Read DDC/CI from the AUX bus, not the EDID bus

With I2C access granted, the helper still found nothing: every connected
monitor was probed on the wrong bus.

A connector has two. The `ddc` symlink points at the classic I2C line
that carries EDID on HDMI and DVI. DisplayPort carries DDC/CI over the
AUX channel instead, which appears as a child directory of the
connector. Both exist on a DP connector and both resolve, so the wrong
choice looks entirely reasonable and simply finds no monitor: this
machine's DP-2 has ddc -> i2c-5, where ddcutil reports "No monitor
detected", while its AUX child i2c-9 answers VCP 0x10 immediately.

This is the guess the previous commit said was unverified, and it was
wrong in the way that mattered. Enumerating from sysfs is still right --
it gives the connector name Hyprland uses and skips empty connectors --
but it has to prefer the AUX child and fall back to the symlink.

The fixture now mirrors sysfs properly: /sys/class/drm/<connector> is a
SYMLINK to the real device directory, and `find` does not follow the
path it is given. A fixture built from plain directories passes whether
or not the code resolves the symlink first, which is a test that agrees
with itself rather than with the kernel.

Verified on hardware: the Kuycon P20 reports 100%, accepts 70 and
returns to 100, and the media keys move it through codex's OSD path.
Both bus-selection mistakes are now caught by the contract.

Claude-Session: https://claude.ai/code/session_01BRvzt4H8XXLPVH5MyYdk9L

config/dot/quickshell/scripts/panama-brightness

@@ -57,6 +57,39 @@ has_accessible_bus() {
     return 1
 }
 
+# The I2C bus that carries DDC/CI for one connector.
+#
+# There are two, and picking the wrong one finds no monitor at all:
+#
+#   DisplayPort carries DDC/CI over the AUX channel. That adapter shows up as a
+#   child directory of the connector -- /sys/class/drm/card1-DP-2/i2c-9 -- and
+#   is the one ddcutil talks to.
+#
+#   The connector's `ddc` symlink points at the classic I2C line used for EDID
+#   on HDMI and DVI. On a DisplayPort connector it still exists and still
+#   resolves, but nothing answers on it: this machine's DP-2 has ddc -> i2c-5,
+#   where ddcutil reports "No monitor detected", while i2c-9 answers VCP 0x10
+#   immediately.
+#
+# So prefer the AUX child and fall back to the symlink. Note the readlink: the
+# entries under /sys/class/drm are symlinks, and `find` does not follow the path
+# it is given, so searching the unresolved path silently finds nothing.
+bus_for_connector() {
+    local path="$1" real aux ddc
+
+    real="$(readlink -f "$path")"
+    aux="$(find "$real" -maxdepth 1 -name 'i2c-*' -printf '%f' -quit 2>/dev/null)"
+    if [[ -n "$aux" ]]; then
+        printf '%s' "${aux#i2c-}"
+        return 0
+    fi
+
+    ddc="$(readlink -f "$path/ddc" 2>/dev/null)" || return 1
+    [[ -n "$ddc" ]] || return 1
+    ddc="$(basename "$ddc")"
+    printf '%s' "${ddc#i2c-}"
+}
+
 cmd_list() {
     has_accessible_bus || emit_error 'no I2C bus is accessible. ddcutil ships a udev rule that grants this, but only to devices created after it was installed. Run: sudo udevadm control --reload-rules && sudo udevadm trigger --subsystem-match=i2c-dev --subsystem-match=drm'
 
@@ -69,8 +102,7 @@ cmd_list() {
         connector="$(basename "$path")"
         connector="${connector#card*-}"
 
-        bus="$(basename "$(readlink -f "$path/ddc")")"
+        bus="$(bus_for_connector "$path")"
-        bus="${bus#i2c-}"
         [[ "$bus" =~ ^[0-9]+$ ]] || continue
 
         # A monitor that does not implement 0x10 is not an error; it simply

tests/quickshell/brightness-helper-contract.sh

@@ -34,15 +34,35 @@ mkdir -p "$fixture/drm" "$fixture/dev" "$fixture/bin" "$fixture/i2c"
 # Two connectors with a monitor, two without. DP-2 answers DDC; DP-3 is
 # connected but does not implement brightness. HDMI-A-1 and DP-1 are empty and
 # must never be probed at all.
+# A connector has an EDID bus (the `ddc` symlink) and, on DisplayPort, an AUX
+# bus that appears as a child directory. DDC/CI rides the AUX channel where one
+# exists, and the `ddc` line answers nothing on a DP connector even though it
+# still resolves -- so the aux argument here is what a real DisplayPort monitor
+# looks like, and omitting it is what HDMI and DVI look like.
 make_connector() {
-    local name="$1" bus="$2" status="$3"
+    local name="$1" ddc_bus="$2" status="$3" aux_bus="${4:-}"
-    mkdir -p "$fixture/drm/$name"
+    local device="$fixture/devices/$name"
-    printf '%s\n' "$status" >"$fixture/drm/$name/status"
+
-    mkdir -p "$fixture/i2c/i2c-$bus"
+    # /sys/class/drm/<connector> is a SYMLINK to the real device directory, and
-    ln -sfn "$fixture/i2c/i2c-$bus" "$fixture/drm/$name/ddc"
+    # this fixture mirrors that rather than using a plain directory. It matters:
+    # `find` does not follow the path it is given, so code that searches the
+    # unresolved path finds nothing while appearing to work anywhere the entry
+    # happens to be a real directory.
+    mkdir -p "$device"
+    printf '%s\n' "$status" >"$device/status"
+    mkdir -p "$fixture/i2c/i2c-$ddc_bus"
+    ln -sfn "$fixture/i2c/i2c-$ddc_bus" "$device/ddc"
+    [[ -n "$aux_bus" ]] && mkdir -p "$device/i2c-$aux_bus"
+
+    mkdir -p "$fixture/drm"
+    ln -sfn "$device" "$fixture/drm/$name"
+    return 0
 }
+# DP-2 is the DisplayPort case: its EDID line is bus 5, which answers nothing,
+# and its AUX child is bus 9, which does. Choosing bus 5 here finds no monitor
+# at all, which is exactly the bug this pins down.
 make_connector card1-DP-1     4  disconnected
-make_connector card1-DP-2     5  connected
+make_connector card1-DP-2     5  connected     9
 make_connector card1-DP-3     6  connected
 make_connector card1-HDMI-A-1 7  disconnected
 
@@ -75,7 +95,7 @@ for arg in "$@"; do
 done
 
 case "$bus" in
-    5) printf 'VCP 10 C 120 200\n'; exit 0 ;;
+    9) printf 'VCP 10 C 120 200\n'; exit 0 ;;
     *) exit 1 ;;
 esac
 STUB
@@ -103,8 +123,9 @@ jq -e . >/dev/null 2>&1 <<<"$listing" || fail "list did not emit JSON: $listing"
 [[ "$(jq -r '.displays[0].connector' <<<"$listing")" == "DP-2" ]] \
     || fail "the connector name must match Hyprland's output name: $listing"
 
-[[ "$(jq -r '.displays[0].bus' <<<"$listing")" == "5" ]] \
+# The AUX bus, not the EDID bus its `ddc` symlink points at.
-    || fail "the display was mapped to the wrong I2C bus: $listing"
+[[ "$(jq -r '.displays[0].bus' <<<"$listing")" == "9" ]] \
+    || fail "the display was mapped to its EDID bus instead of its DisplayPort AUX bus, where nothing answers: $listing"
 
 # 120 of a maximum of 200 is 60%.
 [[ "$(jq -r '.displays[0].value' <<<"$listing")" == "60" ]] \
@@ -123,12 +144,12 @@ if grep -qxE '4|7' "$DDCUTIL_PROBE_LOG"; then
 fi
 
 # ── Writes scale to the reported maximum ─────────────────────────────────────
-run_helper set 5 40
+run_helper set 9 40
-[[ "$(tail -1 "$DDCUTIL_SET_LOG")" == "set 5 80" ]] \
+[[ "$(tail -1 "$DDCUTIL_SET_LOG")" == "set 9 80" ]] \
     || fail "set did not scale to the display's maximum: $(cat "$DDCUTIL_SET_LOG")"
 
-run_helper set 5 500
+run_helper set 9 500
-[[ "$(tail -1 "$DDCUTIL_SET_LOG")" == "set 5 200" ]] \
+[[ "$(tail -1 "$DDCUTIL_SET_LOG")" == "set 9 200" ]] \
     || fail "an out-of-range percent was not clamped: $(cat "$DDCUTIL_SET_LOG")"
 
 # ── No I2C access explains itself ────────────────────────────────────────────