[Yaffs] Can't mount yaffs2 on K9G8G08U0B

Hi guys,

I can't mount yaffs2 with MLC K9G8G08U0B.
The spec of this flash lists below:
Page size: 2048
Spare size: 64
Block size: 256K
ECC: BCH
OOB used:
oob-free:(2, 38)
ecc-pos: 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63

I have a feeling that the my oob used is not correct.
The hardware's generated ecc steals some bytes from my yaffs area.
Does someone have same problem with me?