After updating linux-firmware / amd-gpu-firmware from 20260810-1.fc43 to 20260910-1.fc43 (changelog: "amdgpu: DMCUB updates for various ASICs"), the DMCUB firmware for DCN 3.1.5 (amdgpu/dcn_3_1_5_dmcub.bin) no longer loads on my Lenovo ThinkPad E16 Gen 2 (AMD Ryzen 7 7735HS, Radeon 680M "Rembrandt", PCI 1002:1681 rev 0a, BIOS R2KET36W 1.19).

With the new firmware, amdgpu logs at boot:

  [drm] Loading DMUB firmware via PSP: version=0x0400004A
  [drm] *ERROR* Error getting DMUB FW meta info: 4
  amdgpu 0000:05:00.0: failed to load ucode DMCUB(0x3F)
  [drm] Wait for DMUB auto-load failed: 3
  [drm] DMUB hardware initialized: version=0x0400004A

followed by a continuous stream of (>1000 per boot):

  [drm] *ERROR* Error queueing DMUB command: status=2
  [drm] *ERROR* dc_dmub_srv_log_diagnostic_data: DMCUB error - collecting diagnostic data

Consequence: DisplayPort Alt Mode over USB-C never comes up. When a USB-C hub with two HDMI monitors is plugged in, the kernel logs "[drm] Alt mode has timed out after 77861 ms", all DP-x connectors stay "disconnected" and the monitors show "No signal". The internal eDP panel keeps working. Same result on every boot with kernel 7.2.5-100.fc43.

With the previous firmware (20260810-1.fc43) everything works:

  [drm] Loading DMUB firmware via PSP: version=0x0400004C
  [drm] DMUB hardware initialized: version=0x0400004C

no DMUB errors at all, and both external monitors are detected immediately
(DP-7 1680x1050, DP-8 2560x1440).

Note that the "new" firmware reports a LOWER version (0x4A) than the previous one (0x4C).

The file changed between the two packages:
  20260810: dcn_3_1_5_dmcub.bin  md5 9ea4810cb9c9bed49b1fd06893659f42  (works)
  20260910: dcn_3_1_5_dmcub.bin  md5 ca9a9234394c53e6df198946e5e5608a  (broken) (md5 of the decompressed .bin)

Kernel version is NOT the cause: kernel 7.2.4-100.fc43 still works only because its
initramfs was generated before the firmware update and still contains the old file.
Kernel 7.2.5-100.fc43's initramfs was generated after the update and picks up the broken file.

Workaround: put the 20260810 version of dcn_3_1_5_dmcub.bin.xz into
/lib/firmware/updates/amdgpu/ and regenerate the initramfs (dracut -f) - external
displays work again on kernel 7.2.5.

Reproducible: Always

Steps to Reproduce:
1. Rembrandt / DCN 3.1.5 laptop (Ryzen 6000/7035 series with Radeon 680M).
2. Update to linux-firmware-20260910-1.fc43 and regenerate initramfs (or boot a kernel installed after the update).
3. Check dmesg for "Error getting DMUB FW meta info" and plug in a USB-C DP Alt Mode dock/hub with a monitor.
Actual Results:
1. DMCUB firmware fails to load
2. DMUB command errors flood dmesg
3. USB-C Alt Mode times out
4. external monitors are not detected

Expected Results:
DMCUB loads and external monitors work as with 20260810-1.fc43.

Additional Information:
- Fedora 43, KDE Plasma on Wayland
- kernel 7.2.5-100.fc43.x86_64 (broken) / 7.2.4-100.fc43.x86_64 (works only due to old initramfs)
- linux-firmware-20260910-1.fc43, amd-gpu-firmware-20260910-1.fc43
- previous working: amd-gpu-firmware-20260810-1.fc43
- kernel cmdline: rhgb quiet modprobe.blacklist=ucsi_acpi (this was present with the working
  firmware too, so it is unrelated)
Attached: dmesg amdgpu excerpts from a broken boot (7.2.5) and a working boot (7.2.4).