drm/amd/display: Fix pbn_div Calculation Error

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[ Upstream commit 12cdfb61b3 ]

[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>
Signed-off-by: Sasha Levin <sashal@kernel.org>

...

This commit is contained in:

Fangzhi Zuo 2025-04-08 15:11:43 -04:00 • committed by Greg Kroah-Hartman

parent e4481e73e4

commit b3cc2a33b3

3 changed files with 12 additions and 5 deletions

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2

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c

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13

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_mst_types.c

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2

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_mst_types.h

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