Evaluate compressing the wifi firmware

[peterharperuk]

It looks like we could save ~85KB of flash memory if we compressed the wifi firmware binary. Evaluate if the code size, run time overhead is worth it.

[dpgeorge]

It would be great if a compressed version of the firmware could be sent over SPI to the 43439, so it's faster to copy. But I don't think that's possible, the base firmware on that chip doesn't support any compression. So the benefit would just be smaller firmware (which is still a good benefit).

As for runtime overhead, I don't think it will be much. The time for downloading the firmware is dominated by the SPI transfer.

[dpgeorge]

MicroPython includes a decent gunzip implementation, but for good compression it requires a lot of RAM for the decompression dictionary. One would need to tune the dictionary window size to trade off compression size and RAM usage.

[kripton]

I think @peterharperuk meant to pre-compress the firmware files before including them into the build and then de-compressing them on-the-fly directly buffer-by-buffer before sending them via SPI to the cyw-chip (instead of doing the compression on the uC before sending. That would also not reduce the firmware size). So the RAM usage needed for compression doesn't matter much since it's on a PC, not the uC.

Here are some compression numbers (file size in byte - file name):

225240  43439A0-7.95.49.00.combined
139024  43439A0-7.95.49.00.combined.br
140996  43439A0-7.95.49.00.combined.gz
164856  43439A0-7.95.49.00.combined.lz4
122820  43439A0-7.95.49.00.combined.xz
137444  43439A0-7.95.49.00.combined.zopfli
146782  43439A0-7.95.49.00.combined.zst
426094  4343WA1-7.45.98.50.combined
282192  4343WA1-7.45.98.50.combined.br
286309  4343WA1-7.45.98.50.combined.gz
330717  4343WA1-7.45.98.50.combined.lz4
249680  4343WA1-7.45.98.50.combined.xz
279263  4343WA1-7.45.98.50.combined.zopfli
304148  4343WA1-7.45.98.50.combined.zst

with .br being Brotli (https://github.com/google/brotli)
.gz being gzip -9
.lz4 being LZ4 -9
.xz being XZ (you don't want to decompress that on a RP2040, just for comparison)
.zopfli (https://github.com/google/zopfli) generates a data stream that can be decompressed using gzip/zlib/deflate but is smaller and much slower to compress. Paramter --i1000 was used and it took some time to compress ;)
.zst being ZStandard

Most of the formats can be decompressed on the rp2040 (excluding xz, see https://github.com/kripton/rp2040-compressiontest), however I would personally chose zopfli for the offline/pre-compression and the existing gunzip to decompress on the rp2040 right before sending the file over SPI