PowerMate high-resolution scroll

Userspace C daemon for the classic USB Griffin PowerMate. It reads the in-kernel powermate driver (REL_DIAL, about 94 steps per turn) and injects a virtual mouse wheel using REL_WHEEL_HI_RES — 120 units per traditional notch, so the knob can scroll a browser more smoothly than a notched wheel.

This repository is powermate-ctl. The installed command and systemd unit are powermate-scroll.

Originally prototyped in Python, this daemon was ported to C to drop the interpreter dependency and start faster.

What you get

Action	Result
Turn	High-resolution vertical scroll (clockwise = down)
Hold and turn	Horizontal scroll
Click without turning	Middle-click (open link in a new tab / autoscroll)

Hover the cursor over the page. The compositor treats this like a real high-res mouse wheel, so Firefox, Chromium, and other apps scroll the window under the pointer.

Install

Works on CachyOS / Arch and other systemd Linux desktops. You need a classic USB Griffin PowerMate (077d:0410) and the in-kernel powermate driver (CONFIG_INPUT_POWERMATE).

Run ./install.sh from this project folder (the directory that contains install.sh, Makefile, and src/main.c):

git clone https://github.com/argosyship/powermate-ctl.git
cd powermate-ctl
./install.sh

On CachyOS / Arch the installer will install base-devel, pkgconf, and libevdev if they are missing. On other systemd machines, install a C compiler, pkg-config, and libevdev first.

The installer builds the C binary, copies it to /usr/local/bin/powermate-scroll, installs the udev rule and uinput module load, writes ~/.config/powermate-scroll/config.toml if missing, and enables (and restarts) the user service.

If this was the first time you were added to the input group, log out and back in, then:

lsusb | grep -i griffin          # 077d:0410
powermate-scroll --list          # should mark [PowerMate]
systemctl --user status powermate-scroll

Hover a browser page and turn the knob (do not press it).

Tune the resolution

Edit ~/.config/powermate-scroll/config.toml (created from config.example.toml):

# HI_RES units per PowerMate detent. 120 = one mouse-wheel click.
# Lower is finer. The USB PowerMate has ~94 detents per revolution.
step_size = 24

step_size	Feel
8	Very fine / slower
16	Smooth reading
24	Default (~19 wheel notches per full turn, 94 micro-steps)
48	Fast
120	Coarse, one notch per detent

systemctl --user restart powermate-scroll

Or run once in the foreground:

powermate-scroll --step-size 8 --print-events

--print-events prints mapped events without injecting input.

Run without installing

sudo pacman -S --needed base-devel pkgconf libevdev
make
./powermate-scroll --list
./powermate-scroll --step-size 16

You still need read access to the PowerMate event node and write access to /dev/uinput (udev rule + input group, or run as root for a quick test).

How it works

1. Linux already ships drivers/input/misc/powermate.c. No out-of-tree kernel module.
2. This C daemon (libevdev + uinput) grabs that event device so Plasma/GNOME do not also consume REL_DIAL.
3. Each detent becomes step_size units of REL_WHEEL_HI_RES on a virtual pointer. When the total crosses 120, it also emits legacy REL_WHEEL for apps that ignore high-res events.
4. Wayland compositors expose that as wl_pointer.axis_value120, which Firefox and Chromium use for pixel-smooth scrolling.

Troubleshooting

Service says Waiting for Griffin PowerMate but lsusb shows it
This login does not have the input group yet. Confirm with id (no input) vs getent group input (your user is listed). Log out and back in. Same-session test:

systemctl --user stop powermate-scroll
newgrp input
powermate-scroll

Unplugging and replugging the knob after install can apply the udev uaccess ACL without a logout. A new session is the reliable fix.

powermate-scroll --list omits the device
Same input group issue as above, or the kernel module is missing:

lsmod | grep powermate
sudo modprobe powermate
zgrep CONFIG_INPUT_POWERMATE /proc/config.gz

Knob does nothing in the browser
Confirm --list shows [PowerMate], the user service is running (systemctl --user status powermate-scroll), and the cursor is over the page. Try --step-size 120 to rule out a compositor that only honors discrete notches.

Permission denied on /dev/uinput or /dev/input/event*
Re-run ./install.sh, then log out. Unplug and replug the knob. Quick test: sudo ./powermate-scroll.

Unit could not be found
Run ./install.sh again from this repo, or copy packaging/powermate-scroll.service to /usr/lib/systemd/user/ and then:

systemctl --user daemon-reload
systemctl --user enable --now powermate-scroll

Too fast or too slow
Change step_size. Firefox also has mousewheel.default.delta_multiplier_y in about:config if only Firefox feels wrong.

LED stays dark
The daemon sets a static glow while it is running. A grab or permissions problem on the event node will skip the LED write; scrolling can still work.

Development

make test

No PowerMate is required for the unit tests; they cover the high-res accumulation math, click/hold gestures, and TOML config.