I assigned Radhakrishna directly because the regression was quite likely caused by this patch:

author	Radhakrishna Sripada <radhakrishna.sripada@intel.com>	2018-10-12 11:42:32 -0700
committer	Rodrigo Vivi <rodrigo.vivi@intel.com>	2018-11-02 09:15:58 -0700
commit	47e22ff1a9e0c144611bd063b3e6135f9a269503

drm: Add connector property to limit max bpc

At times 12bpc HDMI cannot be driven due to faulty cables, dongles
level shifters etc. To workaround them we may need to drive the output
at a lower bpc. Currently the user space does not have a way to limit
the bpc. The default bpc to be programmed is decided by the driver and
is run against connector limitations.

Creating a new connector property "max bpc" in order to limit the bpc.
xrandr can make use of this connector property to make sure that bpc does
not exceed the configured value. This property can be used by userspace to
set the bpc.

V2: Initialize max_bpc to satisfy kms_properties
V3: Move the property to drm_connector
V4: Split drm and i915 components(Ville)
V5: Make the property per connector(Ville)
V6: Compare the requested bpc to connector bpc(Daniel)
    Move the attach_property function to core(Ville)
V7: Fix checkpatch warnings
V8: Simplify the connector check code(Ville)
V9: Const display_info(Ville)
V10,V11: Fix CI issues.
V12: Add the Kernel documentation(Daniel)
V14: Crossreference the function name in the doc(Daniel)

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Kishore Kadiyala <kishore.kadiyala@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Manasi Navare <manasi.d.navare@intel.com>
Cc: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Cc: Sunpeng Li <sunpeng.li@amd.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Radhakrishna Sripada <radhakrishna.sripada@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181012184233.29250-1-radhakrishna.sripada@intel.com

Link: https://cgit.freedesktop.org/drm-tip/commit/?id=f1a1217222a24775eaaafbcbd386101dc44f8a81

PS: Thanks for being on fd.o, Radhakrishna!

More machines affected:

https://intel-gfx-ci.01.org/tree/drm-tip/drmtip_137/fi-bdw-5557u/igt@kms_properties@connector-properties-atomic.html

https://intel-gfx-ci.01.org/tree/drm-tip/drmtip_137/fi-bdw-gvtdvm/igt@kms_properties@connector-properties-atomic.html

https://intel-gfx-ci.01.org/tree/drm-tip/drmtip_137/fi-bsw-kefka/igt@kms_properties@connector-properties-atomic.html

https://intel-gfx-ci.01.org/tree/drm-tip/drmtip_137/fi-bxt-j4205/igt@kms_properties@connector-properties-atomic.html

https://intel-gfx-ci.01.org/tree/drm-tip/drmtip_137/fi-byt-clapper/igt@kms_properties@connector-properties-atomic.html

https://intel-gfx-ci.01.org/tree/drm-tip/drmtip_137/fi-cfl-8109u/igt@kms_properties@connector-properties-atomic.html

https://intel-gfx-ci.01.org/tree/drm-tip/drmtip_137/fi-cfl-8700k/igt@kms_properties@connector-properties-atomic.html

https://intel-gfx-ci.01.org/tree/drm-tip/drmtip_137/fi-cfl-guc/igt@kms_properties@connector-properties-atomic.html

https://intel-gfx-ci.01.org/tree/drm-tip/drmtip_137/fi-cfl-s3/igt@kms_properties@connector-properties-atomic.html

https://intel-gfx-ci.01.org/tree/drm-tip/drmtip_137/fi-cnl-u/igt@kms_properties@connector-properties-atomic.html

https://intel-gfx-ci.01.org/tree/drm-tip/drmtip_137/fi-icl-u/igt@kms_properties@connector-properties-atomic.html

https://intel-gfx-ci.01.org/tree/drm-tip/drmtip_137/fi-icl-u2/igt@kms_properties@connector-properties-atomic.html

https://intel-gfx-ci.01.org/tree/drm-tip/drmtip_137/fi-whl-u/igt@kms_properties@connector-properties-atomic.html

Starting subtest: connector-properties-atomic
(kms_properties:1136) CRITICAL: Test assertion failure function max_bpc_prop_test, file ../tests/kms_properties.c:104:
(kms_properties:1136) CRITICAL: Failed assertion: ret == 0
(kms_properties:1136) CRITICAL: Last errno: 22, Invalid argument
(kms_properties:1136) CRITICAL: error: -22 != 0
Subtest connector-properties-atomic failed.

Investigating this issue. It is getting tricky because legacy subtest does not fail whereas atomic connector-properties subtest fails

Problem is quite simple in fact. I enabled atomic debugs for recent kernel and found that this code in drm_atomic.c is executed, thus returning EINVAL:

if (!state->allow_modeset) {
	for_each_new_crtc_in_state(state, crtc, new_crtc_state, i) {
		if (drm_atomic_crtc_needs_modeset(new_crtc_state)) {
			DRM_DEBUG_ATOMIC("[CRTC:%d:%s] requires full modeset\n",
						 crtc->base.id, crtc->name);
			return -EINVAL;
		}
	}
}

So it determines that we have to do a full modeset, while we are not allowing it and returns an error.

state->allow_modeset is set to false, because DRM_MODE_ATOMIC_ALLOW_MODESET bit is not set drmModeAtomicCommit call.

Just merely adding this flag fixes the test case:

ret = drmModeAtomicCommit(fd, req, DRM_MODE_ATOMIC_TEST_ONLY|DRM_MODE_ATOMIC_ALLOW_MODESET, NULL);

Created attachment 142643 [details] [review]
Patch to allow a full modeset if needed

Attaching a patch for igt, which fixes the problem.

Do you also send it for review?

Yes, I've sent it to igt-dev.

Thanks, ref: https://patchwork.freedesktop.org/series/53157/

Fix got merged:
author	Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>	2018-11-28 15:03:34 +0200
committer	Maarten Lankhorst <maarten.lankhorst@linux.intel.com>	2018-12-11 10:08:54 +0100
commit	3b52e8a5809a4e860350c59476a456745cd9fee0
tree	4e347a7c2a6d6910ba2d7034d771af641488d78a
parent	4579ac1d445cf39f6de474071b20db790db575bd

Not seen after the fix, closing.