[U-Boot] [PATCH] nand: fix reading after switching ecc

[Gupta, Pekon]

>From: Jeroen Hofstee [mailto:jeroen at myspectrum.nl]
[...]
>
>The gpmc will fail in hw ecc mode when trying to do subpage reads. Pekon any
>suggestion for the elm mode, or should this bit just be cleared
>unconditionally?
>
There are two reasons for not supporting sub-page feature in OMAP platforms:

(1) ELM h/w engine has limitation that it can support ecc-corrections
only in junks of 512Bytes.  
Now some flash devices (like page-size=4K) have sub-page = 4k/4 = 1k.
Thus, it becomes trivial (though not impossible) to support sub-page.
Hence, for now sub-page feature has been marked as 'un-supported' for OMAP.

(2) Also, we found that sub-page feature was not giving much benefit,
except improving data-packing density on given NAND to some extent.
Like in UBIFS, having sub-page feature packs 'erase-header' and volume-id-header'
into single page, thereby saving 1 page for every erase-block.
Memory Saved = (1x page-size)/ block-size = 2k / 128k = 1/64 ~ 1.5 % (raw calculations).

Hence, in summary, yes you can un-conditionally clear the sub-page read bit.

with regards, pekon