imx-drm: master bind failed: -517 with parallel_display (on i.MX6DL)

[Stefan Birkholz]

Hello,

we are using the mainline kernel (currently on 4.19.128) successfully on an i.MX6DL-based system, but when we try to upgrade to any more recent kernel (>5.1) the display output stops working (screen is blank, backlight works). 

The relevant entries from the kernel log seem to be:
[    8.954942] imx-drm display-subsystem: bound imx-ipuv3-crtc.3 (ops imx_drm_exit [imxdrm])
[    9.028406] imx-drm display-subsystem: failed to bind disp0 (ops imx_pd_ops [parallel_display]): -517
[    9.037873] imx-drm display-subsystem: master bind failed: -517

(I can reproduce those by un-/reloading the parallel-display.ko module.)

On a working 4.19.128 the corresponding entries are:
[    8.879050] imx-drm display-subsystem: bound imx-ipuv3-crtc.3 (ops imx_drm_exit [imxdrm])
[    8.887361] imx-drm display-subsystem: bound disp0 (ops imx_pd_ops [parallel_display])
[    9.131288] imx-drm display-subsystem: fb0: DRM emulated frame buffer device
[    9.139618] [drm] Initialized imx-drm 1.0.0 20120507 for display-subsystem on minor 2

Via git bisection I found the last "good" commit at [044175a06706d516aa42874bb44dbbfc3c4d20eb], and the next "bad" commit at [228cd2dba27cee9956c1af97e6445be056881e41].

I tested both these versions with exactly the same .dtb and our .config (adapted via "make olddefconfig" for both versions) from the 4.19.128 kernel.

Do you have any suggestions what we can do to try and fix this problem?

Kind regards,
 Stefan Birkholz