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

[Ruslan Trofymenko]

Hi Simon,

On Thu, 6 Dec 2018 at 03:31, Simon Glass <sjg at chromium.org> wrote:
>
> Hi,
>
> On Tue, 27 Nov 2018 at 12:57, Ruslan Trofymenko
> <ruslan.trofymenko at linaro.org> wrote:
> >
> > Add a short documentation for A/B enablement and 'android_ab_select'
> > command usage.
> >
> > Signed-off-by: Ruslan Trofymenko <ruslan.trofymenko at 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.
>

Agree with all your previous comments, will send v2 shortly. But this
one I'd like to leave as is (I will drop android_ prefix though). I
think adding "android" command may lead to unneeded architecture
complexity in future, e.g.:
  - we will need to re-work next commands as sub-commands of "android"
command: fastboot, dtimg, mmc_swrite (which can be hard as ab_select
command file has BSD license and other commands have GPL license)
 - we will need to implement sub-sub-commands (e.g. for "android dtimg
dump ..." etc.)
 - having "android" command can be inconvenient for users and may
break existing API (e.g. it will force users to use "android fastboot"
instead of just "fastboot" command)
 - actually I don't see any namespace issues that can be solved by
adding "android" command

Also, in subsequent patch, I will add the dedicated menu option for
Android-related commands and will pull all existing Android commands
(along with ab_select) to that menu entry.

So I hope it's fine with you if I leave this command as "ab_select"?

Thanks.

> > +
> > +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