Add ddcutil fallback for brightness on desktops without backlight device

[felipe3dfx]

Summary

XF86MonBrightnessDown/Up silently fails on systems without /sys/class/backlight — desktops, NUCs, mini-PCs, and laptops where the driver doesn't register a backlight device (common on NVIDIA and some AMD hybrid setups — see #5067). This PR extends omarchy-brightness-display with a DDC/CI fallback via ddcutil, so the same keybindings that work on laptops also work on external monitors, out of the box.

Zero user setup: the Arch ddcutil package already ships modules-load.d/ddcutil.conf (auto-loads i2c-dev at boot) and a 60-ddcutil-i2c.rules udev rule that grants uaccess to display-class i2c buses. This PR leans on that — the migration just installs the package and kicks the module + udev for users whose current session was already running.

What's in the PR

Script (bin/omarchy-brightness-display)

• A third branch after the existing ACPI and Apple paths: iterate ddcutil-detected i2c buses, read/write VCP 0x10, step all monitors together, report a unified percent to swayosd.
• Bus + state cache in $XDG_RUNTIME_DIR (300s TTL) so repeats skip the ~1 s ddcutil detect and the per-monitor getvcp round-trip.
• Blocking flock (not -n) serializes key repeats through the cache read-modify-write. -n silently drops taps during the cold path and users feel it as "one of my three presses didn't register". The debounce worker does not inherit the lock (9>&-).
• Debounced setvcp — rapid taps coalesce into a single setvcp per gesture via a timestamp + atomic target-file + background worker (250 ms window). This is the same pattern KDE PowerDevil uses (D8626), for the same reasons:
  • the OSD stays in sync instead of the monitor chasing stale values over slow i2c,
  • writes to the monitor's NVRAM (limited write cycles per ddcutil docs) are minimized.
• --sleep-multiplier 0.04 --noverify on setvcp for responsive interactive use.
• Bash 5+ EPOCHSECONDS / EPOCHREALTIME builtins — no date forks on the hot path.

Package + migration

• ddcutil added to install/omarchy-base.packages (alphabetical).
• migrations/1776070000.sh — upgrade path for existing users:
  • omarchy-pkg-add ddcutil if missing.
  • modprobe i2c-dev so the current session can use DDC without a reboot.
  • udevadm control --reload-rules && udevadm trigger --subsystem-match=i2c-dev so the upstream package's uaccess rule applies to already-enumerated i2c buses in the active seat.
  • set -e so a failing omarchy-pkg-add or modprobe doesn't get silently marked as applied by omarchy-migrate.

No fresh-install hook is needed: pacman running during install drops modules-load.d/ddcutil.conf, which systemd-modules-load.service picks up on the first boot into the installed system, and the bundled udev rule processes all i2c buses on that same boot — so first login comes up with /dev/i2c-* already ACL-tagged for the active seat user.

Measurements

BenQ EX2780Q + Dell U2415, NVIDIA proprietary on Hyprland:

Path	Time
Warm (cached bus + state, debounced)	21–22 ms
Cold (TTL expired, full ddcutil detect)	~750 ms, once per 5-min window
getvcp physical floor, BenQ, multiplier 0.04	465 ms
setvcp physical floor, BenQ, multiplier 0.04	487 ms

The 300–500 ms per VCP op is physical DDC/CI wire time — that is why the cache + debounce pattern matters: it keeps that time off the hot path.

Tested

• Multi-monitor DDC step together, unified percent to swayosd: verified live.
• Rapid Fn+F1 burst (10 taps): no drops, single setvcp per gesture, monitor slews to the final value once: verified live.
• Cold cache (rm /run/user/$UID/omarchy-ddc-*): re-detects buses, re-reads state, one slow tap then warm path: verified live.
• Apple ASdControl path and native backlight path: inspection only — the new branch runs only after both short-circuit, no behavior change on laptops.

Review iteration

An earlier revision of this PR shipped its own default/udev/ddcutil-i2c.rules + an i2c group + install/config/ddcutil-i2c.sh that copied the rule and added the user to the group (requiring a logout). The Copilot review on install/config/ddcutil-i2c.sh and migrations/1776070000.sh pushed on the i2c-dev module loading question, and digging into that surfaced that the Arch ddcutil package already ships both a modules-load.d entry and its own 60-ddcutil-i2c.rules with TAG+="uaccess" — which is a strictly better approach than the group-based one (no logout, handled by systemd-logind, filtered to display-class buses via ATTRS{class}=="0x03*"). This revision leans on that instead. Net result: 3 files + 140 lines instead of 6 files + 169 lines, and no more logout notice.

Related

• feat: transparent support for ddcutil on desktop #5201 — @jamerrq's DDC PR, open. Same problem, different architecture (this PR adds blocking-flock drop fix + debounce + multi-monitor iteration). Not trying to replace it; wanted to offer a more end-to-end take.
• Display brightness control not working on ASUS TUF (Hyprland / Omarchy) – no backlight device exposed #5067 — ASUS TUF driver-level backlight registration bug. This PR is a workaround path for users in that situation.
• Fall back to DDC/CI for brightness on external monitors #4841 — self-closed by the author, not a technical rejection.

[Copilot]

Pull request overview

Adds a DDC/CI (ddcutil) fallback to omarchy-brightness-display so brightness keybindings work on systems without a /sys/class/backlight device (e.g., desktops/external monitors), and wires up install + migration hooks to grant i2c device access.

Changes:

• Extend bin/omarchy-brightness-display with a DDC/CI fallback, including caching + debounced setvcp.
• Add ddcutil to base packages and introduce an install hook to create the i2c group + install a udev rule.
• Add a migration to apply the same udev/group setup for existing installs.

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Reviewed changes

Copilot reviewed 5 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
bin/omarchy-brightness-display	Adds DDC/CI fallback path with caching, locking, unified OSD reporting, and debounced writes.
default/udev/ddcutil-i2c.rules	New udev rule to grant group access to /dev/i2c-* devices.
install/omarchy-base.packages	Installs ddcutil by default.
install/config/ddcutil-i2c.sh	Fresh-install hook to create i2c group, add user, and install udev rule.
install/config/all.sh	Runs the new ddcutil-i2c.sh hook during install.
migrations/1776070000.sh	Upgrade migration to install ddcutil and apply udev/group access for existing users.

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Copilot reviewed 5 out of 6 changed files in this pull request and generated 2 comments.

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