SPIConfig design concerns

[bmuessig]

I have just began using TinyGo for RP2040 after previously always using the Pico SDK.
Since the concerns are with the abstraction, they don't only concern the RP2040 though.

There are a few things I found strange configuring SPI:

• SPI config uses pin defaults when pin is zero
  Doesn't this clash when setting one of the SPI pins to pin 0, if the default is not zero?

• SPI config does not support omitting MOSI or MISO
  Especially on designs where all pins are used and one channel (e.g. receive on a display) is not used, this causes major issues. Is there an official way to handle this?

I hope these could be addressed, since I really like the idea of moving future firmware projects from C to Go.

[bmuessig]

• SPI config uses pin defaults when pin is zero
  Doesn't this clash when setting one of the SPI pins to pin 0, if the default is not zero?

Here it seems that the code is already prepared to handle this well, by checking if all set pins are zero. For me, it didn't initially click with the documentation, that once at least one pin of the peripheral is changed, the manual override takes effect.

• SPI config does not support omitting MOSI or MISO
  Especially on designs where all pins are used and one channel (e.g. receive on a display) is not used, this causes major issues. Is there an official way to handle this?

I did in the meantime find machine.NoPin, which does not seem to be implemented for the RP2xxx platform. Seems like it's time to submit a PR there.

[bmuessig]

Will submit a PR soon, but this should do it: bmuessig@444bdff