ba457a858f36ac2110a6fdd91d8a1570b58e1..6398a6b50bd1b7be256c8fc07e18fbfabc6bdb00 100644 (file)
--- 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;