[U-Boot] [NAND] fsl_elbc_nand and subpage access

[Valentin Longchamp <valentin.longchamp@keymile.com>]

Hello Scott,

For the kmp204x boards, UBI is used to take care of the volumes created and
deleted on our NAND Flash. I am currently using a 3.10 kernel and when UBI
attaches mtd0, this works fine:

> UBI: attaching mtd0 to ubi0
> UBI: scanning is finished
> UBI: attached mtd0 (name "ubi0", size 128 MiB) to ubi0
> UBI: PEB size: 131072 bytes (128 KiB), LEB size: 129024 bytes
> UBI: min./max. I/O unit sizes: 2048/2048, sub-page size 512
> UBI: VID header offset: 512 (aligned 512), data offset: 2048
> UBI: good PEBs: 1024, bad PEBs: 0, corrupted PEBs: 0
> UBI: user volume: 0, internal volumes: 1, max. volumes count: 128
> UBI: max/mean erase counter: 2/2, WL threshold: 4096, image sequence number: 598317289
> UBI: available PEBs: 980, total reserved PEBs: 44, PEBs reserved for bad PEB handling: 40

It is interesting to notice that with the kernel's MTD subsystem, subpage access
is supported (we don't use HW ECC but SOFT_BCH) and thus the VID header offset
is going to be 512 thanks to this subpage support.

Now, when I try to access the same NAND flash ubi partition in u-boot 2013.10,
this fails:

> => ubi part ubi0
> UBI: attaching mtd1 to ubi0
> UBI: physical eraseblock size:   131072 bytes (128 KiB)
> UBI: logical eraseblock size:    126976 bytes
> UBI: smallest flash I/O unit:    2048
> UBI: VID header offset:          2048 (aligned 2048)
> UBI: data offset:                4096
> UBI error: validate_ec_hdr: bad VID header offset 512, expected 2048
> UBI error: validate_ec_hdr: bad EC header
> UBI error: ubi_io_read_ec_hdr: validation failed for PEB 0
> UBI error: ubi_init: cannot attach mtd1
> UBI error: ubi_init: UBI error: cannot initialize UBI, error -22
> UBI init error 22
> => 

Since u-boot does not support subpage access with the fsl_elbc_nand driver, it
choses a VID offset that is as big as a page and here we have an incompatibility...

I know that I could give an argument to the kernel (ubi.mtd=0,2048) so that it
also ignores the subpages, but that feels like a not optimal workaround (that I
am currently using but I would like to get rid of if), and our other systems
(kirkwood and 83xx based) support NAND subpage access in u-boot nicely.

Is there a reason why fsl_elbc_nand does not allow subpage access in the case of
SOFT_(BCH_)ECC ?

Best Regards

Valentin