Boot to the desktop or Gaming Mode
Some SteamOS-style images start in Gaming Mode (Gamescope) by default. On these images you can choose the session the next boot starts.
Choosing Switch to Desktop inside Steam's Gaming Mode can leave the display dead until the next full restart. Switching the other way, from the desktop to Gaming Mode, works. Set the session before you boot instead of switching while Linux runs.
This section was reported on firmware 13.60 with the official CachyOS image. PS5 Bazzite Deck has its own session handling, see Built-in commands.
Choose the next session
The image reads a one-word file at boot. Run this on the PS5 in Linux:
echo plasma > ~/.config/ps5-next-session
To return to Gaming Mode:
echo gamescope > ~/.config/ps5-next-session
Restart, then run your jailbreak and the Linux loader again. Linux returns to the PS5 OS on every restart, see Booting Linux.
Set it before the first boot
While the drive is connected to a PC with a Linux system, mount the root partition (partition 1) and create home/steam/.config/ps5-next-session containing plasma. The file must be owned by user ID 1000.
Log out of the desktop
Logging out of Plasma returns to Gaming Mode and resets the file to gamescope. Write plasma to the file again before the next boot. A restart or shutdown from the desktop keeps the setting.
Steam Big Picture still runs inside the desktop session.