[U-Boot] [PATCH] fastboot: Fix slot names reported by getvar

[Eugeniu Rosca]

Hi Sam,

Thanks for the detailed answer. Some comments below.

On Thu, Jun 13, 2019 at 04:59:40PM +0300, Sam Protsenko wrote:
> Hi Eugeniu,
> 
> On Thu, Jun 13, 2019 at 12:31 PM Eugeniu Rosca <erosca at de.adit-jv.com> wrote:
> >
> > Hi Sam,
> >
> > On Thu, Jun 13, 2019 at 12:49:45AM +0300, Sam Protsenko wrote:
> > > In commit [1] fastboot tool was changed w.r.t. new A/B specification [2],
> > > and now we should report slot names in "a" format instead of "_a".
> > > Latter is now considered legacy and we shouldn't rely on that anymore.
> >
> > This looks like a change which advantages the users who are always on
> > the tip/HEAD of all relevant components (fastboot and U-Boot), but that
> > rarely happens in the industry. Suppliers and hardening vendors often
> > deliver obsoleted versions because they can't keep up with upstream
> > development. Can you please document the behavior of 'fastboot flash'
> > (and anything else relying on
> > 'fastboot getvar (current-slot|slot-suffixes) in below scenarios:
> >
> > A. fastboot >= [1] && U-Boot + this patch
> > B. fastboot >= [1] && U-Boot - this patch
> > C. fastboot <  [1] && U-Boot + this patch
> > D. fastboot <  [1] && U-Boot - this patch
> >
> > Would it be possible to keep U-Boot backward-compatible, such that
> > regardless of the scenario enumerated above, 'fastboot flash' will
> > always succeed?
> >
> 
> I'm afraid in this particular case we weren't given any choice, and we
> won't be able to provide backward-compatibility for older Android
> releases. After this commit:
> 
>     [3] https://android.googlesource.com/platform/system/core/+/42b18a518bac85c3eea14206f6cbafbd1e98a31f

Thumbs up for pointing out this specific commit.

> 
> they dropped support for "_a" format completely (in fastboot tool). So:
>   * if user runs new fastboot tool ( >= [3]), then the only way to
> make "fastboot flash" work is to return slot in "a" format from
> bootloader
>   * if user runs old fastboot tool (< [1]), then the only way to make
> "fastboot flash" work is to return slot in "_a" format from bootloader

This interface change seems to carry no semantic value, so it's
unfortunate to see such kind of non-backward-compatible update
contributed by the maintainers of the user-space tool.

> Good news is that user can basically downgrade or upgrade fastboot
> tool, to be in sync with U-Boot version in use. Bad news, we need to
> decide which Android version to break in U-Boot/master.
> 
> I suggest we track AOSP/master in U-Boot/master. Please let me know if
> you agree, or maybe there is some better way I'm missing.

I can't come up with a better proposal.

Reflecting on fastboot in general, I think there are many more
questions which lack a straightforward answer:
 - How accurately the fastboot protocol [4] reflects the AOSP
   user-space implementation (and viceversa)?
 - Some basic git grepping reveals that the fastboot protocol [4]
   was once upgraded from v0.3 to v0.4, but that's pretty much it. It
   definitely doesn't cover the evolution of fastboot usage/parameters.
   For example, none of the AB/slot-related parameters (e.g.
   "slot-suffixes", "current-slot") has ever been documented in [4].
 - Since the protocol and the actual development are out of sync,
   staying aligned to AOSP means asserting about the fastboot
   interface updates by reviewing the AOSP patches (which is time
   consuming and cannot be easily automated).
 - OTOH, it is also not clear to which degree U-Boot/master is aligned
   to AOSP/master even after integrating this patch. As example,
   commit [3] mentioned by you dropped the support for 'slot-suffixes'
   altogether while the option is still being processed in U-Boot.

In a nutshell, I think we have no other choice but to apply this fix
(still not clear if w/ or w/o keeping the support for "slot-suffixes").
I think it's also a must to warn the users that this patch
assumes/depends on AOSP fastboot version/commit [3] or higher.

> > > [1] https://android.googlesource.com/platform/system/core/+/8091947847d5e5130b09d2ac0a4bdc900f3b77c5
> > > [2] https://source.android.com/devices/tech/ota/ab/ab_implement#partitions

[4] https://android.googlesource.com/platform/system/core/+/master/fastboot/README.md

-- 
Best Regards,
Eugeniu.