I am working through a pull-request to add a new eMMC driver for the STM32 family, and much of it is based on the sdmmc_stm32 driver for SD cards. EMMC chips use a different protocol over the SDIO bus, though it is electrically identical. A suggested approach is to pull all of the common code out of the existing sdmmc_stm32 driver, and have 3 files: sdmmc_stm32_common.c, sdmmc_stm32.c, and emmc_stm32.c, with all the code that is common in one place and not copied.

I agree that this approach would be best, but it will mean significant refactoring to the existing driver that I do not have the hardware to test. Anyone willing to help?
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Nick