Add ALSA loopback audio bridge for native_sim ↔ QEMU integration

[peterus]

Context

We can now run the full station stack on a single PC:

• QEMU x64 — Yocto image with Station Agent (A/B rootfs, GRUB, read-only)
• native_sim — Zephyr firmware running as Linux process (SA818 driver, GPIO/ADC emulators, UART PTY)

Serial communication (Shell, AT commands) works via UART PTY → QEMU USB-Serial. Audio is the missing link.

Goal

Station Agent in QEMU captures/plays audio from a virtual USB soundcard, and the audio data flows to/from native_sim's audio pipeline — just like on real hardware where the STM32 presents as a USB Audio Class device.

Architecture

native_sim (Zephyr)          Host Kernel            QEMU Guest (Yocto)
┌──────────────┐           ┌─────────────┐        ┌──────────────────┐
│ RX Audio ────┼─► play ──►│  snd-aloop   │►─ cap ─┼► Station Agent   │
│ (DAC write)  │  hw:L,0,0 │  (loopback)  │ hw:L,1 │  (ALSA capture)  │
│              │           │              │        │                  │
│ TX Audio ◄───┼─◄ cap ───◄│              │◄─play ─┼◄ Station Agent   │
│ (ADC read)   │  hw:L,1,1 │              │ hw:L,0 │  (ALSA playback) │
└──────────────┘           └─────────────┘        └──────────────────┘

Implementation steps

Zephyr side (this repo)

• New DAC backend: dac_alsa — similar to existing dac_wav but writes PCM to an ALSA device instead of a WAV file. Config: CONFIG_DAC_ALSA=y, device tree property output-device = "hw:Loopback,0,0".
• ADC ALSA source (optional) — capture from hw:Loopback,1,1 to feed TX audio into the radio pipeline. Lower priority since TX testing is less common.
• native_sim overlay update — add native_sim_native_64.conf entries for CONFIG_DAC_ALSA=y and update the DT overlay to use alsa-dac instead of wav-dac.

QEMU/run.sh side (linux-image repo)

• Add to run.sh:

  -audiodev alsa,id=snd0,in.dev=hw:Loopback,1,0,out.dev=hw:Loopback,0,1 \
  -device usb-audio,audiodev=snd0

• Document: sudo modprobe snd-aloop as prerequisite.

Station Agent side (station-manager repo)

• Implement ALSA capture/playback for audio streaming to/from server.
• The agent code should be identical to production (ALSA device, not mocks).

Host prerequisites

sudo modprobe snd-aloop
# Verify: aplay -l should show "Loopback" card

Acceptance criteria

• native_sim RX audio (e.g., sine tone) appears as capture data on QEMU guest's ALSA device
• QEMU guest playback reaches native_sim ADC input
• Full duplex works simultaneously
• Station Agent can stream audio to Station Manager server via WebSocket (once server is deployed)

Not in scope

• USB Audio Class emulation in native_sim (real STM32 USB stack not needed for simulation)
• DFU over USB (separate ticket if needed)
• Real-time latency guarantees (best-effort is fine for testing)

References

• Existing WAV DAC driver: drivers/dac/ in this repo
• ALSA loopback module: snd-aloop kernel module
• QEMU audio: https://www.qemu.org/docs/master/system/audio.html