[SRU][Q][PATCH 0/1] Got black screen after clicked logout button

[AceLan Kao]

From: "Chia-Lin Kao (AceLan)" <acelan.kao at canonical.com>

BugLink: https://bugs.launchpad.net/bugs/2143100

[Impact]
HP and Dell systems with Intel Panther Lake-H (PTL-H) CPU and eDP panel
configured for Panel Replay suffer a permanent black screen after logout
from the desktop. Only a hard reboot restores the display.

[Fix]
Fix the intel_alpm_disable() function in drivers/gpu/drm/i915/display/intel_alpm.c:
1. Remove the PORT_ALPM_CTL write from intel_alpm_disable() — this register
   must only be written before link training, not during disable.
2. Stop clearing ALPM_CTL_ALPM_AUX_LESS_ENABLE in intel_alpm_disable() — this
   bit is about mode switching (AUX-Less vs AUX-Wake), not ALPM enable/disable.

The fix is in linux-next as of 2026-02-27:
  008304c9ae75c772d3460040de56e12112cdf5e6 drm/i915/alpm: ALPM disable fixes
In mainline v7.0-rc2
  eb4a7139e9737 drm/i915/alpm: ALPM disable fixes

Patchwork: https://patchwork.freedesktop.org/patch/704253/

[Test Plan]
Requires a system with Intel integrated graphics and an eDP panel that supports
Panel Replay (typically recent Intel platforms with Panel Replay-capable eDP displays).

Click logout button from the top-right menu and then the GDM shows up.

[Where problems could occur]
The change modifies the ALPM disable path in the Intel i915 display driver, which
is exercised whenever PSR2 or Panel Replay is active on eDP panels.
If the removal of the PORT_ALPM_CTL write is incorrect (i.e., if some hardware
requires it to be cleared at disable time), ALPM may fail to disable properly,
leaving the link in a low-power state that prevents display from recovering. This
would manifest as a black screen or display not coming back after suspend/resume.
Similarly, if removing the ALPM_CTL_ALPM_AUX_LESS_ENABLE clear has unintended
side effects on some panel configurations, the ALPM mode (AUX-Less vs AUX-Wake)
could be stuck in the wrong state, causing link training failures or display
flicker on affected eDP panels.

[Other Info]
Remaining open issue: a ~20-second GDM login delay after logout (unrelated to ALPM) is
tracked upstream at https://gitlab.freedesktop.org/drm/xe/kernel/-/issues/7497 and is
outside the scope of this SRU.

Jouni Högander (1):
  drm/i915/alpm: ALPM disable fixes

 drivers/gpu/drm/i915/display/intel_alpm.c | 7 +------
 1 file changed, 1 insertion(+), 6 deletions(-)

--
2.53.0