USB: serial: ftdi_sio: Add MTP NVM support

Hi Johan,

Thanks for the review.

Quoted 8 lines · Johan Hovold

The CBUS configuration is stored in just a couple of words at a known offset and could easily be retrieved without depending on the nvmem subsystem when needed.

As I assume fiddling with the EEPROM should be rare (e.g. done during development or by hobbyists) and would still depend user-space tools (e.g. for layouts, checksums and external EEPROMs), I think we should just leave it all to user space to deal with.

I'm fine with that, to be honest I'm interested in adding GPIO control but wanted to progress step by step, EEPROM access being the first one. I thought it could have been valuable to expose it via nvmem, but like you said, we can keep this internal for now.

So, I'll come back with new patche(s) for CBUS GPIOs.