Using Steam Big Picture Mode on Ubuntu 15.10, with a PS3 Sixaxis controller over Bluetooth, was proving difficult. Every time I would restart BPM, it would lose all of my controller mappings. The normal solution of using xboxdrv didn’t work, since this was via Bluetooth and not wired.
I configured the controller as normal via BPM (the default A and B buttons for me were left-stick-click and right-stick-click respectively), then closed Steam.
After exiting Steam, open ~/.local/share/Steam/config/config.vdf, and look for the line that begins with “SDL_GamepadBind”. Copy the large value in quotes.
Edit the Steam startup script (this may be overwritten on Steam updates)
sudo vi /usr/bin/steam
And before the ‘export STEAMSCRIPT’ line, add a line with
making sure to replace CONTROLLERSTRING with the large value in quotes.