When using SDDM in either KDE4 or KDE5 there are random input freezes of keyboard and mouse.  Furthermore the machine cannot be ssh'd to from another machine in this state or rebooted.  The only solution is to hard reboot.  After disabling SDDM and enabling KDM the lockups do not recur.  I attempted to use SDDM in FC20 and the problem first appeared.  Upgrade to FC21 and still the same problem.  Did a bios upgrade - that did not resolve it.  Installed KDE5 Plasma (very nice) - but that has not resolved the problem.  Cannot find anything specific in the logs.  Have used different keyboards and mouse to no avail.
Also noticed on the last crash that Cap Lock was on in SDDM but not on the keyboard.  Had to ssh to the machine and enable KDM and reboot.

Reproducible: Always

Steps to Reproduce:
1. Use SDDM
2.  Move mouse
3. Random lockup

Actual Results:  
Total freeze of computer.  Fan still running.

Expected Results:  
SDDM should not cause imput lockup