drm/amd/display: Fix pbn_div Calculation Error

[Why]
dm_mst_get_pbn_divider() returns value integer coming from
the cast from fixed point, but the casted integer will then be used
in dfixed_const to be multiplied by 4096. The cast from fixed point to integer
causes the calculation error becomes bigger when multiplied by 4096.

That makes the calculated pbn_div value becomes smaller than
it should be, which leads to the req_slot number becomes bigger.

Such error is getting reflected in 8k30 timing,
where the correct and incorrect calculated req_slot 62.9 Vs 63.1.
That makes the wrong calculation failed to light up 8k30
after a dock under HBR3 x 4.

[How]
Restore the accuracy by keeping the fraction part
calculated for the left shift operation.

Reviewed-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Fangzhi Zuo <Jerry.Zuo@amd.com>
Signed-off-by: Wayne Lin <wayne.lin@amd.com>
Tested-by: Dan Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

1 files changed, 1 insertions, 1 deletions

diff --git a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
index 5ae01b0ac093..e34d98a945f2 100644
--- a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
+++ b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
@@ -8015,7 +8015,7 @@ static int dm_encoder_helper_atomic_check(struct drm_encoder *encoder,
 	if (IS_ERR(mst_state))
 		return PTR_ERR(mst_state);
 
-	mst_state->pbn_div.full = dfixed_const(dm_mst_get_pbn_divider(aconnector->mst_root->dc_link));
+	mst_state->pbn_div.full = dm_mst_get_pbn_divider(aconnector->mst_root->dc_link);
 
 	if (!state->duplicated) {
 		int max_bpc = conn_state->max_requested_bpc;