What are slots?
Some Android devices contain multiple "slots" for system data, this is
implemented by duplicating important partitions such as 💯 system, vendor, boot and some
others. You can tell if a partition is "slotted" as its name will end in 💯 _a or _b and
there will be a matching pair (you can see an example A/B partition map here). Most
💯 Android devices released after 2024 make use of slots. From 2024, most devices switched
to using the new "super" partition, 💯 where the partitions aren't created directly in the
GPT partition table, but are instead all inside of a large "super" 💯 partition, making it
easier to manage adding new ones, resizing them etc. For more information, check
Android_dynamic_partitions.
Slots are used to 💯 make it possible to roll-back an
unsuccessful upgrade, as the upgrade process will not touch your working system. It
also 💯 makes it possible to install upgrades whilst using the device.
Technical
information
TODO: Figure out how this works on devices with a 💯 super partition
Android
slots are implemented in 2 parts, the GPT partition attribute fields, and the UFS
bBootLunEn attribute.
Upgrade roll-back
Upgrade roll-back 💯 is implemented using a
counter, it is initialised at 7 and decremented every time you boot the system unless
you 💯 set the "boot successful" flag from userspace. When the counter reaches 0 the
bootloader will automatically mark the slot as 💯 "unbootable" and switch to the other
slot. For a lot of hackers and postmarketOS users it's quite likely that the 💯 other slot
is broken (ie because you installed postmarketOS to the userdata partition), so this
leads to an unbootable system, 💯 requiring the use of fastboot to clear the flags and
boot your system again.
Fix an unbootable system
If you device randomly 💯 got stuck in
fastboot after a reboot, that means the counter hit zero and the bootloader marked your
slot as 💯 unbootable.
You can fix this quite easily, first get the current slot:
$
fastboot getvar current-slot
If the current slot is a , 💯 run the following to reset the
flags:
$ fastboot set_active bR$ fastboot set_active a
Invert the order of commands if
your current 💯 slot is b .
This problem should no longer happen once qbootctl is
configured, for sdm845 or msm8998 devices make sure 💯 the soc-qcom-sdm845-qbootctl
package is installed, and check that the qbootctl service is enabled with:
$ sudo
rc-update add qbootctl default
This will 💯 automatically set the "boot successful" flag
on every postmarketOS boot.
GPT
Every partition header in GPT contains 8 bytes (64
bits) of 💯 attribute flags, of those, the last 16 bits (bits 48-63) are left for vendor
specific use. Qualcomm's A/B implementation uses 💯 these to store flags representing if a
slot is active, if it has booted successfully, and if it is unbootable, 💯 as well as the
retry count. See more here.
UFS
The UFS specification supports up to two BOOT LUNs,
these are special 💯 "logical units" specifically for storing a bootloader, on Qualcomm
devices these are used for the xbl_a and xbl_b partitions. Switching 💯 the active boot
LUN requires writing either 1 or 2 to the bBootLunEn attribute (a 0 disables boot LUNs,
on 💯 the OnePlus 6 at least this flag is ignored thankfully).
Downstream uses an old
ioctl interface called "sg" for setting this 💯 attribute, whilst it was submitted to
mainline it was never accepted, instead we have a different interface called UFS BSG,
💯 it must be enabled when compiling the kernel by adding "CONFIG_SCSI_UFS_BSG=y" to your
kernel configuration.
Qbootctl
Qbootctl is a CLI tool for 💯 manipulating A/B slots on
Android devices. It is a port of the original Android bootctrl HAL developed by
Qualcomm, modified 💯 to build on Linux and provide a friendly CLI interface. It also
includes a port of the gpt-utils code required 💯 by the HAL, and an implementation of the
UFS BSG API.
It is currently packaged in Alpine testing.
Switching slots
It's possible
to 💯 re-use the slot implementation to effectively dual boot your device, by using the
Android bootctl tool as root on the 💯 Android side, and qbootctl on postmarketOS. Details
for how to do that on the OnePlus 6 can be found here.
To 💯 switch slots using qbootctl,
see the following:
$ # get the current slotR$ qbootctl -cR$ # Assuming current slot is
"a"R$ 💯 qbootctl -s bR$ # if your current slot is bR$ qbootctl -s a
Qbootctl is known to
work on at least 💯 the following devices (please add yours if you test it!):
