I tested all 4 methods in this article. But I still cannot get into download mode.
The second method get me into a factory mode:
- Power phone off
- Plug USB into Computer
- Plug microUSB into phone
- Hold pwr + vol up + vol down
- When Batt icon goes away, release pwr, still holding vol up + vol down
- Yellow /_\ (triangle, Downloading, Andy shoveling, Do not turn off Target!!! symbol)
You should now be in Download mode.
turns out there is no such mode. The adb shell default is root shell. To root the device, I can simply
adb pull <system partition>, then mount on PC to add su and other stuff, then just flash it back.