drm/imx: 'flip_done timed out' on i.MX6D+ and kernel v4.19.48

[MOESL.Hannes]

Hi,

We observed the following error on an i.MX6D+ CPU during start of X. As a result, the screen goes blank.

-------
[ 3599.200886] [drm:drm_atomic_helper_wait_for_flip_done] *ERROR* [CRTC:30:crtc-0] flip_done timed out
[ 3610.080885] [drm:drm_atomic_helper_wait_for_dependencies] *ERROR* [CRTC:30:crtc-0] flip_done timed out
[ 3620.320849] [drm:drm_atomic_helper_wait_for_dependencies] *ERROR* [CONNECTOR:45:LVDS-1] flip_done timed out
[ 3630.560864] [drm:drm_atomic_helper_wait_for_dependencies] *ERROR* [PLANE:28:plane-0] flip_done timed out
-------

The error happens quite randomly but it can be reliably reproduced by repeatedly restarting X . On our system, the error occurs somewhere between 80 and 300 restarts of X.
We first observed the issue using kernel 4.14.123. Updating to 4.19.48 did not solve the issue unfortunately.

Cheers,
Hannes Moesl

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