Originally reported by Phoronix's Michael Larabel, the support for the Steam Deck controller will be added to the Steam HID driver (originally developed for the Steam Controller) inside the Linux 6.3 kernel. This looks to initially be basic functionality while advanced features like force feedback will come in the future. Vicki Pfau who is working for Valve on this project gives the following quote explain the details:
HID: hid-steam: Add Steam Deck support
Add preliminary support for the Steam Deck's controller interface. Currently, this only supports the controller inputs and toggling lizard mode (Valve's name for a hardware kb/m emulation mode). It does not support any of the advanced features, such as the motion sensors or force-feedback.
The Steam Deck also includes a heartbeat for lizard mode that switches it back on if no reports have been received within a few milliseconds. The official Steam client handles this by sending a handful of configuration reports every few ms, so we copy this behavior by sending configuration reports to disable the mouse and reset the digital mappings every 5ms. As this isn't needed for the older Steam Controller, this is only done on the Steam Deck.
It's good to see Valve continue to contribute to the open source Linux world. Should you decide to run a different Linux Distro on your Deck, as long as it has the Linux kernel 6.3 it should have better compatibility with the Steam Deck controller out of the box.