Kernel 5.10.4 i915 and dual screen freeze on intel GPU

I understand there are two topics opened in the last days about dual screen freezes with kernel 5.10.4, but they are all related to nvidia hardware.

• last slackpkg update stopped dual screen
• Occasional system freeze after the latest update in Slackware64 --curent

So I opened this one as this is related to i915 (Intel GPU). I have a thinkpad x250 with Intel HD Graphics 5500. After kernel 5.10.4 landed, dual monitor setup is no longer an option. No matter what I do, anything using X or xrandr will freeze the system. The only way to regain a responsive system is by rebooting. I've already tried many configuration options. None was successful.

Right now, my environment is as follow:

• Boot cmdline with no mention to any of i915* parameter
• Removed xf86-video* packages
• No /etc/X11/xorg.conf.d/20-intel.conf
• Hence the system is using KMS with mesa detected driver (i915)

The settings I had before and that was providing a working dual head configuration since kernel 5.4 series was:

• Boot cmdline with i915.fastboot=1
• All xf86-video* packages installed
• No /etc/X11/xorg.conf.d/20-intel.conf

While debugging, I've tried many variants of /etc/X11/xorg.conf.d/20-intel.conf, including (but not limited to):

Code:Section "Device"
Identifier "Intel Graphics"
Driver "intel"
Option "AccelMethod" "uxa"
EndSectionCode:Section "Device"
Identifier "Intel"
Driver "modesetting"
Option "AccelMethod" "glamor"
Option "Backlight" "intel_backlight"
EndSectionNone of these provided a working setup for the second head.

I've also tried plasma.wayland too. It works fine with the second monitor attached, but as soon as I end a session and sddm take over (using X server), system freezes again.

I can see that kernel 5.10.4 changelog brings many i915 and drm related commits, but I've not tested the role of these changes into the issue I'm now reporting.

I've also tried using xrandr in the CLI to setup screens once the external monitor is connected. It also freezes as soon as I issue `xrandr --auto` or any other xrandr option that triggers external monitor.

Right now, the only workaround to have a stable system is to disconnect the external monitor and do not connect it again.

I'm running out of ideas on how to properly configure my box to work with dual screens after kernel 5.10.4. That's why I'm starting this topic so other can share their experience.