Adding subpage support to NAND driver -- backwards compatibility concerns

From: Ben Shelton <ben.shelton@ni.com>
To: dwmw2@infradead.org, computersforpeace@gmail.com,
	dedekind1@gmail.com, adrian.hunter@intel.com,
	linux-mtd@lists.infradead.org, linux-kernel@vger.kernel.org
Cc: joshc@ni.com, xander.huff@ni.com, punnaiah.choudary.kalluri@xilinx.com
Subject: Adding subpage support to NAND driver -- backwards compatibility concerns
Date: Wed, 22 Apr 2015 12:29:44 -0500	[thread overview]
Message-ID: <20150422172944.GA4134@bshelton-desktop> (raw)

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