hi, how to set gnome3 in fallback mode?

["shiyao.ma" <i@introo.me>]

I did a fresh installation of debian tesing on my laptop. After first 
login, the gnome3 told me that due to hardware problem, gnome3 was in 
fallback mode. Thus, I installed the linux-firmware-nonfree. After a 
restart, the gnome3 was in standard mode automatically. However, I found 
myself like the fallback mode more.  I found "System settings->System 
info->Graphics has no setting of fallback mode. It just shows that I am 
in standard experience.
How can I set it to fallback mode again?
Thanks.