Comment # 55 on bug 1028575 from Michal Srb

(In reply to Mircea Kitsune from comment #53)
> How do I get the full log? "dmesg -w" prints the data in the console, which
> has a character limit by default. If it's simply the output of "dmesg" after
> rebooting and logging back in, I already attached that here... however it is
> pretty old, so next time it happens I will make a new one.

You can redirect the output to a file, then get the file after reboot:
  dmesg -w > dmesg.txt

Alternatively use journalctl to retrieve some of the older logs:
  journalctl --list-boots
  journalctl --boot=<ID> --dmesg > dmesg.txt