Adding subpage support to NAND driver -- backwards compatibility concerns

[Ben Shelton]

Hi all,

We're currently carrying a patch out of tree to add subpage read and
write support to the pl353_nand driver.  Xilinx is currently working to
mainline this driver; see

http://www.spinics.net/lists/devicetree/msg76307.html

We'd like to upstream our patch, but my concern is that UBIFS behaves
differently when it knows that the flash device supports subpages.  I
have a couple of questions related to that:

- I know from experience that bad things happen when you use a kernel
  without subpage support with an UBIFS filesystem that was formatted
  with subpage support.  Is it safe to do the opposite (kernel with
  subpage support / UBIFS filesystem formatted without subpage support)?

- Assuming that it isn't safe, what's the best way to add subpage
  support to this driver in an upstreamable way / without breaking
  people?  Would it be sufficient to add subpage support as a Kconfig
  option that's disabled by default with a strongly-worded message
  describing the consequences of enabling it?

Thanks,
Ben