Comment 2 for bug 1903969

$ git bisect log
git bisect start
# old: [f8394f232b1eab649ce2df5c5f15b0e528c92091] Linux 5.10-rc3
git bisect old f8394f232b1eab649ce2df5c5f15b0e528c92091
# new: [c0f98d2f8b076bf3e3183aa547395f919c943a14] Merge tag 'drm-misc-next-2020-11-05' of git://anongit.freedesktop.org/drm/drm-misc into drm-next
git bisect new c0f98d2f8b076bf3e3183aa547395f919c943a14
# old: [3cea11cd5e3b00d91caf0b4730194039b45c5891] Linux 5.10-rc2
git bisect old 3cea11cd5e3b00d91caf0b4730194039b45c5891
# old: [6a6e5988a2657cd0c91f6f1a3e7d194599248b6d] drm/ttm: replace last move_notify with delete_mem_notify
git bisect old 6a6e5988a2657cd0c91f6f1a3e7d194599248b6d
# new: [f566fdcd6cc49a9d5b5d782f56e3e7cb243f01b8] drm/i915: Force VT'd workarounds when running as a guest OS
git bisect new f566fdcd6cc49a9d5b5d782f56e3e7cb243f01b8
# old: [b50b7991b739c6d63658e3324a01eaa0fafe8b7f] drm/i915/dg1: add more PCI ids
git bisect old b50b7991b739c6d63658e3324a01eaa0fafe8b7f
# old: [0a5991f5d022235ba085001e2a3926f83a0aed1b] drm/i915: Simplify the link training functions
git bisect old 0a5991f5d022235ba085001e2a3926f83a0aed1b
# new: [0dac17af0ab4e55b6b18a762141f255d3eea1e37] drm/i915/dg1: Enable DPLL for DG1
git bisect new 0dac17af0ab4e55b6b18a762141f255d3eea1e37
# new: [055f8458d95a38c20e8360634255f684378179eb] drm/i915: Force DPCD backlight mode for BOE 2270 panel
git bisect new 055f8458d95a38c20e8360634255f684378179eb
# old: [7b2a4ab8b0ef2a6dbec0b15d2cabf5a9e7acdf4c] drm/i915: Switch to LTTPR transparent mode link training
git bisect old 7b2a4ab8b0ef2a6dbec0b15d2cabf5a9e7acdf4c
# new: [b30edfd8d0b4b22f9c17c208064b39a3d93e6971] drm/i915: Switch to LTTPR non-transparent mode link training
git bisect new b30edfd8d0b4b22f9c17c208064b39a3d93e6971
# first new commit: [b30edfd8d0b4b22f9c17c208064b39a3d93e6971] drm/i915: Switch to LTTPR non-transparent mode link training

So this, https://patchwork.freedesktop.org/series/82449/ , patchset is the fix.

* b30edfd8d0b4 drm/i915: Switch to LTTPR non-transparent mode link training
* 7b2a4ab8b0ef drm/i915: Switch to LTTPR transparent mode link training
* 9782f52ab5d6 drm/dp: Add LTTPR helpers
* c43027a9a375 drm/i915: Factor out a helper to disable the DPCD training pattern
* 0a5991f5d022 drm/i915: Simplify the link training functions
* 6777a855d66d drm/i915: Fix DP link training pattern mask

These six commits are required with additional dependency to https://patchwork.freedesktop.org/series/76993/ as well. Verified by cherry-picking these to a Ubuntu-5.10-5.10.0-3.4 kernel.