Support claiming unused SM/channels for audio

[Daft-Freak]

Uses a dma_channel/pio_sm of 0xFF to mean "claim unused". (Considered changing the configs to signed and/or adding some defines, but thought I should try the smallest change first...)

This change was motivated by finding that I'd written this in a project:

uint8_t dma_channel = dma_claim_unused_channel(true);
uint8_t pio_sm = pio_claim_unused_sm(audio_pio, true);

dma_channel_unclaim(dma_channel);
pio_sm_unclaim(audio_pio, pio_sm);

struct audio_i2s_config config = {
  .data_pin = PICO_AUDIO_I2S_DATA_PIN,
  .clock_pin_base = PICO_AUDIO_I2S_CLOCK_PIN_BASE,
  .dma_channel = dma_channel,
  .pio_sm = pio_sm,
};

Ugh.

[Gadgetoid]

Oh hello, I have written this same code 😆

https://github.com/Gadgetoid/pico-tinyusb-i2s-speaker/blob/28b0c9a034615d38c22e05df83c8f74009fd0f85/src/i2s_audio.c#L42-L46

Probably borrowed the idea from you.

[MichaelBell]

+1 for this. Or I would support just removing them from the config entirely and always claiming - it's a pretty small change to existing projects that would also flag for people to remove their workarounds.