utagboot for Motorola Droid 4 xt894

With utagboot we can configure droid 4 utags partition to boot to any
partition on the device using the stock Linux kernel that comes with the
device. This tool just generates a utags partition file.

The way utagboot works is by modifying the stock kernel command line arguments
using the utags partition on the device. The utags partition seems to be
for custom ATAGs that also show up on the kernel command line. The utags
partition can be easily erased and flashed with fastboot.


USAGE

Typically utagboot is used together with a kexec based minimal rootfs,
such as droid4-kexecboot. Please see the README for droid4-kexecboot for
more information:

https://github.com/tmlind/droid4-kexecboot


DEPENDENCIES

Shell, Make and fastboot on your PC. Usable Linux rootfs on a partition
with recent kexec-tools on the device.


LIMITATIONS USING THE STOCK KERNEL

With the stock kernel, CONFIG_VT is disabled making it not very friendly for
distro use. And the LCD seems to operate only in manual update mode, so using
the LCD needs customized software. Further, the stock kernel seems to have some
issues, for example warm reset can cause the device to reboot multiple times.
And there are issues trying to use the stock kernel and a newer kernel with the
same distro with stock kernel wanting to use /dev/ttyO2 while newer should be
using /dev/ttyS2 for serial console.

To work around these issues, it's best to use the stock kernel to boot to a
Linux distro on a selected root partition enough to just kexec boot a newer
kernel.

Also note that kexec booting older v3.0.8 based kernels needs to be done
with the custom kexec binary that comes with SafeStrap. And it seems that
Lineage OS currently has a dependency to the SafeStrap loop devices being
configured.


REFERENCES

For more information how utags work, see "fastboot oem vuln: Android Bootloader
Vulnerabilities in Vendor Customizations" by Roee Hay. I don't think the
related fastboot oem to modify utags are available on droid 4 though.