[U-Boot] [PATCH v2 2/2] cmd: Add 'bcb' command to read/modify/write BCB fields

[Sam Protsenko]

Hi Eugeniu,


On Mon, May 20, 2019 at 10:23 AM Eugeniu Rosca <erosca at de.adit-jv.com> wrote:
>
> Hi Simon
> cc: Sam, Igor, feel free to correct/augment anything of below
>
> On Sat, May 18, 2019 at 10:33:02AM -0600, Simon Glass wrote:
> > Hi Eugeniu,
> >
> > On Fri, 17 May 2019 at 08:46, Eugeniu Rosca <erosca at de.adit-jv.com> wrote:
> > >
> > > 'Bootloader Control Block' (BCB) is a well established term/acronym in
> > > the Android namespace which refers to a location in a dedicated raw
> > > (i.e. FS-unaware) flash (e.g. eMMC) partition, usually called "misc",
> > > which is used as media for exchanging messages between Android userspace
> > > (particularly recovery [1]) and an Android-capable bootloader.
> > >
> [..]
> >
> > Where is this documented? Perhaps it should go in README.avb2?
>
> README.avb2 is solely about the "verified/secure" booting of Android,
> while the 'bcb' command proposed in this patch can be useful both in
> verified boot scenarios (e.g. full-featured U-Boot builds), as well
> as in non-avb ones (e.g. development, PoC, demos, custom
> configurations). Thus, I think that README.avb2 is not the best
> place for 'bcb'.
>
> More generally, as somebody who uses git as an extension of himself, I
> am quite happy with commit-only documentation. OTOH, for those who
> receive U-Boot in tarballs or expect source-level docs/tutorials, I
> agree that having the 'bcb' described in a README might be helpful.
>
> I can identify two Android-dedicated README files, but none of them
> seems to be suitable for the new command:
>  - doc/README.android-fastboot
>  - doc/README.avb2
>
> Igor, Sam, what's your view on the above? Would you suggest creating
> a doc/README.android-bcb or there is a more elegant solution to it?
>

Documentation would be nice. Although you already provided a generic
description of 'bcb' command in 'help bcb', the user often wants to
read more high-level documentation. I'd say, you can copy the
description from commit message to doc/README.android-bcb, extending
it with usual use-cases, and how this command can be used in those
use-cases. For example, your pseudo-code for reboot reason you
provided to me earlier, etc. Also, it can be useful to mention if
Google have any requirements (mandatory or optional) for the
bootloader (misc partition, reboot reason, recovery, etc), and how
this 'bcb' command can help in those requirements implementation.

All that said, IMHO documentation/test wise: it's not critical in this
case, you can add that later, just to speed-up the whole development
process and divide it into iterations. But that's for maintainers to
decide, of course.

Also, I've a feeling that we have another problem, more common than
just a documentation. At the moment we have a bunch of Android related
features, which don't have namespace separation on several levels:
 - file/directories: we may want to move all Android related commands
to sub-directory
 - commands: we may want to add main command called "android" for all
Android-related commands, or maybe just a prefix
 - Kconfig: we may want to have some generic naming scheme for all
Android-related commands
 - Documentation, tests: the same here

So at some point we will probably need to discuss and fix that
somehow. Not here, of course :)

> >
> > Should it default to enabled if avb is used?
>
> I think at this specific moment in time, 'bcb' is orthogonal (meaning it
> is neither a direct, nor a reverse dependency) to any other Android
> feature in U-Boot. This could be re-assessed, if platform maintainers
> start to rely on 'bcb' in their U-Boot environments on regular basis.
>
> --
> Best Regards,
> Eugeniu.