X1 Carbon 3rd Gen, multi-monitor broken after Big Sur upgrade

Hi. It's weekend and I have a bit of time so I took a plunge and finally updated to Big Sur (after backing up Catalina install).

It kind of went ok but I'm having serious issues getting multi-monitor to work.

My setup is:

  • Lenovo X1 Carbon 3rd Gen / 2015
  • i5-5200u, Intel HD5500 / Broadwell
  • Two HDMI external displays, one connected via HDMI->HDMI, other HDMI->MiniDP with adapter

This has been working fine in Catalina but in Big Sur I'm seeing all kinds of craziness:

  • Booting with one monitor connected with HDMI will result in black screen on both after reaching point where Apple logo would appear (same with both external ones)
  • Booting with one monitor connected with MiniDP goes to login screen, I can enter password but then fans go into overdrive and I'm left with spinning beach ball of death
  • I can connect one of monitors after I'm logged in but connecting another gives black screen on all and system freeze. Monitor that was connected second goes to sleep, first one remains on.
  • Disconnecting any monitor after connecting gives black screen on internal one and system freeze
  • While I was checking if it works with some kind of order of connecting I got all to work once but can't repeat that.

Im on latest OC, Lilu, Whatevergreen. Below are device properties I'm injecting:

 <dict> <key>AAPL,ig-platform-id</key> <data>BgAmFg==</data> <key>AAPL,slot-name</key> <string>Internal@0,2,0</string> <key>device_type</key> <string>VGA compatible controller</string> <key>framebuffer-fbmem</key> <data>AACQAA==</data> <key>framebuffer-patch-enable</key> <data>AQAAAA==</data> <key>framebuffer-stolenmem</key> <data>AAAwAQ==</data> <key>hda-gfx</key> <string>onboard-1</string> <key>model</key> <string>Intel HD Graphics 5500</string> </dict> 

Some other issues I'm seeing and didnt get to troubleshoot yet, not sure if related:

  • Every other boot my mouse gets pretty choppy (skips every sec or so), tried different one with same result. Touchpad is fine.
  • Every now and then whole system has massive delays on opening apps and websites
submitted by /u/nisk
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