640: Skip EasyDMA slice location check for empty slices and copy data if necessary r=Dirbaio a=TilBlechschmidt
As discussed, this PR makes the following changes:
- Ignore pointer location of zero-length slices (fixes#631)
- Change default functions so they copy the tx buffer if it does not reside in RAM
- Introduce new variants for `write`, `transfer`, and their blocking versions which fails instead of copying
- Add documentation about the motivation behind all these variants
<img width="984" alt="image" src="https://user-images.githubusercontent.com/5037967/155415788-c2cd1055-9289-4004-959d-be3b1934a439.png">
Remaining TODOs:
- [x] Change copying behaviour for other peripherals
- [x] TWI
- [x] UART
- [x] Add module-level documentation regarding EasyDMA and `_from_ram` method variants
`@Dirbaio` it probably makes sense for you to review it now before I "copy" over the changes to the other two peripherals.
Co-authored-by: Til Blechschmidt <til@blechschmidt.de>
13247897b0
