[LEDE-DEV] Why my kernel does not find the rootfs in a ubi partition?

[Alberto Bursi]

I'm trying to add support for a ZyXEL NSA310 (kirkwood-based NAS), but 
the kernel does not find the rootfs in the ubi.

The image has kernel+rootfs(squashfs)+rootfs_data(ubifs?) in ubi, built with
FILESYSTEMS := squashfs
IMAGES += factory.bin
IMAGE/factory.bin := append-ubi
KERNEL_IN_UBI := 1

If I boot the initramfs (ramdisk) system image while there is a ubi 
image flashed to nand, it gives this error while booting

[ 1.070706] UBI: auto-attach mtd2
[ 1.073868] ubi0: attaching mtd2
[ 1.083812] UBI: EOF marker found, PEBs from 28 will be erased
[ 1.089971] ubi0: scanning is finished
[ 1.093994] ubi0 error: ubi_read_volume_table: the layout volume was not 
found
[ 1.101458] ubi0 error: ubi_attach_mtd_dev: failed to attach mtd2, error -22
[ 1.108559] UBI error: cannot attach mtd2

but since ubi isn't necessary for the initramfs image, it works fine and 
I get to system console.

If I flash the image to nand (and set uboot for booting from ubi, with 
NO bootargs passed to the kernel), the kernel boots until it finds the 
same error above, and then it posts this

[ 1.605331] VFS: Cannot open root device "(null)" or unknown-block(0,0): 
error -6
[ 1.612865] Please append a correct "root=" boot option; here are the 
available partitions:
[ 1.621268] 1f00 1024 mtdblock0 (driver?)
[ 1.626346] 1f01 512 mtdblock1 (driver?)
[ 1.631439] 1f02 129024 mtdblock2 (driver?)
[ 1.636521] Kernel panic - not syncing: VFS: Unable to mount root fs on 
unknown-block(0,0)

And of course auto-reboots.

I've tried with bootargs like ubi.mtd=2 but with any bootarg about root= 
or ubi it simply hangs and does not even start the kernel (shortly after 
uboot hands over to the kernel the device resets).

the ubi partition seems to be OK according to uboot. Any help 
appreciated. :confounded:

NSA310> ubi part ubi
ubi0: attaching mtd1
ubi0: scanning is finished
ubi0: attached mtd1 (name "mtd=2", size 126 MiB)
ubi0: PEB size: 131072 bytes (128 KiB), LEB size: 129024 bytes
ubi0: min./max. I/O unit sizes: 2048/2048, sub-page size 512
ubi0: VID header offset: 512 (aligned 512), data offset: 2048
ubi0: good PEBs: 1008, bad PEBs: 0, corrupted PEBs: 0
ubi0: user volume: 3, internal volumes: 1, max. volumes count: 128
ubi0: max/mean erase counter: 1/0, WL threshold: 4096, image sequence 
number: 1235565284
ubi0: available PEBs: 0, total reserved PEBs: 1008, PEBs reserved for 
bad PEB handling: 20

NSA310> ubi info
UBI: MTD device name: "mtd=2"
UBI: MTD device size: 126 MiB
UBI: physical eraseblock size: 131072 bytes (128 KiB)
UBI: logical eraseblock size: 129024 bytes
UBI: number of good PEBs: 1008
UBI: number of bad PEBs: 0
UBI: smallest flash I/O unit: 2048
UBI: VID header offset: 512 (aligned 512)
UBI: data offset: 2048
UBI: max. allowed volumes: 128
UBI: wear-leveling threshold: 4096
UBI: number of internal volumes: 1
UBI: number of user volumes: 3
UBI: available PEBs: 0
UBI: total number of reserved PEBs: 1008
UBI: number of PEBs reserved for bad PEB handling: 20
UBI: max/mean erase counter: 1/0

-Alberto