[REGRESSION] Panther Lake 8086:b082 internal eDP black after xe takeover since 7.1.6, still present in 7.1.8

Hello,

I am reporting an Intel xe display regression on a Panther Lake laptop. The
problem started with Linux 7.1.6 and is still reproducible with Linux 7.1.8.

System:

   - Laptop: Lenovo ThinkPad T14p Gen 4 (2026)
   - CPU: Intel Core Ultra 9 388H, Panther Lake
   - GPU: Intel integrated graphics, PCI ID 8086
   - Driver: xe
   - Display: internal eDP panel
   - Distribution: Arch Linux x86_64
   - Desktop: KDE Plasma on Wayland

Regression status:

   - Known good: 7.1.5-arch1-2
   - First known bad: 7.1.6-arch1-1
   - Latest tested bad: 7.1.8

The issue is fully reproducible with both 7.1.6 and 7.1.8. Downgrading only
the kernel to 7.1.5-arch1-2 makes the problem disappear.

Steps to reproduce:

   1. Disconnect all external displays.
   2. Cold boot the laptop.
   3. The Lenovo firmware logo and systemd-boot menu are displayed normally.
   4. Select the Arch Linux boot entry.
   5. When the xe driver takes over from the firmware framebuffer, the
   internal panel becomes completely black.
   6. Close and reopen the laptop lid.
   7. The internal panel starts working normally.

The issue occurs on every boot. Connecting an external display changes the
behavior and may allow the internal panel to initialize.

The failure coincides with the following kernel messages:

fbcon: xedrmfb (fb0) is primary device
xe 0000:00:02.0: [drm] *ERROR* CPU pipe A FIFO underrun
xe 0000:00:02.0: [drm] fb0: xedrmfb frame buffer device

Tests performed:

   - Adding xe.enable_psr=0 does not help.
   - Disabling my custom xe SR-IOV service and booting without creating any
   VF does not help.
   - Linux 7.1.5-arch1-2 works with the same firmware and userspace.
   - Linux 7.1.6 and Linux 7.1.8 consistently reproduce the problem.
   - I have not bisected the exact offending commit.

Full debug dmesg and system information are available in the linked drm/xe
issue:

https://gitlab.freedesktop.org/drm/xe/kernel/-/work_items/8902

#regzbot introduced: v7.1.5..v7.1.6

Regards,
rpwtsama