Adding a data point to help pin down the exact blob.

I am still holding linux-firmware at 0ubuntu2.26, which is stable on my
machine. For comparison with the hash posted in comment #6:

Working (0ubuntu2.26):
  dcn_3_5_dmcub.bin SHA-256: 
ce5dc8ef543ce71a0ef772d4d821872fbfaa48afd0634e00f814e04ed991c431
  Loaded DMCUB version: 0x09002C01
  No lockups, no Oops.

Broken (reported in #6 with linux-firmware-amd-graphics 0ubuntu3.2):
  dcn_3_5_dmcub.bin SHA-256: 
3849f6e2dd33c24977254f486710be9ee8f51fb5dad4b442444e119ced0c55d5
  Loaded DMCUB version: 0x09000D00

So 0ubuntu3.2 still ships the regressed DCN 3.5 blob. The fix released
for the Navi 21 case (LP #2163303) does not address DCN 3.5 / Strix
Point.

I can also confirm that the test build at
people.canonical.com/~juergh/lp2163303/ (0ubuntu2.29+lp2163303) still
loads DMCUB 0x09000D00 here, so it does not fix this case either. I
reverted to 0ubuntu2.26 and re-applied the hold.

Two more notes:

- Comment #6 reports the same failure on a Lenovo ThinkPad P16s Gen 4
AMD (Ryzen AI 9 HX PRO 370, Radeon 890M), so this is not vendor-
specific: it affects Strix Point / DCN 3.5 generally, on the same
6.17.0-1032-oem kernel.

- The stack trace in #6 is worth noting: amdgpu_device_rreg ->
dmub_srv_wait_for_idle -> dmub_abm_set_pipe ->
amdgpu_dm_atomic_commit_tail. That is the Adaptive Backlight Management
path, which suggests the regression is in the ABM handling of the DMCUB
firmware rather than something broader.

- Also relevant for the package split: the hold on the monolithic linux-
firmware package does not cover the new linux-firmware-amd-graphics
package. Users who let the split happen may silently get the regressed
blob back.

My system: SLIMBOOK EXCALIBUR-AMD-AI, Ryzen AI 9 365 / Radeon 890M
(gfx1150, DCN 3.5), kernel 6.17.0-1032-oem, Zorin OS 18.1 (Ubuntu 24.04
base), X11.

Happy to test any build that targets the DCN 3.5 blob specifically.

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https://bugs.launchpad.net/bugs/2163311

Title:
  linux-firmware 0ubuntu2.29 regresses DMCUB on DCN 3.5 (Strix Point)
  causing SDMA0 fence timeout and kernel Oops

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