[2.0.x] First things first: Sort out SPI header spaghetti.

[thinkyhead]

@MarlinFirmware/32bit-maintainers !

At the moment the inter-dependency, most especially around the SPI-oriented headers in the HAL is distressing (BitOrder not defined, etc.). See #9476, etc. Let's figure out how to disentangle these –and any similar circular dependencies– so that we can move forward.

[teemuatlut]

I think the clearest way is Marlin -> Possible library -> External SPI library -> Platform core.
Not everything can be included in Marlin since outside resources will not have access to the Marlin defined implementations.

BitOrder can be problematic since some define MSB/LSBFIRST as #define MSBFIRST and some use an enum of the two.

[thinkyhead]

I mean, we can keep adding workarounds, like defining BitOrder in HAL_SPI.h differently for each platform. But ultimately, it just keeps digging us deeper.

[boelle]

@thinkyhead still working on this one?

[boelle]

this one has gone stale and discussion could as well be held on discord

Marlin Discord server. Join link: https://discord.gg/n5NJ59y

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