Hi Ben,
Add another nand write. variant, ubi. This command will request of nand_write_skip_bad() that all trailing all-0xff pages will be dropped from eraseblocks as they are written as-per the reccommended behaviour of the UBI FAQ.
If I understand the code correctly, then the assumption is that writing FFs to an erased flash is essentially a no-op, right? This is not really UBI specific, so why don't we use a name like e.g. "trimffs" for the new functionality?
Moreover now that I think about it, I can imagine a corner case where the flash is not erased at positions where the image contains ffs. As I read your code, the ffs will silently be dropped and no error will be generated, although the contents of the image will _not_ correpsond to the contents in flash.
If this is right, then this has potential for great confusion. Maybe we should check that the flash is really erased at the positions corresponding to ffs?
Cheers Detlev