[SRU][N:OEM-6.17][PATCH 0/1] Fix i915 DP MST FEC support check for uncompressed streams

[ChunAn Wu]

BugLink: https://bugs.launchpad.net/bugs/2150523
BugLink: https://bugs.launchpad.net/bugs/2150524

SRU Justification:

[ Impact ]
On systems with hybrid graphics (Intel iGPU + NVIDIA dGPU), including
Lenovo T1g/P16s and Dell Pro Max 14/16 Premium, the i915 driver
incorrectly enables FEC for uncompressed streams on MST links when DSC
compression is enabled for another stream on the same link. The
downstream branch device rejects the invalid FEC configuration, causing
the DP MST display pipeline to stall. This blocks NVIDIA modeset from
completing its display operations, resulting in continuous timeout
errors (nvidia-modeset: ERROR: GPU:0: Error while waiting for GPU
progress) and preventing the NVIDIA GPU from going idle. When suspend
is triggered, the NVIDIA driver cannot suspend the GPU due to the
stuck channel, causing the system to fail to enter suspend or hang
entirely.

[ Fix ]
Cherry-pick upstream commit:
- 06a691f0e7d0 ("drm/i915/dp_mst: Fix check for FEC support for an
  uncompressed stream")

The fix adds a `dsc` condition check so the FEC support verification
is only performed for compressed (DSC) streams, not for uncompressed
streams that happen to have FEC enabled due to another stream on the
same MST link.

[ Test Plan ]
1. Boot affected system (Lenovo T1g/P16s or Dell Pro Max 14/16
   Premium) with oem-6.17 kernel (no external displays needed)
2. Use the system under normal workload
3. Trigger suspend: systemctl suspend
4. Verify system enters suspend successfully
5. Resume and check dmesg for absence of nvidia-modeset timeout errors
6. Repeat suspend/resume cycles to confirm stability

[ Where problems could occur ]
The change only affects the FEC support check path for uncompressed
streams on MST links. Risk is low as it relaxes an overly strict
check that was already validated for the compressed stream case.

Imre Deak (1):
  drm/i915/dp_mst: Fix check for FEC support for an uncompressed stream

 drivers/gpu/drm/i915/display/intel_dp_mst.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

-- 
2.34.1