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>
This commit is contained in:
ergosteur
co-authored by
Claude Opus 5.5
Claude Opus 5.5
parent
78e52ffb29
commit
9c3b8e48ee
@@ -3,7 +3,7 @@
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## Agent-accessible host
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- **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.
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- 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`.
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- 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.
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- 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`).
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- Host inventory/specs: see `~/AGENT_INFO.md` on the host itself. Original provisioning prompt saved at `~/AGENT_PROMPT.txt` on the host.
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- Groups: adm,cdrom,sudo,dip,plugdev,lpadmin,sambashare,docker. Has docker, tailscale0 interface (not connected as of provisioning), python3.12, git, jq, rsync, tmux.
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@@ -71,8 +71,37 @@ selects the newest installed kernel, so `7.0.0-30-generic` is what boots
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by default now; `6.17.0-42-generic` remains available via the GRUB
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"Advanced options" submenu as a known-good fallback if ever needed.
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**2026-09-27, SDDM → Plasma Login**: an `apt upgrade` (which also
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brought kernel `7.0.0-34-generic`) switched the display manager to
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`plasmalogin` (`/etc/X11/default-display-manager`). Its greeter runs
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under `kwin_wayland` (user `plasmalogin`, uid 984, `plasma-login*.service`
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in its systemd *user* manager), so the SDDM `Xsetup` hook above no
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longer fires and the black screen came back. Findings:
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- A resolution change via `kscreen-doctor` (1600x900 and back) is **not**
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enough under KWin: the eDP panel fitter absorbs it (kernel adjusted
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mode stays native 3200x1800, no real pipe modeset). Going to 1600x900
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and staying there would also be unusable anyway — KWin keeps the 1.85x
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scale, giving an 865x487 logical greeter.
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- A DPMS cycle (`kscreen-doctor --dpms off` then `--dpms on`) fully
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disables the CRTC (`i915_display_info` shows pipe A `active=no`) and
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its re-enable is a genuine full modeset — that un-sticks the pipe.
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- `kscreen-doctor` needs the greeter's session bus to *apply* changes
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(`DBUS_SESSION_BUS_ADDRESS=unix:path=/run/user/984/bus`,
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`WAYLAND_DISPLAY=wayland-0`); with `KSCREEN_BACKEND_INPROCESS=1` it
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reads outputs fine but silently fails to set ("Cannot find output
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with id 1"). Mode specs must match exactly (`1600x900@59.95`, not
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`@60`) or use mode ids.
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- i915 debugfs is under `/sys/kernel/debug/dri/1/` on this box (DRM
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minor 1), not `dri/0`.
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Fix: `/usr/local/libexec/hp-chell-i915-dpms-cycle` (no-op unless kernel
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`7.*` and user `plasmalogin`; waits for KWin, then cycles DPMS), run via
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`/etc/systemd/user/plasma-login.service.d/10-i915-kernel7-dpms-cycle.conf`
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as `ExecStartPre` (plasma-login.service is ordered after
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plasma-login-kwin_wayland.service, so KWin is already up).
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Setup script for a fresh install: `setup-i915-kernel7-fix.sh` (in this
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repo) installs the `Xsetup` fix and sets `GRUB_TIMEOUT=5` /
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repo) installs both the SDDM `Xsetup` fix and the Plasma Login drop-in, and sets `GRUB_TIMEOUT=5` /
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`GRUB_TIMEOUT_STYLE=menu` so the boot menu is visible for 5s. Idempotent,
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safe to re-run.
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@@ -4,10 +4,11 @@
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# Run on a fresh hp-chell (HP Chromebook 13 "chell", coreboot/MrChromebox
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# firmware, KDE neon) install to apply two things:
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#
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# 1. The kernel 7.x cold-boot black-screen workaround: a
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# /usr/share/sddm/scripts/Xsetup hook that forces a second modeset
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# on eDP-1 to un-stick i915's pipe A on 7.x kernels. See project
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# CLAUDE.md for the full diagnosis writeup.
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# 1. The kernel 7.x cold-boot black-screen workaround, which forces a
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# second modeset on eDP-1 to un-stick i915's pipe A on 7.x kernels:
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# a /usr/share/sddm/scripts/Xsetup hook (SDDM) and a
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# plasma-login.service drop-in doing a DPMS cycle (Plasma Login).
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# See project CLAUDE.md for the full diagnosis writeup.
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# 2. GRUB_TIMEOUT=5 / GRUB_TIMEOUT_STYLE=menu, so the boot menu is
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# visible for 5 seconds (useful for picking a fallback kernel like
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# 6.17.0-42-generic if a future 7.x point release regresses again).
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@@ -50,6 +51,50 @@ esac
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EOF
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chmod 755 /usr/share/sddm/scripts/Xsetup
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# Plasma Login (plasmalogin, replaced SDDM on neon in 2026-09) runs its
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# greeter under kwin_wayland, so the SDDM Xsetup hook above never fires.
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# Equivalent fix there: a DPMS off/on cycle via kscreen-doctor, run from
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# the greeter's own systemd user manager right after KWin comes up. A
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# plain resolution change is NOT enough under KWin -- the eDP panel
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# fitter absorbs it as a fastset without a real pipe modeset; DPMS off
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# fully disables the CRTC so the "on" is a genuine full modeset.
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echo "==> installing plasmalogin greeter i915 kernel-7.x workaround"
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install -d /usr/local/libexec
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cat > /usr/local/libexec/hp-chell-i915-dpms-cycle << 'EOF'
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#!/bin/sh
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# Run as the plasmalogin greeter user (from plasma-login.service's
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# ExecStartPre). Forces a full modeset on kernel 7.x to un-stick i915
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# pipe A after the cold-boot first modeset. See hp-chell CLAUDE.md.
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case "$(uname -r)" in
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7.*) ;;
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*) exit 0 ;;
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esac
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[ "$(id -un)" = plasmalogin ] || exit 0
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export WAYLAND_DISPLAY="${WAYLAND_DISPLAY:-wayland-0}"
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export QT_QPA_PLATFORM=wayland
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# wait (up to ~20s) for KWin's output management to answer
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i=0
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until kscreen-doctor -o >/dev/null 2>&1; do
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i=$((i + 1))
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[ "$i" -ge 40 ] && { echo "kscreen-doctor never came up" >&2; exit 0; }
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sleep 0.5
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done
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kscreen-doctor --dpms off >/dev/null 2>&1
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sleep 1
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kscreen-doctor --dpms on >/dev/null 2>&1
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echo "cycled DPMS off/on (i915 kernel 7.x cold-boot workaround)"
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EOF
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chmod 755 /usr/local/libexec/hp-chell-i915-dpms-cycle
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install -d /etc/systemd/user/plasma-login.service.d
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cat > /etc/systemd/user/plasma-login.service.d/10-i915-kernel7-dpms-cycle.conf << 'EOF'
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[Service]
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ExecStartPre=-/usr/local/libexec/hp-chell-i915-dpms-cycle
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EOF
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echo "==> setting GRUB_TIMEOUT=5 / GRUB_TIMEOUT_STYLE=menu"
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grub_conf=/etc/default/grub
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