From: Ruslan Trofymenko ruslan.trofymenko@linaro.org
Add a short documentation for A/B enablement and 'ab_select' command usage.
Signed-off-by: Ruslan Trofymenko ruslan.trofymenko@linaro.org Signed-off-by: Igor Opaniuk igor.opaniuk@gmail.com Signed-off-by: Sam Protsenko semen.protsenko@linaro.org Reviewed-by: Alistair Strachan astrachan@google.com Reviewed-by: Sam Protsenko semen.protsenko@linaro.org Reviewed-by: Simon Glass sjg@chromium.org ---
doc/android/ab.txt | 67 ++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 67 insertions(+) create mode 100644 doc/android/ab.txt
diff --git a/doc/android/ab.txt b/doc/android/ab.txt new file mode 100644 index 0000000000..9f37ed5c58 --- /dev/null +++ b/doc/android/ab.txt @@ -0,0 +1,67 @@ +Android A/B updates +=================== + +Overview +-------- + +A/B system updates ensures modern approach for system update. This feature +allows one to use two sets (or more) of partitions referred to as slots +(normally slot A and slot B). The system runs from the current slot while the +partitions in the unused slot can be updated [1]. + +A/B enablement +-------------- + +The A/B updates support can be activated by specifying next options in +your board configuration file: + + CONFIG_ANDROID_AB=y + CONFIG_CMD_AB_SELECT=y + +The disk space on target device must be partitioned in a way so that each +partition which needs to be updated has two or more instances. The name of +each instance must be formed by adding suffixes: _a, _b, _c, etc. +For example: boot_a, boot_b, system_a, system_b, vendor_a, vendor_b. + +As a result you can use 'ab_select' command to ensure A/B boot process in your +boot script. This command analyzes and processes A/B metadata stored on a +special partition (e.g. "misc") and determines which slot should be used for +booting up. + +Command usage +------------- + + ab_select <slot_var_name> <interface> <dev[:part_number|#part_name]> + +for example: + + => ab_select slot_name mmc 1:4 + +or + + => ab_select slot_name mmc 1#misc + +Result: + + => printenv slot_name + slot_name=a + +Based on this slot information, the current boot partition should be defined, +and next kernel command line parameters should be generated: + + - androidboot.slot_suffix= + - root= + +For example: + + androidboot.slot_suffix=_a root=/dev/mmcblk1p12 + +A/B metadata is organized according to AOSP reference [2]. On the first system +start with A/B enabled, when 'misc' partition doesn't contain required data, +the default A/B metadata will be created and written to 'misc' partition. + +References +---------- + +[1] https://source.android.com/devices/tech/ota/ab +[2] bootable/recovery/bootloader_message/include/bootloader_message/bootloader_message.h