[Intel-gfx] Commit a22c074fd1dd52a8b41dd6789220409b64093e9c broke Intel Corporation HD Graphics (rev 08)

[Frej Drejhammar <frej@stacken.kth.se>]

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Hi,

Upgrading an Acer Chromebook C740 from a stable 5.14.18 to a 5.15.2
kernel leads to a hang/crash very early during startup. With 5.15.2, the
kernel fails before any video output and bootup continues to the
recovery kernel. Bisecting points to commit
a22c074fd1dd52a8b41dd6789220409b64093e9c "Merge tag
'drm-intel-next-2021-08-10-1' of
git://anongit.freedesktop.org/drm/drm-intel into drm-next" as the
culprit, but here the symptoms are slightly different, as bootup just
hangs and no fallback to the recovery kernel happens.

Lspci says that the graphics chipset is a:

00:02.0 VGA compatible controller: Intel Corporation HD Graphics (rev 08)

# dmesg  | grep -e "\(drm\)\|\(i915\)"
[    0.335241] [drm] Initialized i915 1.6.0 20201103 for 0000:00:02.0 on minor 0
[    0.335604] [drm] Initialized vgem 1.0.0 20120112 for vgem on minor 1
[    0.341795] fbcon: i915 (fb0) is primary device
[    1.650456] i915 0000:00:02.0: [drm] fb0: i915 frame buffer device

Both parents of a22c074fd1dd52a8b41dd6789220409b64093e9c:
cb22f12f302584a974a68034350bd48798843ffc and
3bfa7d40ce736ffbbfe07127061f54b359ee2b12 boots and provides video output
without any problems.

I would be grateful for suggestions on how to debug this further.

Regards,

--Frej Drejhammar