brightnessctl drives the kernel backlight class, which laptop panels
have and this desktop does not -- it reports only keyboard and NIC LEDs.
So BrightnessControl removed itself and there was no way to dim the
screen from Panama at all. DDC/CI is the channel the buttons on a
monitor's bezel drive, and it is the only brightness an external display
has. Both sources now render a row each, so a machine gets whichever it
actually has, or none.
Displays are enumerated from sysfs rather than `ddcutil detect`. The
kernel publishes the connector-to-bus mapping as
/sys/class/drm/<card>-<connector>/ddc along with whether anything is
plugged in, which beats parsing detect's undocumented brief output,
yields the connector name spelled exactly as Hyprland spells it, and
probes only connectors with a monitor attached -- one bus on this
machine rather than fourteen, where each empty bus costs a timeout.
No model name is read: Hyprland already knows what every output is
called, so the UI joins on the connector instead of keeping a second
source of truth that could disagree with the Displays page.
Writes are debounced, serial, and read back. Serial because DDC/CI has
no arbitration and two ddcutil processes on one bus interleave their
exchanges and both return garbage. Read back because a write is not a
promise: panels clamp to their own range, ignore values while waking
from standby, and drop writes that arrive too fast. Without the read the
slider would show what Panama asked for rather than what the monitor
did, which is the same class of lie as trusting `hyprctl keyword`.
Brightness is deliberately not a stored preference. The monitor
remembers it and the bezel buttons change it behind Panama's back, so
persisting it would mean restoring a value the panel had moved past.
The contract runs against fixtures with ddcutil stubbed and both sysfs
roots redirected, so it never touches a real monitor. Its fixture
reports a maximum of 200 rather than 100 on purpose -- at 100 the
scaling arithmetic is the identity and a helper that ignored the
reported maximum would pass everything. Verified it catches that, plus a
dropped connection-status filter and an unstripped connector prefix.
Not yet confirmed against hardware: this machine cannot open any I2C bus
yet. ddcutil's udev rule grants that through uaccess but only to devices
created after it was installed, so it needs one udevadm trigger. The
helper detects exactly that case and returns the command as its error
rather than reporting "no displays".
Claude-Session: https://claude.ai/code/session_01BRvzt4H8XXLPVH5MyYdk9L
Gabriel Brown
7541a45e77