Re: [PATCH] android_ab: fix slot selection

[Mattijs Korpershoek <mkorpershoek@kernel.org>]

Hi Colin,

Thank you for the patch.
Sorry for the late review. For some reason I have not been cc'ed so I
missed this.

Please use ./scripts/get_maintainer.pl -f boot/android_ab.c next time
you send a patch.
See: https://docs.u-boot.org/en/latest/CONTRIBUTE.html#contributions

On Fri, Mar 13, 2026 at 15:14, Colin Pinnell McAllister <colin.mcallister@garmin.com> wrote:

> The boot selection rules state that a slot is bootable if it is not
> corrupted and either has tries remaining or has already booted
> successfully. However, slots that have tries_remaining == 0 and
> successful_boot == 1 will be disregarded when picking the slot to
> attempt.

Can you please give me more references to the boot selection rules?
Is this just the code comment that's mentioned (boot/android_ab.c:276) ?

I've checked:
* https://source.android.com/docs/core/ota/ab
* https://source.android.com/docs/core/ota/ab/ab_implement
and it's unclear to me from that.

Then, I've looked a bit at the default bootcontrol AIDL hal at:
https://android.googlesource.com/platform/hardware/interfaces/+/refs/heads/main/boot/aidl/default/Android.bp

It seems to be using libboot_control as a static library.
libboot_control tells us that a slot is considered unbootable when:
tries_remaining != 0;

See:
https://cs.android.com/android/platform/superproject/+/android-latest-release:hardware/interfaces/boot/1.1/default/boot_control/libboot_control.cpp;l=332?q=libboot_control

>
> Update the selection logic so slots marked successful remain eligible
> even when their tries counter is zero. Also include the successful_boot
> state in the unbootable-slot debug output.

What happens if the tries counter is zero and we attempt to boot with
dec_tries=true? In that case, we will decrement tries_remaining which is
an uint8_t (see: android_bootloader_message.h).

>
> Signed-off-by: Colin Pinnell McAllister <colin.mcallister@garmin.com>
> ---
>
> An alternative would be to keep the existing behavior and update the
> comment in boot/android_ab.c:276 to remove the successful_boot
> condition. I do prefer updating the logic to match the documented rules.
> Currently, the userspace AB code will need to increment the tries
> counter for a slot that has booted successfully but has no tries left.
>
>  boot/android_ab.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
>
> diff --git a/boot/android_ab.c b/boot/android_ab.c
> index 13e82dbcb7f..93e74f81d4b 100644
> --- a/boot/android_ab.c
> +++ b/boot/android_ab.c
> @@ -289,11 +289,14 @@ int ab_select_slot(struct blk_desc *dev_desc, struct disk_partition *part_info,
>         slot = -1;
>         for (i = 0; i < abc->nb_slot; ++i) {
>                 if (abc->slot_info[i].verity_corrupted ||
> -                   !abc->slot_info[i].tries_remaining) {
> +                   (!abc->slot_info[i].tries_remaining &&
> +                        !abc->slot_info[i].successful_boot)) {
>                         log_debug("ANDROID: unbootable slot %d tries: %d, ",
>                                   i, abc->slot_info[i].tries_remaining);
> -                       log_debug("corrupt: %d\n",
> +                       log_debug("corrupt: %d, ",
>                                   abc->slot_info[i].verity_corrupted);
> +                       log_debug("successful: %d\n",
> +                                 abc->slot_info[i].successful_boot);
>                         continue;
>                 }
>                 log_debug("ANDROID: bootable slot %d pri: %d, tries: %d, ",
> --
> 2.53.0
>
>
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