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https://github.com/kuhyx/testsAndMisc.git
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Krzysztof kuhy Rudnicki
7e2d78c7db
Live on this host since 2026-09-11 but never committed. The wheel firmware decodes the shifter's hall sensor itself and this unit's top row rests at Y 180-193, below the ~195 engage threshold, so 1st/3rd often did not register. g29_shifter.bpf.c re-decodes those two gates from the raw X/Y bytes with hysteresis (engage >= 160, release <= 140) and ORs the bit in before hid-logitech parses the report; it never clears a bit, so the firmware's working gates cannot regress. fix_g29_shifter.sh builds, attaches and installs it persistently (udev-hid-bpf rule + a systemd safety-net unit pulled in on every re-enumeration); g29_shifter_capture.py records and grades raw captures per gate. Two files were over the 250-line cap: the test file is split at the record/CLI boundary with the report builders moved into a conftest fixture (the naming hook allows only test_*.py in tests/), and the grader's docstrings are tightened. No behaviour change to the installed fix. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01SS8M5wk88nJTXxDWz61KKk
105 lines
5.1 KiB
Markdown
105 lines
5.1 KiB
Markdown
# Logitech G29 Driving Force Shifter: 1st/3rd not engaging (HID-BPF fix)
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Files: `fixes/g29_shifter.bpf.c` (the fix), `fixes/fix_g29_shifter.sh`
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(build/attach/install), `fixes/g29_shifter_capture.py` (grades a raw capture).
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Force feedback (a separate problem, SDL-side): `DOCS-g29-ffb.md`.
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## Symptom
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The stick physically locks into every gate, but 1st and 3rd often do not
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register in any game unless the stick is shoved hard forward. 2/4/6/R are fine.
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## Cause (measured 2026-09-11)
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The wheel firmware decodes the shifter's hall sensor into gear buttons itself.
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The G29 input report (12 bytes, no report id) carries the raw values too:
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| byte | content |
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| ---- | --------------------------------------------------------------------------- |
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| 0–3 | hat (4 bits) + 25 buttons; gears 1–6, R = byte 2 bits 0–6 |
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| 4–5 | wheel, 6–8 pedals |
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| 9 | shifter X: left ≈47–90 (1/2), centre ≈104–139 (3/4), right ≈166–191 (5/6/R) |
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| 10 | shifter Y: bottom row ≈30–60, neutral ≈105, top row ≈180–233 |
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| 11 | flags; 0x40 = stick pushed down (reverse) |
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Firmware thresholds, read off the on/off edges of a raw capture:
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| row | engages | releases | where this stick actually rests |
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| ---------------- | -------- | -------- | ------------------------------------ |
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| top (1/3/5) | Y ≥ ~195 | Y ≤ ~179 | 180–233, often **180–193 → no gear** |
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| bottom (2/4/6/R) | Y ≤ ~56 | Y ≥ ~72 | 30–60 |
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The thresholds are symmetric around Y=125 but this unit's neutral is ≈105
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(sensor/magnet offset ~20 rearward), so the top row has no margin. The bottom
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row keeps ~15–25 and occasionally drops for a second when the stick relaxes to
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Y 58–71 (seen once on 4th, once on 2nd in the baseline captures).
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## Fix
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`g29_shifter.bpf.c` is a HID-BPF `hid_device_event` program attached by
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`udev-hid-bpf`. It re-decodes 1st/3rd from bytes 9/10 with **engage Y ≥ 160,
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release Y ≤ 140** (hysteresis kept in a BPF global) and ORs the bit into byte
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2 before `hid-logitech` parses the report. It never clears a bit, so the
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firmware's own decode is untouched — the working gates cannot regress. It is
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transparent to every game, Proton and Steam Input, and FFB is unaffected.
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5th is left to the firmware: no capture data for the top-right gate.
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The Arch `udev-hid-bpf` package ships no headers; the script shallow-clones the
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matching upstream tag into `~/.cache/g29_shifter/` and compiles with the same
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clang flags as upstream's `meson.build`.
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## Surviving reboots
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Three layers, because a missed attach is silent — the wheel still works, just
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with the bad decode, which is indistinguishable from "the fix stopped working":
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1. `udev-hid-bpf install` → `/etc/udev-hid-bpf/g29_shifter.bpf.o` plus
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`/etc/udev/rules.d/99-hid-bpf-g29_shifter.rules`, which attaches on `add`.
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2. `g29-shifter-bpf.service` (`systemd/`), enabled at `multi-user.target`, runs
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`fix_g29_shifter.sh --ensure`: a no-op when attached, re-attaches when not.
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3. `/etc/udev/rules.d/99-g29-shifter-ensure.rules` pulls that unit in on every
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re-enumeration, so hotplug is covered as well as boot.
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`--status` reports all of it. Verified 2026-09-12 by detaching and starting the
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unit: it re-attached and exited cleanly. That test also caught the unit failing
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with `HOME: unbound variable` — systemd has no `$HOME`, so the cache path now
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falls back.
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## If the symptom returns after a boot
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Check `--status` first. If it says attached and gears still misbehave, the
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sensor's zero has drifted and the thresholds need recalibrating — the fixed
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`Y_TOP_RELEASE` (140) assumes the stick rests near Y≈105. A drift upward of
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~+40 would park the rest position _above_ the release line, so a gear would
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engage and never let go (the car stays in gear). Measure the rest value:
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```bash
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sudo ~/src/testsAndMisc/linux_configuration/fixes/g29_shifter_capture.py \
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--record 20 ~/data/g29-captures/neutral.txt --runs
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```
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with hands off the stick, and compare the reported Y against 105.
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## Usage
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```bash
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~/src/testsAndMisc/linux_configuration/fixes/fix_g29_shifter.sh --test # attach until unplug
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~/src/testsAndMisc/linux_configuration/fixes/fix_g29_shifter.sh # persistent (udev rule)
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~/src/testsAndMisc/linux_configuration/fixes/fix_g29_shifter.sh --status
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~/src/testsAndMisc/linux_configuration/fixes/fix_g29_shifter.sh --remove
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```
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## Verifying / recalibrating
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hidraw sees the report _after_ the BPF program, so a raw capture shows the fix:
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```bash
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sudo timeout 60 xxd -c 12 /dev/hidraw$(basename "$(readlink -f /dev/input/by-id/usb-Logitech_G29_Driving_Force_Racing_Wheel-hidraw)" | tr -dc 0-9) > /tmp/g29.txt
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~/src/testsAndMisc/linux_configuration/fixes/g29_shifter_capture.py /tmp/g29.txt --edges
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```
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Exit code 1 means some gate had a dwell ≥0.25 s with the bit clear. `--edges`
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prints X/Y at every on/off transition — that is where the thresholds come
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from. If the sensor drifts further, edit the `#define`s at the top of the
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`.bpf.c` and re-run the script.
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