[MTL][SDW] No HDMI audio via Thunderbolt - monitor on converter 3, only 3 PCMs in topology

[kochka]

Hello !

Hardware: Dell XPS 14 9440, Meteor Lake-P
External display: Dell U4025QW via Thunderbolt 4
Kernel: 6.18.9-arch1-2
sof-firmware: 2025.12.2-1
Codecs: CS42L43 + 4x CS35L56 (SoundWire)

Problem:
HDMI/DP audio does not work on the external display connected via Thunderbolt.
The monitor is detected (ELD valid, monitor_present=1) but assigned to
converter 3 (cvt_nid=0x3). The function topology sof-hdmi-pcm5-id5.tplg
only creates 3 HDMI PCMs, covering converters 0-2. Converter 3 has no PCM.

Speakers (CS35L56) and headphone jack (CS42L43) work correctly with SOF driver.

dmesg (relevant):
sof-audio-pci-intel-mtl 0000:00:1f.3: hda codecs found, mask 4
sof-audio-pci-intel-mtl 0000:00:1f.3: loading topology 3: intel/sof-ace-tplg/sof-hdmi-pcm5-id5.tplg
sof_sdw sof_sdw: hda_dsp_hdmi_build_controls: no PCM in topology for HDMI converter 3

ELD for the monitor:
monitor_present 1
eld_valid 1
codec_pin_nid 0xa
codec_dev_id 0x0
codec_cvt_nid 0x3
monitor_name DELL U4025QW
connection_type DisplayPort

aplay -l shows: HDMI1 (dev 5), HDMI2 (dev 6), HDMI3 (dev 7)
None of them produce sound on the external display (speaker-test confirmed).

Expected: A 4th HDMI PCM mapped to converter 3, or dynamic
converter-to-PCM assignment that covers the active monitor.

Thanks