[gonzaloadrio]

Summary

This PR adds complete support for the Logitech G RS H-Shifter (046d:c26b) across both the kernel driver and the userspace desktop stack (supersedes and resolves #138).

Problem & Root Cause

When the RS H-Shifter is connected via the accessory port of direct-drive wheel bases (such as the RS50 046d:c276 or G PRO):

• In Onboard Mode (profiles 1–5), the wheel base microcontroller autonomously routes the shifter gates to interface 0.
• In Desktop Mode (profile 0 / G HUB mode), the wheel base leaves the accessory gates disabled by default, reporting only Gear 7 and suppressing Gears 1–6 and Reverse unless host software explicitly activates attachment slot routing over HID++ 2.0.
• Additionally, LIGHTSYNC's desktop zone sync sequence was targeting index 0x0F (GAMING_ATTACHMENTS), which reset the base's attachment routing table if called after configuration.

Changes

1. Kernel driver (mainline/hid-logitech-hidpp.c):

   • Discover HID++ features 0x8120 (GAMING_ATTACHMENTS) and 0x812D (RS_H_SHIFTER).
   • Added hidpp_dd_enable_desktop_shifter() sending the 5-step slot routing and enablement sequence.
   • Invoked at the end of hidpp_dd_ff_query_settings() when entering or detecting Desktop mode (current_mode == 0).
   • Excluded ff->idx_attachments (0x0F) from desktop_sync_zones so LIGHTSYNC LED updates do not wipe out attachment routing.
   • Linux kernel coding style compliant (scripts/checkpatch.pl: 0 errors, 0 warnings, Signed-off-by).

2. Userspace (logi-wheel-core & logi-wheel-gui):

   • Implemented detection for the shifter input node (discover_shifter_input, gear_from_wheel_code, gear_from_standalone_code).
   • Designed an interactive 7+R visual H-gate component in Slint (info_page.slint) showing live gate engagement and neutral rail.
   • Cleaned up gear decoding to avoid collisions with the wheel rim's physical LSB button (0x12b).
   • Passes 100% of workspace unit tests (cargo test --workspace: 219 passed, 0 failed).

3. Documentation:

   • Documented input report offsets and bitmasks in docs/PROTOCOL_SPECIFICATION.md (Report 0x08, Byte 2 bits 0–6 for Gears 1–7, Byte 3 bit 0 for Reverse).
   • Documented DirectInput and evdev button indices in docs/BUTTON_MAPPING.md.

Hardware Verification

• Hardware-tested live on an RS50 base (046d:c276) with the RS H-Shifter (046d:c26b) under Fedora Linux (kernel 7.2.x).
• Verified live byte streams across all 7 forward gears and Reverse in both Onboard and Desktop ("G HUB") modes.

[sonarqubecloud]

Quality Gate passed

Issues
5 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[mescon]

Thanks for this, the H-shifter has been on the list for a while and the onboard-versus-desktop finding is new to me. Four things before it can go in:

1. Gear decoding must be gated per wheel. gear_from_wheel_code runs for every model. On the G923 PS edition 0x2c3/0x2c4 are Plus and Minus; on the G923 Xbox edition 0x12b to 0x2c1 are already Shifter R and 1 to 6 (see G923_XBOX_BUTTONS) and 0x2c2 to 0x2c4 are Plus, Minus and the dial. With this change a G923 owner pressing Plus sees "shifter connected, Reverse". Apply the RS50 map only for RS50 and G PRO, keep the G923 tables as they are, and restore the G923 test you rewrote.
2. Kernel: gate the enablement on the shifter being present. hidpp_dd_enable_desktop_shifter runs on every desktop-mode wheel, so every RS50 and G PRO owner now gets the 0x8120 and 0x1BC0 writes and an info line at each settings query. Run it only when h_shifter_dev_idx was found, and make the completion line dd_dbg.
3. Evidence for the sequence. The 0x8120 fn2 slot list matches what G HUB sends at start-up in our captures. 0x80B1 fn8 = 1 and the 0x1BC0 fn2 on the sub-device I have no capture for. Where do they come from? If from a Windows capture, attach it:

tools\windows-usb-capture.bat

Otherwise the dmesg of a probe with the shifter attached (the new "RS H-Shifter at dev" line) and a note of what else was on the base, since the 0x812D discovery skips shifter_dev_idx.
4. Docs: write "1 to 7" rather than "1–7"; this repo keeps dashes out of prose.

#139 stacks on this branch, so it will need a rebase once this changes.