Comment # 36 on bug 1173733 from Stefan Dirsch

(In reply to Nikolai Nikolaevskii from comment #35)
> So is this issue solved or not?

Honestly I can't say.

> My wild shot: GDM starts with root privileges, SDDM starts with ordinary
> user privileges. 
> Maybe I am wrong.

I'm afraid you are. AFAIK sddm chooser runs as root, but then gets replaced by
the user Xsession running as regular user, so with autologin enabled it may
look like X not working from beginnning when permissions to /dev/nvidia0 are
not available.

It's similar with gdm, which chooser is being run as gdm user and then starts a
second Xserver running the Xsession under regular user.