[RFC] mmc: add types + traits for SD/MMC peripherals

[rmsyn]

SD/MMC Trait

This RFC proposal is to add a set of traits for abstracting common functionality for handling SD/MMC peripherals.

Why

Currently, drivers based on embedded-hal that implement the SD/MMC protocol, like embedded-sdmmc-rs, are based on the Spi traits.

However, traits that are more generally abstract over SD/MMC functionality will allow for a wider range of driver implementations.

For example, drivers for SD/SDIO/MMC peripherals, in various modes and specification versions.

Reference Implementation Requirements

This tracking issue is accompanied with a reference implementation to add the traits to embedded-hal:

• sdmmc: add HAL traits for SD/MMC peripherals
  • SD/SDIO intialization
    • variants for SPI quirks
  • MMC initialization
    • variants for SPI quirks
  • Sending commands to the device
  • Reading responses from the device
  • Data transfers (read/write)
  • Separate, related async implementation (?)
  • Delay/wait functionality
  • Tuning
  • Interrupt configuration + handling
  • FIFO configuration + handling
  • DMA configuration + handling