Comment # 5 on bug 1222156 from Togan Muftuoglu

I have the same issue and in the beginning I thought that was sth to do with
the kde6 upgrade but looks more likely to radeon in Mesa I you suggest.

kernel: show_signal_msg: 59 callbacks suppressed
Mar 31 12:20:56 desktop.dinamizm.com kernel: QSGRenderThread[1998]: segfault at
40 ip 00007ff12e32b98d sp 00007ff0d8dff1d0 error 4 in
radeonsi_dri.so[7ff12dc91000+1476000] likely on CPU 1 >
Mar 31 12:20:56 desktop.dinamizm.com kernel: Code: 8b 6f 30 85 ed 0f 84 e5 00
00 00 31 db 49 89 e5 eb 34 0f 1f 00 4d 03 77 38 49 8b 36 48 85 f6 74 0e f0 83
2e 01 75 08 4c 89 e7 <41> ff 54 >
Mar 31 12:20:56 desktop.dinamizm.com sddm-helper[1885]: 


As a workaround I used the following cmdline options radeon.si_support=0
amdgpu.si_support=1

This enables at least the graphical environment but it is not the ideal
solution