STM32H735 SDMMC Error FR_NO_FILESYS

Sounds like a cache coherency issue.

Brilliant!! Turned off data cache and voila, it works! Thanks.

Do you think this means I cannot use the data cache at all, or should I selectively disable it while doing SD card IO?

Thanks again! Turned on the macro and turned back on the data cache. This now results in a different error (FR_INT_ERR). But I'm sure I'll find out quickly what is going on, now that you put me on the right track. Wonderful to have smart and helpful folks around.

To conclude this thread, it is worth explaining why it still didn't work with the  "ENABLE_SD_DMA_CACHE_MAINTENANCE" set. The macro invalidates the cache of a 32-byte aligned address range and makes visible what has been transferred by DMA into the underlying memory and with the macro set, all data read from disk shows up properly. But now the inverse problem occurs, invalidating cache content that was not related to the transfer but part of the 32-byte aligned invalidation. The filesystem FATS structure has at its very end the window buffer for the last read sector, but the buffer doesn't start at an aligned address. So the invalidation reaches back into the preceding part of the structure and destroys recently updated variables.

A buffer allocation via malloc would be ok as it would allocate at 32 byte boundaries, but in this case it is just the beginning of the FATS struct that is aligned, the buffer at the end is not. For now I fixed it by putting some filler words into the struct, after the last variable and before the buffer declaration, this way pushing the buffer over to the next 32-byte boundary. The down site is that I need to save the modified ff.h before generating code with CubeMX and then restore it.