UBIFS seeing corrupt blank pages when image flashed via u-boot

[Gupta, Pekon]

Hi Calvin,

>From: Artem Bityutskiy [mailto:artem.bityutskiy at linux.intel.com]
>>On Mon, 2014-01-13 at 17:49 +0530, Calvin Johnson wrote:
>> If the 4th to last-page are left blank and not covered with ECC, what
>> will happen in case of bit flips on the blank pages? There was an
>> issue reported some time back.
>> http://lists.infradead.org/pipermail/linux-mtd/2012-January/039256.html
>>
>> Does UBI/UBIFS take care of this now?
>
>No. UBIFS still assumes that blank pages are ECC-protected by the
>driver. No one stepped in and took care of changing this yet.
>
Yes, it's true that in newer technologies (specially < 28nm flash), we are seeing
lot of erased-pages having bit-flips. And due to which UBIFS is cribbing.
But as there is no ECC stored in an erased-page, bit-flips in erased-page
cannot be corrected, unless you compare each byte of read_data.
However, there are other way of handling bit-flips in erased-page.
Following are few ways in which OMAP NAND driver handles bit-flips
in erased-page:

*Case-1*: If bit-flips are found in data-region of an erased-page.
(1) An erased-page implicitly means that its data-region should *only* contain 0xff,
   so its safe to fill read_buf() with 0xff.
  But the controller driver should report the correctable/un-correctable bit-flips
  to upper-layer, so that upper-layers like UBI take corrective action by re-erase
  this block before using it.
(Refer) http://lists.infradead.org/pipermail/linux-mtd/2014-January/051368.html


*Case-2*: If bit-flips are found in oob-region of an erased-page.
This is bit trivial, because if there are bit-flips in ecc-layout (OOB region) of
erased-page, it would be difficult to differentiate between an
erased-page v/s programmed-page.
Though you can keep a 'marker' in ecc-layout reserved for detecting
Programmed-pages, but that marker byte itself can be subjected to
bit-flips (assuming on MLC and newer technology NAND bit-flips are common).
So, OMAP NAND driver takes probabilistic approach.
(Refer) http://lists.infradead.org/pipermail/linux-mtd/2014-January/051367.html
-----------------------
This patch 'assumes' any page to be 'erased':
		(a) if        all(read_ecc)  == 0xff
		(b) else if   all(read_data) == 0xff
-----------------------


Currently both UBI and UBIFS layer checks for erased-page to be all(0xff),
But I think its over-kill to put this burden on UBI or UBIFS layer, because
low-level controller drivers can handle this easily.
So, if Artem and Brian agree to above approaches, then I can a submit patch
for removal of:
 - "ubi_self_check_all_ff()" from UBI layer.
 - checking of 'buf == 0xff' from ubifs_scan_leb() in UBIFS layer.


with regards, pekon