Thank you @doniks and @kugiigi for caring my topic. This seem be my special use case, or people using this mode to be a HUD device in car. With Iphone, there is a jaibreak tweak for this option called "Heads Up Display Mode" in Cydia.
My purpose is to pratice getting used to mirror texts when using computer and phone. I 'm a left-hander and I have a kind of Obsessive-Compulsive Disorder that I have to write by left hand. I belive that is the way people rebalance both hand and both halfs of body (in controlling). We all know that the balance of body is very important, in allmost kind of physical sport activities.
I don't know how to make it flip mirror screen permanently in android rom, so I find some other os, that closer to pure linux. But I found that all of them was base on Wayland, not X system. And document about Wayland seem not popular and easy enough for me to program st for that.
If I fail in android devices, maybe the simplest way is get a Iphone. However, I don't like Iphone much!