Fix kernel 7.x black screen under Plasma Login via DPMS cycle

An apt upgrade switched the display manager from SDDM to plasmalogin,
whose kwin_wayland greeter never runs the SDDM Xsetup hook. Add a
plasma-login.service user drop-in that cycles DPMS off/on with
kscreen-doctor on 7.x kernels, forcing the full modeset that un-sticks
i915 pipe A (a plain resolution change is absorbed by the eDP panel
fitter and doesn't help). Verified across a cold reboot.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

CLAUDE.md

@@ -3,7 +3,7 @@
 ## Agent-accessible host
 
 - **hp-chell** (`hp-chell.lan.1qaz.ca`) — KDE neon (Debian-based) x86_64 laptop (Intel Core m7-6Y75, 4 cores, 15GiB RAM, 117GB disk, Intel HD Graphics 515), full sudo (nopasswd) account `agent-user` (uid 60123). Provisioned 2026-09-03.
-  - Auth: SSH key preferred (this machine's `~/.ssh/id_ed25519.pub` is already in the host's `authorized_keys`) — `ssh agent-user@hp-chell.lan.1qaz.ca`.
+  - Auth: SSH key preferred (this machine's `~/.ssh/id_ed25519.pub` is already in the host's `authorized_keys`) — `ssh agent-user@hp-chell.lan.1qaz.ca`. When working *on* hp-chell itself as `matt` (no nopasswd sudo), `matt`'s own `~/.ssh/id_ed25519.pub` is also authorized (added 2026-09-27) — use `ssh agent-user@localhost 'sudo ...'` for root.
   - Fallback password: stored in local Bitwarden vault, item id `1db3b9f6-a82e-4692-846a-b4bb002a5be6` (name "agent-user@hp-chell.lan.1qaz.ca (SSH)") — fetch with `bw get password 1db3b9f6-a82e-4692-846a-b4bb002a5be6` (vault must be unlocked; `bw unlock`).
   - Host inventory/specs: see `~/AGENT_INFO.md` on the host itself. Original provisioning prompt saved at `~/AGENT_PROMPT.txt` on the host.
   - Groups: adm,cdrom,sudo,dip,plugdev,lpadmin,sambashare,docker. Has docker, tailscale0 interface (not connected as of provisioning), python3.12, git, jq, rsync, tmux.
@@ -71,8 +71,37 @@ selects the newest installed kernel, so `7.0.0-30-generic` is what boots
 by default now; `6.17.0-42-generic` remains available via the GRUB
 "Advanced options" submenu as a known-good fallback if ever needed.
 
+**2026-09-27, SDDM → Plasma Login**: an `apt upgrade` (which also
+brought kernel `7.0.0-34-generic`) switched the display manager to
+`plasmalogin` (`/etc/X11/default-display-manager`). Its greeter runs
+under `kwin_wayland` (user `plasmalogin`, uid 984, `plasma-login*.service`
+in its systemd *user* manager), so the SDDM `Xsetup` hook above no
+longer fires and the black screen came back. Findings:
+- A resolution change via `kscreen-doctor` (1600x900 and back) is **not**
+  enough under KWin: the eDP panel fitter absorbs it (kernel adjusted
+  mode stays native 3200x1800, no real pipe modeset). Going to 1600x900
+  and staying there would also be unusable anyway — KWin keeps the 1.85x
+  scale, giving an 865x487 logical greeter.
+- A DPMS cycle (`kscreen-doctor --dpms off` then `--dpms on`) fully
+  disables the CRTC (`i915_display_info` shows pipe A `active=no`) and
+  its re-enable is a genuine full modeset — that un-sticks the pipe.
+- `kscreen-doctor` needs the greeter's session bus to *apply* changes
+  (`DBUS_SESSION_BUS_ADDRESS=unix:path=/run/user/984/bus`,
+  `WAYLAND_DISPLAY=wayland-0`); with `KSCREEN_BACKEND_INPROCESS=1` it
+  reads outputs fine but silently fails to set ("Cannot find output
+  with id 1"). Mode specs must match exactly (`1600x900@59.95`, not
+  `@60`) or use mode ids.
+- i915 debugfs is under `/sys/kernel/debug/dri/1/` on this box (DRM
+  minor 1), not `dri/0`.
+
+Fix: `/usr/local/libexec/hp-chell-i915-dpms-cycle` (no-op unless kernel
+`7.*` and user `plasmalogin`; waits for KWin, then cycles DPMS), run via
+`/etc/systemd/user/plasma-login.service.d/10-i915-kernel7-dpms-cycle.conf`
+as `ExecStartPre` (plasma-login.service is ordered after
+plasma-login-kwin_wayland.service, so KWin is already up).
+
 Setup script for a fresh install: `setup-i915-kernel7-fix.sh` (in this
-repo) installs the `Xsetup` fix and sets `GRUB_TIMEOUT=5` /
+repo) installs both the SDDM `Xsetup` fix and the Plasma Login drop-in, and sets `GRUB_TIMEOUT=5` /
 `GRUB_TIMEOUT_STYLE=menu` so the boot menu is visible for 5s. Idempotent,
 safe to re-run.
 

setup-i915-kernel7-fix.sh

@@ -4,10 +4,11 @@
 # Run on a fresh hp-chell (HP Chromebook 13 "chell", coreboot/MrChromebox
 # firmware, KDE neon) install to apply two things:
 #
-#   1. The kernel 7.x cold-boot black-screen workaround: a
+#   1. The kernel 7.x cold-boot black-screen workaround, which forces a
-#      /usr/share/sddm/scripts/Xsetup hook that forces a second modeset
+#      second modeset on eDP-1 to un-stick i915's pipe A on 7.x kernels:
-#      on eDP-1 to un-stick i915's pipe A on 7.x kernels. See project
+#      a /usr/share/sddm/scripts/Xsetup hook (SDDM) and a
-#      CLAUDE.md for the full diagnosis writeup.
+#      plasma-login.service drop-in doing a DPMS cycle (Plasma Login).
+#      See project CLAUDE.md for the full diagnosis writeup.
 #   2. GRUB_TIMEOUT=5 / GRUB_TIMEOUT_STYLE=menu, so the boot menu is
 #      visible for 5 seconds (useful for picking a fallback kernel like
 #      6.17.0-42-generic if a future 7.x point release regresses again).
@@ -50,6 +51,50 @@ esac
 EOF
 chmod 755 /usr/share/sddm/scripts/Xsetup
 
+# Plasma Login (plasmalogin, replaced SDDM on neon in 2026-09) runs its
+# greeter under kwin_wayland, so the SDDM Xsetup hook above never fires.
+# Equivalent fix there: a DPMS off/on cycle via kscreen-doctor, run from
+# the greeter's own systemd user manager right after KWin comes up. A
+# plain resolution change is NOT enough under KWin -- the eDP panel
+# fitter absorbs it as a fastset without a real pipe modeset; DPMS off
+# fully disables the CRTC so the "on" is a genuine full modeset.
+echo "==> installing plasmalogin greeter i915 kernel-7.x workaround"
+install -d /usr/local/libexec
+cat > /usr/local/libexec/hp-chell-i915-dpms-cycle << 'EOF'
+#!/bin/sh
+# Run as the plasmalogin greeter user (from plasma-login.service's
+# ExecStartPre). Forces a full modeset on kernel 7.x to un-stick i915
+# pipe A after the cold-boot first modeset. See hp-chell CLAUDE.md.
+case "$(uname -r)" in
+    7.*) ;;
+    *) exit 0 ;;
+esac
+[ "$(id -un)" = plasmalogin ] || exit 0
+
+export WAYLAND_DISPLAY="${WAYLAND_DISPLAY:-wayland-0}"
+export QT_QPA_PLATFORM=wayland
+
+# wait (up to ~20s) for KWin's output management to answer
+i=0
+until kscreen-doctor -o >/dev/null 2>&1; do
+    i=$((i + 1))
+    [ "$i" -ge 40 ] && { echo "kscreen-doctor never came up" >&2; exit 0; }
+    sleep 0.5
+done
+
+kscreen-doctor --dpms off >/dev/null 2>&1
+sleep 1
+kscreen-doctor --dpms on >/dev/null 2>&1
+echo "cycled DPMS off/on (i915 kernel 7.x cold-boot workaround)"
+EOF
+chmod 755 /usr/local/libexec/hp-chell-i915-dpms-cycle
+
+install -d /etc/systemd/user/plasma-login.service.d
+cat > /etc/systemd/user/plasma-login.service.d/10-i915-kernel7-dpms-cycle.conf << 'EOF'
+[Service]
+ExecStartPre=-/usr/local/libexec/hp-chell-i915-dpms-cycle
+EOF
+
 echo "==> setting GRUB_TIMEOUT=5 / GRUB_TIMEOUT_STYLE=menu"
 grub_conf=/etc/default/grub