Cannot use sof-firmware-signed with pc-kernel snap / sof-firmware not included in pc-kernel (no audio)

[Lukas Mayer]

# System
This concerns Ubuntu 24.04 LTS running the pc-kernel snap with TPM-based full disk encryption.
The device uses an Intel Core Ultra 5 125U .

# Description

When using the pc-kernel snap, no audio is available to the system:

```
[ 6.200060] sof-audio-pci-intel-mtl 0000:00:1f.3: DSP detected with PCI class/subclass/prog-if info 0x040100
[ 6.200111] sof-audio-pci-intel-mtl 0000:00:1f.3: Digital mics found on Skylake+ platform, using SOF driver
[ 6.200388] sof-audio-pci-intel-mtl 0000:00:1f.3: DSP detected with PCI class/subclass/prog-if 0x040100
[ 6.200827] sof-audio-pci-intel-mtl 0000:00:1f.3: bound 0000:00:02.0 (ops i915_audio_component_bind_ops [i915])
[ 6.207483] sof-audio-pci-intel-mtl 0000:00:1f.3: use msi interrupt mode
[ 6.293899] sof-audio-pci-intel-mtl 0000:00:1f.3: hda codecs found, mask 5
[ 6.293909] sof-audio-pci-intel-mtl 0000:00:1f.3: using HDA machine driver skl_hda_dsp_generic now
[ 6.293918] sof-audio-pci-intel-mtl 0000:00:1f.3: DMICs detected in NHLT tables: 2
[ 6.294613] sof-audio-pci-intel-mtl 0000:00:1f.3: SOF firmware and/or topology file not found.
[ 6.295594] sof-audio-pci-intel-mtl 0000:00:1f.3: Supported default profiles
[ 6.295596] sof-audio-pci-intel-mtl 0000:00:1f.3: - ipc type 1 (Requested):
[ 6.295599] sof-audio-pci-intel-mtl 0000:00:1f.3: Firmware file: intel/sof-ipc4/mtl/sof-mtl.ri
[ 6.295601] sof-audio-pci-intel-mtl 0000:00:1f.3: Topology file: intel/sof-ace-tplg/sof-hda-generic-2ch.tplg
[ 6.295603] sof-audio-pci-intel-mtl 0000:00:1f.3: Check if you have 'sof-firmware' package installed.
[ 6.295606] sof-audio-pci-intel-mtl 0000:00:1f.3: Optionally it can be manually downloaded from:
[ 6.295607] sof-audio-pci-intel-mtl 0000:00:1f.3: https://github.com/thesofproject/sof-bin/
[ 6.297818] sof-audio-pci-intel-mtl 0000:00:1f.3: error: sof_probe_work failed err: -2
```[ 6.200060] sof-audio-pci-intel-mtl 0000:00:1f.3: DSP detected with PCI class/subclass/prog-if info 0x040100
[ 6.200111] sof-audio-pci-intel-mtl 0000:00:1f.3: Digital mics found on Skylake+ platform, using SOF driver
[ 6.200388] sof-audio-pci-intel-mtl 0000:00:1f.3: DSP detected with PCI class/subclass/prog-if 0x040100
[ 6.200827] sof-audio-pci-intel-mtl 0000:00:1f.3: bound 0000:00:02.0 (ops i915_audio_component_bind_ops [i915])
[ 6.207483] sof-audio-pci-intel-mtl 0000:00:1f.3: use msi interrupt mode
[ 6.293899] sof-audio-pci-intel-mtl 0000:00:1f.3: hda codecs found, mask 5
[ 6.293909] sof-audio-pci-intel-mtl 0000:00:1f.3: using HDA machine driver skl_hda_dsp_generic now
[ 6.293918] sof-audio-pci-intel-mtl 0000:00:1f.3: DMICs detected in NHLT tables: 2
[ 6.294613] sof-audio-pci-intel-mtl 0000:00:1f.3: SOF firmware and/or topology file not found.
[ 6.295594] sof-audio-pci-intel-mtl 0000:00:1f.3: Supported default profiles
[ 6.295596] sof-audio-pci-intel-mtl 0000:00:1f.3: - ipc type 1 (Requested):
[ 6.295599] sof-audio-pci-intel-mtl 0000:00:1f.3: Firmware file: intel/sof-ipc4/mtl/sof-mtl.ri
[ 6.295601] sof-audio-pci-intel-mtl 0000:00:1f.3: Topology file: intel/sof-ace-tplg/sof-hda-generic-2ch.tplg
[ 6.295603] sof-audio-pci-intel-mtl 0000:00:1f.3: Check if you have 'sof-firmware' package installed.
[ 6.295606] sof-audio-pci-intel-mtl 0000:00:1f.3: Optionally it can be manually downloaded from:
[ 6.295607] sof-audio-pci-intel-mtl 0000:00:1f.3: https://github.com/thesofproject/sof-bin/
[ 6.297818] sof-audio-pci-intel-mtl 0000:00:1f.3: error: sof_probe_work failed err: -2

```

When trying to install firmware-sof-signed using apt, the installation fails to pc-kernel's read-only filesystem:

```
dpkg: error processing archive /var/cache/apt/archives/firmware-sof-signed_2023.12.1-1ubuntu1.4_all.deb (--unpack):
 unable to create '/lib/firmware/intel/sof/community/sof-apl.ri.zst.dpkg-new' (while processing './lib/firmware/intel/sof/community/sof-apl.ri.zst'): Read-only file system
dpkg: error while cleaning up:
 unable to remove newly-extracted version of '/lib/firmware/intel/sof/community/sof-apl.ri.zst': Read-only file system
dpkg-deb: error: paste subprocess was killed by signal (Broken pipe)
Errors were encountered while processing:
 /var/cache/apt/archives/firmware-sof-signed_2023.12.1-1ubuntu1.4_all.deb
E: Sub-process /usr/bin/dpkg returned an error code (1)
```

No audio devices will be available to the system.

# Expected behaviour
Referenced firmware such as sof-audio-pci-intel-mtl should be included into the pc-kernel snap.

ProblemType: Bug
DistroRelease: Ubuntu 24.04
Package: apt 2.7.14build2
ProcVersionSignature: Ubuntu 6.8.0-54.56-generic 6.8.12
Uname: Linux 6.8.0-54-generic x86_64
ApportVersion: 2.28.1-0ubuntu3.3
Architecture: amd64
CasperMD5CheckResult: pass
CurrentDesktop: ubuntu:GNOME
Date: Wed Mar 12 17:43:11 2025
ProcEnviron:
 LANG=de_DE.UTF-8
 PATH=(custom, no user)
 SHELL=/bin/bash
 TERM=xterm-256color
 XDG_RUNTIME_DIR=<set>
SourcePackage: apt
UpgradeStatus: No upgrade log present (probably fresh install)