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drm/amd/display: Fix pbn_div Calculation Error

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[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>
This commit is contained in:
Fangzhi Zuo
2025-04-08 15:11:43 -04:00
committed by Alex Deucher
parent 1bde5584e2
commit 12cdfb61b3
3 changed files with 12 additions and 5 deletions
Download Patch File Download Diff File Expand all files Collapse all files

2
drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
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@@ -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;