All info current as of 01/22/2021I'm editing this original post to update what I've cobbled together from various forums. I do not take credit for any of the below. Nor the responsibility if you brick your phone. So, proceed with caution and at your own risk.
Rooting your stock ROM:
F2 rooting process works on Power 3 as well. Attached is the patched boot.img (magisk 22.0) or patch boot.img yourself.
The only TWRP version available is an unofficial version. This version doesn't mount super/system and there's no reason to install it.
Installing a GSI:
To install a GSI on your phone, below are the steps that works on Windows 10. I haven't identified any optional steps. So, make sure you follow the steps below, in order. Don't skip steps or ignore errors. Installing Magisk on any GSI results in a boot loop.
01. Download an AB GSI. If it a zip file, extract the img from it. Flashing an A-Only GSI will result in a boot loop.
02. Flash the stock ROM. I used UMIDIGI_Power_3.E_V1.2_202003282206. Keep the stock firmware and SP flash tool folders.
03. Boot into the stock rom. In developer settings enable OEM unlock, USB debugging and authorize it from your computer. If unfamiliar, google it. Don't bother setting up wifi or installing apps. The phone will be factory reset shortly. Ensure that you have the latest version of drivers.
04. Perform the below commands in the latest version of adb.
05. adb reboot recovery
06. Select Factory reset and reset.
07. adb reboot bootloader (or use the menu option). The screen should show "fastboot mode" at the bottom of the screen in small font.
08. fastboot flashing unlock. Read phone screen for instructions. You will have to press volume up key to continue to unlock.
09. fastboot --disable-verity --disable-verification flash vbmeta vbmeta.img (from stock ROM)
10. fastboot --disable-verity --disable-verification flash vbmeta_system vbmeta_system.img (from stock ROM)
11. fastboot --disable-verity --disable-verification flash vbmeta_vendor vbmeta_vendor.img (from stock ROM)
12. fastboot -w
13. fastboot reboot fastboot (you MUST be at the fastbootd menu now. If not, you will not be able to flash the GSI successfully)
14. fastboot erase system
15. You will need a larger system partition to flash a GSI. There are 2 ways to achieve this
15a. fastboot delete-logical-partition product - deletes the product partition, allowing a larger system partition
15b. fastboot resize-logical-partition product 509344768
15c. fastboot resize-logical-partition system 3089272064
resizes product and system partitions. This gives you about 3GB of system partitions. Adjust the numbers as required.
16. fastboot --disable-verity --disable-verification flash system <GSI file with .img extention>
17. Reboot to system.
18. Hold your breath and wait to see your GSI's bootscreen.
Other useful info:
The boot.img is already present in the ROM. The below command will recover the boot.img from your phone and save it in your phone's Downloads folder. Execute it from a terminal on your phone and as super user. You will need root.
dd if=/dev/block/by-name/boot of=/storage/emulated/0/Download/boot.img
For retrieving super partition to your device's download folder,
dd if=/dev/block/platform/bootdevice/by-name/super of=/storage/emulated/0/Download/super.img
Unpacking and repacking super.img is a bit involved. I've only gotten till step 06 so far.
01 You WILL need a linux environment. If you are on windows, you can install a linux distro from the microsoft store.
02 Download lpunpack_and_lpmake-masterz.zip. Unzip it to a folder and execute the below commands in the linux shell
x86 or x86_64:
sudo apt install clang zlib1g-dev
arm or arm64:
sudo apt install clang
03 You should see 2 files in the folder - lpmake and lpunpack.
04 Copy stock super.img into the folder and in the linux shell, execute ./lpunpack super.img.
05 After a few minutes, you will see vendor, product and system images in the folder now.
06 Replace system.img with a gsi of your choice. The file name MUST be system.img.
07 Repack super.img using ./lpmake. Syntax.
08 You should now be able to flash super.img using the SPFlash tool.
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