Controlling which partition gets booted to in an A/B partitioning scheme using a boot partition

[Luther Goh Lu Feng]

I have a multi paritioned microSD card as follows. P1 is the boot partition, P2 and P3 are rootfs partitions. Based on my investigations, I have ensured both P2 and P3 are bootable rootfs. By default, the BBB will boot to P2. By adding mmcpart=3 in /boot/uEnv.txt, I can let the BBB boot to P3. The partition booted to was verified using output from df and seeing which device / was mounted on

I would like to control which partition gets booted to using P1 instead of P2. I remove mmcpart=3 in /boot/uEnv.txt in P2. Then in 1 attempt I added a /uEnv.txt with mmcpart=3 in P1, and in another, I added a /boot/uEnv.txt with mmcpart=3 in P1. Both attempts did not work.

I have referenced the uboot patch[1] by Robert, but perhaps I have missed something

# fdisk -l /dev/mmcblk0

Disk /dev/mmcblk0: 14.5 GiB, 15523119104 bytes, 30318592 sectors

Units: sectors of 1 * 512 = 512 bytes

Sector size (logical/physical): 512 bytes / 512 bytes

I/O size (minimum/optimal): 512 bytes / 512 bytes

Disklabel type: dos

Disk identifier: 0x9582e2f8

Device         Boot   Start      End Sectors  Size Id Type

/dev/mmcblk0p1         8192   212991  204800  100M 83 Linux

/dev/mmcblk0p2       212992  4308991 4096000    2G 83 Linux

/dev/mmcblk0p3      4308992  8404991 4096000    2G 83 Linux

/dev/mmcblk0p4      8404992 12503039 4098048    2G  5 Extended

/dev/mmcblk0p5      8407040 12503039 4096000    2G 83 Linux

[1] https://github.com/RobertCNelson/Bootloader-Builder/blob/master/patches/v2017.11/0001-am335x_evm-uEnv.txt-bootz-n-fixes.patch