With the previous change, stereo mp3 playback changed from needing
4 2304-byte allocations to needing 2 4604-byte allocations. This was
enough to cause MemoryErrors with regularity.
By using m_realloc() here, the existing memory region can be used.
m_realloc() also works on the first invocation, because m_realloc(NULL, sz)
just calls m_malloc of sz.
.. the documentation doesn't make this clear, but in practice it works
to write both of the DATABUF registers at the same time. This should
also reduce the amount of wear and tear DMA puts on the system, as the
number of transfers is cut in half. (the number of bytes transferred
remains the same, though)
In principle, this could cover all stereo cases if audio_dma_convert_signed
also learned to 16-bit extend and swap values. However, this is the
case that matters for stereo mp3 playback on PyGamer.
Testing performed: Listened to some tracks with good stereo separation.
Introduces a way to place CircuitPython code and data into
tightly coupled memory (TCM) which is accessible by the CPU in a
single cycle. It also frees up room in the corresponding cache for
intermittent data. Loading from external flash is slow!
The data cache is also now enabled.
Adds support for the iMX RT 1021 chip. Adds three new boards:
* iMX RT 1020 EVK
* iMX RT 1060 EVK
* Teensy 4.0
Related to #2492, #2472 and #2477. Fixes#2475.
This removes downscaling (halving-add) when multiple voices are
being mixed. To avoid clipping, and get similar behavior to before,
set the "level" of each voice to (1/voice_count).
Slow paths that were applicable to only M0 chips were removed.
As a side effect, the internal volume representation is now 0 ..
0x8000 (inclusive), which additionally makes a level of exactly 0.5
representable.
Testing performed, on PyGamer: For all 4 data cases, for stereo and
mono, for 1 and 2 voices, play pure sign waves represented as
RawSamples and view the result on a scope and through headphones.
Also, scope the amount of time spent in background tasks.
Code size: growth of +272 bytes
Performance (time in background task when mixing 2 stereo 16-bit voices):
76us per down from 135us (once per ~2.9ms long term average)
(Decrease from 4.7% to 2.4% of all CPU time)
