libdmaudio and the dmaudio service: mixing streams for the audio devices

[JaxCalder]

Summary

The top layer of the audio stack: a mixer for the audio devices.

• libdmaudio (library, one in the system) holds everything:
  • Streams. An application opens a stream on a device (by name, or the first one) in the format of its own samples: any rate, 1 to 8 channels, S16/S24/S32. It writes into the stream's buffer (200 ms by default), which is lock-free with one writer and one reader. A write waits while the buffer is full.
  • Mixing. Each stream is converted to the device's format (linear interpolation of the rate, channel mapping), scaled by its volume (square curve) and summed in 24 bits, clamped on output.
  • Serving a device. libdmaudio_serve() configures the node (AUDIO_CONFIGURE; a fixed-format SAI keeps its own) and takes the format from AUDIO_GET_STREAM. It then mixes and writes 10 ms periods, paced by the node's DMA rather than a timer. After 2 s with nothing to play it drains and stops the node; the next samples start it again. The master volume and mute go to the node, or to the mix when the node has none.
  • Streams of processes that end without closing them are dropped.
• dmaudio (service) is main() calling libdmaudio_serve(), under 1 kB. It runs at priority 3, above the applications. configs/dmaudio.rules starts dmaudio@<name> for every node reported as audio; options go in the unit's args=.
• Tools: audiotone (sine tones, one stream each), audioplay (PCM WAV), audioctl (list devices, volume, mute).
• docs/api-reference.md, README.

Not implemented yet: capture (recording streams), and a better resampler than linear interpolation.

Testing

• test_libdmaudio (host, 16 steps) mixes a device registered without a node. It covers conversions, mono/stereo, S24/S32 output, sums and clamping, volume, 24→48 kHz interpolation, full buffers and timeouts, drain, underruns, pause, and a device that goes away.
• STM32F746G-DISCO with dmsai 0.1, dmwm8994 0.2 and these modules placed in dmod-boot's build/dmf by hand (not released yet):
  • dmaudio wm8994 /dev/wm8994 started by the rule. This needs Report the nodes after their friends, on the hot-plug worker dmdevfs#69; with dmdevfs 1.5 the codec is not reported.
  • audiotone -f 440 -t 3: 22050 Hz mono into 48 kHz stereo, 3.45 s, 0 underruns.
  • audiotone with three tones at once: three streams mixed, 0 underruns.
  • audioplay of a 3 s 44.1 kHz 24-bit stereo WAV from the SD card: 3.48 s.
  • I checked the timing and the underrun counters only; I did not listen to the output.

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