[U-Boot] [PATCH 6/7] doc: android: Add simple guide for A/B updates

[Simon Glass <sjg@chromium.org>]

Hi,

On Tue, 27 Nov 2018 at 12:57, Ruslan Trofymenko
<ruslan.trofymenko@linaro.org> wrote:
>
> Add a short documentation for A/B enablement and 'android_ab_select'
> command usage.
>
> Signed-off-by: Ruslan Trofymenko <ruslan.trofymenko@linaro.org>
> ---
>  doc/README.android-ab | 67 +++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 67 insertions(+)
>  create mode 100644 doc/README.android-ab
>
> diff --git a/doc/README.android-ab b/doc/README.android-ab
> new file mode 100644
> index 0000000..230088c
> --- /dev/null
> +++ b/doc/README.android-ab
> @@ -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_ANDROID_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 'android_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
> +-------------
> +
> +    android_ab_select <slot_var_name> <interface> <dev[:part_number|#part_name]>

Can we have a shorter command?

Perhaps we need a new 'android' command with an 'ab_select'
subcommand? Then the automatica abbreviation will work.

> +
> +for example:
> +
> +    => android_ab_select slot_name mmc 1:4
> +
> +or
> +
> +    => android_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
> --
> 2.7.4
>

Regards,
Simon