[PATCH] drm/dp_mst: Fix drm RAD print

[Wayne Lin <Wayne.Lin@xxxxxxx>]

[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, usd 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)

Fixes: 2f015ec6eab6 ("drm/dp_mst: Add sideband down request tracing + selftests")
Cc: Imre Deak <imre.deak@xxxxxxxxx>
Cc: Ville Syrjälä <ville.syrjala@xxxxxxxxxxxxxxx>
Cc: Harry Wentland <hwentlan@xxxxxxx>
Cc: Lyude Paul <lyude@xxxxxxxxxx>
Signed-off-by: Wayne Lin <Wayne.Lin@xxxxxxx>
---
 drivers/gpu/drm/display/drm_dp_mst_topology.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/display/drm_dp_mst_topology.c b/drivers/gpu/drm/display/drm_dp_mst_topology.c
index f7c6b60629c2..152c60f1e80f 100644
--- a/drivers/gpu/drm/display/drm_dp_mst_topology.c
+++ b/drivers/gpu/drm/display/drm_dp_mst_topology.c
@@ -179,13 +179,13 @@ 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] = {0};
 
-	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
-- 
2.37.3