utagboot for Motorola Droid 4 xt894

With utagboot we can boot droid 4 directly to any partition on the device
using the stock Linux kernel that comes with the device.

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.


CURRENT LIMITATIONS

For some reason /dev/fb0 is not working and there is no console on the LCD. It
might be related to the LCD only accepting 2 for display0/update_mode instead
of 1 for automatic updates.

Also it seems that a warm reset of the device with fastboot reboot can cause
the device to keep rebooting, this does not seem to happen after a proper
power cycle.


DEPENDENCIES

Shell, Make and fastboot on your PC


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.


INSTALL

First root your device, then reformat the webtop partition and untar the Linux
distro of your choice there. See also the notes below regarding necessary
kernel modules for the new root file system.

Then to install utags.bin to the device utags partition by connecting the
device over USB in fastboot mode and typing:

$ Make install

This will generate a utags.bin file and then write it to the device utags
partition. On reboot, your device will attempt to boot the stock kernel
without initrd and run the standard /sbin/init on the rootfs. If you want
to use a custom kernel command line, you can do it with:

$ make cmdline="console=ttyO2,115200 debug earlyprintk earlycon"

The fastboot mode can be enabled by holding down Volume Down button while
pressing the power button to turn it on.


UNINSTALL

To uninstall utagboot, connect the device in fastboot mode over USB and type:

$ make uninstall

This will erase the utags partition and allow booting the stock Android or
SafeStrap if that is installed.


NOTES

SafeStrap can still be used together with utagboot, but it will be inactive
when utagboot is active. To use SafeStrap instead, see the uninstall notes.

By default utagboot uses the 1.3GB /dev/mmcblk1p23 webtop partition as the
root file system, and then you can kexec from there to a newer kernel and
to a larger partition on the external SD card for example.

Using the 8GB /dev/mmcblk1p25 might work too after re-formatting it to
ext4. The stock Android kernel will show notes about it though, so please
make proper back-ups. Using the external SD card should work too.

In order to use serial console and kexec, you need to load the kexec modules
uart.ko arm_kexec.ko and kexec.ko when the distro installed to rootfs starts
booting. For information about using the serial console and the related
kernel modules, please see:

http://muru.com/linux/d4/

You probably also want copy the modules from the stock file system to the
new rootfs /lib/modules/3.0.8-g448a95f/kernel to have working WLAN.