Testing performed: I used a Particle Xenon with a HDA1334 I2S DAC.
I played a variety of mono 16-bit samples at 11025 and 22050Hz nominal
bit rates. With this setup, all the 11025Hz samples sound good.
I tested play, pause, and loop functionality.
During some runs with 22050Hz samples, there were glitches. However,
these may have only occurred during runs where I had set breakpoints
and watchpoints in gdb.
I also tested with a MAX98357A I2S amplifier. On this device, everything
sounded "scratchy". I was powering it from 5V and the 5V rail seemed
steady, so I don't have an explanation for this. However, I haven't
tried it with a SAMD board.
Previously, we depended on allocated channels to always be
"dma_channel_enabled". However, (A) sometimes, many operations
would take place between find_free_audio_dma_channel and
audio_dma_enable_channel, and (B) some debugging I did led me to believe
that "dma_channel_enabled" would become false when the hardware ended
a scheduled DMA transaction, but while a CP object would still think it
owned the DMA channel.
((B) is not documented in the datasheet and I am not 100% convinced that
my debugging session was not simply missing where we were disabling the
channel, but in either case, it shows a need to directly track allocated
separately from enabled)
Therefore,
* Add audio_dma_{allocate,free}_channel.
* audio_dma_free_channel implies audio_dma_disable_channel
* track via a new array audio_dma_allocated[]
* clear all allocated flags on soft-reboot
* Convert find_free_audio_dma_channel to audio_dma_allocate_channel
* use audio_dma_allocated[] instead of dma_channel_enabled() to check
availability
* remove find_free_audio_dma_channel
* For each one, find a matching audio_dma_disable_channel to convert
to audio_dma_free_channel
Closes: #2058
.. otherwise, a sequence like
>>> a = audioio.AudioOut(board.A0)
>>> a.play(sample, loop=True)
>>> a.deinit()
would potentially leave related DMA channel(s) active.
