ALC4080: S/PDIF disabled because `spdif_nodev` checks PCM `subname` instead of `name`

ALC4080: S/PDIF disabled because `spdif_nodev` checks PCM `subname` instead of `name`

[GitHub issues - opened]

alsa-project/alsa-ucm-conf issue #779 was opened from djglxxii:

On an MSI MAG X870 Tomahawk WiFi motherboard with Realtek ALC4080 USB audio (`0db0:cd0e`), optical S/PDIF output disappears with `alsa-ucm-conf` 1.2.16.

The kernel/driver side appears to expose the hardware correctly: direct ALSA playback to the raw PCM device works and is audible over optical. The regression appears to be in `ucm2/USB-Audio/Realtek/ALC4080-HiFi.conf`, where the S/PDIF device is disabled because `find-device` searches `field=subname` for `Audio #3`, but this device exposes `Audio #3` in the PCM `name` field instead.

## Affected Hardware

- Motherboard: MSI MAG X870 Tomahawk WiFi
- Codec: Realtek ALC4080 USB audio
- USB ID: `0db0:cd0e`
- Driver: `snd_usb_audio`
- `CardComponents`: `USB0db0:cd0e`

## Regression

- Working: `alsa-ucm-conf` 1.2.15.3
- Broken: `alsa-ucm-conf` 1.2.16

After upgrading `alsa-ucm-conf`, PipeWire/WirePlumber no longer exposes an S/PDIF sink. Since the analog outputs are jack-gated and unplugged on this setup, the card ends up with no usable output profile.

## Evidence That The Kernel/Hardware Path Works

Raw ALSA playback works:

```sh
speaker-test -D hw:0,3 -c2 -t wav
speaker-test -D hw:0,2 -c2 -t wav
```

Both are audible through the optical speakers.

The relevant PCM exists:

```text
$ cat /proc/asound/card0/pcm3p/info
id:      USB Audio
name:    USB Audio #3
subname: subdevice #0
```

`aplay -l` shows the same:

```text
card 0: Audio [USB Audio], device 3: USB Audio [USB Audio #3]
  Subdevice #0: subdevice #0
```

## Suspected Root Cause

`ALC4080-HiFi.conf` contains this logic:

```conf
If.spdif_dev2 {
    Condition {
        Type RegexMatch
        String "${CardComponents}"
        Regex "USB(0b05:(1996|1a5[23c])|0db0:1feb)"
    }
    True.Define {
        SpdifName "S/PDIF Output"
        SpdifPCM "2"
    }
    False.If.spdif_nodev {
        Condition {
            Type String
            Empty "$${find-device:type=pcm,stream=playback,field=subname,regex='Audio #3'}"
        }
        True.Define.SpdifName ""
    }
}
```

On this board, the regex `Audio #3` matches the PCM `name`, not the `subname`:

```text
name:    USB Audio #3
subname: subdevice #0
```

So this lookup returns empty:

```conf
field=subname,regex='Audio #3'
```

That causes:

```conf
True.Define.SpdifName ""
```

which removes the S/PDIF `SectionDevice` from the HiFi verb even though the PCM exists and works.

## Suggested Fix

Search the PCM `name` field instead:

```diff
- Empty "$${find-device:type=pcm,stream=playback,field=subname,regex='Audio #3'}"
+ Empty "$${find-device:type=pcm,stream=playback,field=name,regex='Audio #3'}"
```

Alternatively, detect the fourth playback PCM by index/device number rather than matching this string in `subname`.

## Workaround

Downgrading `alsa-ucm-conf` to 1.2.15.3 restores S/PDIF output. Applying the `field=name` change locally also restores the optical sink.

## Environment

- Distribution: CachyOS
- Upgrade command that exposed the regression: `sudo pacman -Syu`
- `alsa-ucm-conf`: 1.2.16 broken, 1.2.15.3 working
- `alsa-lib`: 1.2.16
- Kernel: 7.0.11-1-cachyos
- ALSA driver string: `k7.0.11-1-cachyos`
- Audio server: PipeWire/WirePlumber
- S/PDIF output terminal present in USB descriptors: `bTerminalID 24`, `wTerminalType 0x0605` / S/PDIF interface

Issue URL     : https://github.com/alsa-project/alsa-ucm-conf/issues/779
Repository URL: https://github.com/alsa-project/alsa-ucm-conf