SPI NAND Partitioning

The 128 MiB SPI NAND is split into four MTD partitions. The first holds the SPL + U-Boot; the others are UBI volumes used at runtime.

Name	Size	MTD	Size (hex)	Offset (hex)	Content
boot	1 MiB	mtd0	`0x0100000`	`0x0000000`	SPL + U-Boot (`spi-nand.bin`)
fota	15 MiB	mtd1	`0x0f00000`	`0x0100000`	UBI: recovery `uImage`, DTB and `uboot.env`
rootfs	56 MiB	mtd2	`0x3800000`	`0x1000000`	UBI: main `rootfs.ubifs` (the actual operating system)
data	56 MiB	mtd3	`0x3800000`	`0x4800000`	UBI: scratch space for downloaded firmware images
Σ	128 MiB		`0x8000000`

The exact byte sizes come from kernel_defconfig and the device-tree partitions node; double-check both if you change the layout.

Why this split

This layout supports the A/B firmware update strategy:

boot is updated in place when reflashing the bootloader.
fota carries a minimal recovery image (initramfs + kernel) able to reflash rootfs even when the main system is broken.
rootfs is the running system; it is mounted read-write as UBIFS.
data is used as a staging area to drop a new rootfs.ubifs before applying it (over ubiupdatevol).

UBI parameters

Measured on the board:

PEB size:        131072 bytes (128 KiB)
LEB size:        126976 bytes
min I/O size:    2048
sub-page size:   2048
VID header off:  2048
data offset:     4096
good PEBs:       448 (rootfs / data), 1016 (full chip)

When generating a UBIFS image in Buildroot, the LEB size of the image must match the LEB size of the runtime UBI device, otherwise mounting fails with:

UBIFS error: LEB size mismatch: 129024 in superblock, 126976 real

Set the Buildroot UBIFS LEB size accordingly under Filesystem images → ubifs.