Refer to follows:
here u can search all Environment variable meaning
https://codereview.qt.nokia.com/q/status:open+-is:wip

https://codereview.qt.nokia.com/c/qt/qtbase/+/280681
click show all

http://blog.iotwrt.com/tips/2017/06/04/swap-layer/

Drm = (gem + kms)
https://man.archlinux.org/man/drm-kms.7.en

trace drm source and change layer
~/SDK/kernel/drivers/gpu/drm/rockchip

Env. variable reference
https://github.com/MythTV/mythtv/blob/master/mythtv/libs/libmythui/platforms/mythdrmdevice.cpp

export QT_QPA_PLATFORM="eglfs"
export QT_QPA_EGLFS_KMS_ATOMIC=1
export QT_QPA_EGLFS_INTEGRATION="eglfs_kms"
export QT_QPA_EGLFS_DEBUG="1"
expoet QT_QPA_EGLFS_KMS_ZPOS=(select 1,2,3)
export QT_QPA_EGLFS_FORCE888=1