testsAndMisc/linux_configuration/docs/DOCS-g29-ffb.md

Logitech G29: no force feedback in BeamNG.drive (native Linux)

Files: fixes/install_new_lg4ff.sh (optional driver upgrade), ~/src/steam-backlog-enforcer/steam_backlog_enforcer/_desktop_env.py (the host-level env var). Shifter companion: DOCS-g29-shifter.md.

Symptom

BeamNG.drive 0.39.x, native Linux build (not Proton): the G29 steers, pedals and buttons work, but the wheel is limp — no resistance, no self-centring. Other games with FFB are unaffected.

Cause (measured 2026-09-14, BeamNG 0.39.4.0, SDL 3.4.12 static)

~/.local/share/BeamNG/BeamNG.drive/current/beamng.log:

D|logHapticFeatures| Device joystick3 supports the following ffb features:
    Constant Sine Triangle SawToothUp SawToothDown Spring Damper Friction Status Query
I|inputRegistry| Created ffbid 0, ... on device joystick3 with axis id=0
E|updateFrame| Failed to get joystick3 FFB effect status:     (x12,201 per session)

That feature list is SDL3's HIDAPI lg4ff driver (SDL_hidapi_lg4ff.c), which talks to the wheel over hidraw and bypasses the kernel hid-logitech driver — the kernel driver advertises only constant/gain/autocenter (capabilities/ff = 300040000 0). The hidapi haptic backend advertises a status query it cannot answer; BeamNG polls it every frame, gets an error, and treats the effect as dead. Known regression: https://www.beamng.com/threads/ffb-not-working-on-linux.110807/.

Not involved: the shifter HID-BPF (input direction only), hidraw permissions (SDL had already opened the device), Proton (not used).

Fix: make SDL use the kernel driver

Two env vars exist; they are not interchangeable:

var	scope	use
SDL_JOYSTICK_HIDAPI_LG4FF=0	only the Logitech-wheel HIDAPI driver	host-level: set for every game, gamepads keep HIDAPI (BT rumble, LEDs)
SDL_JOYSTICK_HIDAPI=0	all of SDL's HIDAPI joystick drivers	forum-proven fallback, per-game only

Both names verified against strings in BinLinux/BeamNG.drive.x64.

1. Per game — Steam → BeamNG.drive → Properties → Launch Options:

   SDL_JOYSTICK_HIDAPI_LG4FF=0 LSFG_PROCESS=BeamNG.Drive gamemoderun %command%
   

   The previous value had no %command%, so Steam appended LSFG_PROCESS=... gamemoderun ./BinLinux/BeamNG.drive.x64 as arguments to the game (visible in pgrep -a BeamNG.drive.x64) and neither gamemode nor LSFG was ever applied.

2. Host level — Steam on this machine is started by steam-backlog-enforcer through sudo -u kuhy env <allowlist> steam …. sudo's env_reset makes that list the entire environment Steam and every game sees, so /etc/environment, ~/.profile or ~/.pam_environment never reach it. The variable lives in desktop_env_args() in the enforcer's _desktop_env.py. Check it landed:

   tr '\0' '\n' < /proc/$(pgrep -x steam)/environ | grep LG4FF
   

   A missing line means Steam predates the enforcer change — it takes effect on the next Steam launch (systemctl restart steam-backlog-enforcer kills a running game; check pgrep -f BeamNG.drive.x64 first).

Verify

• Game-independent motor check, BeamNG closed: fftest /dev/input/by-id/usb-Logitech_G29_Driving_Force_Racing_Wheel-event-joystick — effects 0 (constant) and 4 (autocenter) must move the wheel.
• In a fresh beamng.log the haptic feature line becomes the kernel-style list and grep -c 'FFB effect status' beamng.log drops to 0. A non-zero count with working force is fine; a zero count with no force is not.
• The gate is physical: drive a car, feel resistance and centring.

Optional: new-lg4ff instead of the in-tree driver

fixes/install_new_lg4ff.sh installs new-lg4ff-dkms-git (AUR). Its dkms.conf sets DEST_MODULE_NAME="hid-logitech", so DKMS places it in updates/dkms/ and it shadows the in-tree module by name — no blacklist, no modules-load.d. Once loaded it registers as hid_logitech_new. Adds spring/damper/friction/periodic effects and the spring_level / damper_level / friction_level / gain / autocenter sysfs knobs.

Traps the script handles:

• mkinitcpio's autodetect hook bundles hid-logitech into the initramfs because the wheel is plugged in at build time; a stale in-tree copy there would load first at boot. The script regenerates the presets when an image carries the module.
• dkms status <name> is a prefix filter that prints everything on a miss; always grep dkms status output.
• USB interface 1 of the G29 (0003:046D:C24F.0007) is unbound by design.

~/src/testsAndMisc/linux_configuration/fixes/install_new_lg4ff.sh           # install (runs yay; needs a TTY)
~/src/testsAndMisc/linux_configuration/fixes/install_new_lg4ff.sh --status  # which module owns the wheel + ff caps
~/src/testsAndMisc/linux_configuration/fixes/install_new_lg4ff.sh --revert  # pacman -Rns + reload, no reboot