lightdm if it is not installed yet:
# yum install lightdm lightdm-gtk
gdm service and make the lightdm service be started by systemd
# systemctl disable gdm.service
# systemctl enable lightdm.service
After rebooting your Fedora Linux, lightdm should be the dm.
Thank you!!! I am running Fedora 22 in virtualbox, and Cinnamon was complaining about running in “software rendering mode”. I tried downgrading GDM, downgrading mesa, and all the other advice on the web, but the above info fixed it.