Monitor Doesn't Connect On The Latest Kernel

[ElmoTheWizard]

Chromebook: Acer Chromebook Spin 11 (CP311-H1, CP311-1HN)
Firmware: MrChromebook.tech
Installation Method: ISO

Problem:

Using Ubuntu Mainline Kernel Installer, I'd installed kernel 5.10.16 out of curiosity.
I had reboot my Chromebook. Everything looked normal as I logged in, however my
monitor wasn't detected by my Chromebook.

Since the Spin 11 doesn't have a HDMI port, I've been using a USB-C to HDMI Cable
with my external monitor.

I rebooted my Chromebook and logged into kernel 4.16.18 and my monitor was immediately
recognized again. This is not a problem specific problem to GalliumOS however. I've installed Ubuntu 20.4 and
Arch Linux on the latest kernel and my monitor still doesn't connect.

I'm not certain if this applies to all Apollo Lake Chromebooks but since GalliumOS 4.0 is in the works, on issue #566
you're aiming for kernel 5.4/5.6+. I'm not sure if external monitors would work on those patches as well but it's something to
keep in mind. It would be a major deal breaker if my monitor doesn't work on 4.0.

[RodBarnes]

I confirm this behavior. My Acer CB515-1HT has working USB-C/HDMI video from both ports when running GalliumOS 3.1 (kernel 4.16.18). When I run later kernels (5.4, 5.13) this no longer works. Note that the USB-C ports continue to work -- I can connect my phone to them and the phone is mounted successfully. But when I try to use video, all I get is a black screen on the external monitor. xrandr reports disconnected for both parts regardless of which is actually connected to a monitor.

[RodBarnes]

What is involved in moving this code forward into the main Linux kernel so that it is available to all later releases? I've found several *buntu-based distros that all work fine (on my Acer CB515-1HT, Apollo Lake) with the exception of this issue and #364.

I have lots of time, am willing and able to help. If someone with the necessary experience can provide guidance I'm willing to put in the work to resolve this. I have lots of experience with Windows internals but am new to Linux, though learning a much about the kernel as I investigate issues.