DVI output on i.MX51 EVP board not working?

["Wladimir J. van der Laan" <laanwj@gmail.com>]

Hello,

I can't get the DVI output to work on a i.MX51 EVP board (babbage). There
seems to be no signal at all. The board is running the mainline kernel 4.11.0-rc4 as of a
day ago (89970a0).

- The connector and cable works. u-boot (v2016.07-rc2) shows a penguin at boot.
  However while the Linux kernel boots, the signal goes away: monitor (a fairly
  old Dell 24" panel, max resolution 1600x1200, works fine with Linux/DRM with other
  hardware) goes to sleep mode.

- Monitor is connected through HDMI<->DVI cable, could try another monitor if
  that's potentially the issue (Could it be EDID related? I'd guess not as the timings
  are hardcoded in the device tree).

- Haven't changed anything in the dtb with regard to port config or resolution
  or timings. The default ones in imx51-babbage.dts should theoretically just
  work. I checked and these are the same as in u-boot:

        timing0: dvi {
                clock-frequency = <65000000>;
                hactive = <1024>;
                vactive = <768>;
                hback-porch = <220>;
                hfront-porch = <40>;
                vback-porch = <21>;
                vfront-porch = <7>;
                hsync-len = <60>;
                vsync-len = <10>;
        };

- Went as far back as kernel v4.0, even to v3.12 or so (commit 493a863, "ARM:
  dts: imx51-babbage: Make DVI and WVGA panel functional"). No difference. So nothing to
  bisect, unfortunately.

- In dmesg this is all the output I get:

[    0.801700] imx-ipuv3 40000000.ipu: no port@0 node in /soc/ipu@40000000, not using CSI0
[    0.809826] imx-ipuv3 40000000.ipu: no port@1 node in /soc/ipu@40000000, not using CSI1
[    0.818600] [drm] Supports vblank timestamp caching Rev 2 (21.10.2013).
[    0.825230] [drm] No driver support for vblank timestamp query.
[    0.831481] imx-drm display-subsystem: bound imx-ipuv3-crtc.0 (ops ipu_crtc_ops)
[    0.839132] imx-drm display-subsystem: bound imx-ipuv3-crtc.1 (ops ipu_crtc_ops)
[    0.846617] imx-drm display-subsystem: bound display@di0 (ops imx_pd_ops)
[    0.986410] Console: switching to colour frame buffer device 128x48
[    1.032306] imx-drm display-subsystem: fb0:  frame buffer device
[    1.039362] [drm] Initialized imxdrm 1.0.0 20120507 for display-subsystem on minor 0
[    1.047276] imx-ipuv3 40000000.ipu: IPUv3EX probed

As far as I know this is nothing unexpected. CSI0 and CSI1 are not connected but DI0 should be the
DVI port, which does get initialized. The size of the framebuffer device seems to indicate the resolution
is set correctly (assuming 16x8 characters).

There was also a "timeout waiting for vsync" error, 

[    0.917254] WARNING: CPU: 0 PID: 1 at drivers/gpu/drm/drm_atomic_helper.c:1122 drm_atomic_helper_wait_for_vblanks.part.0+0x264/0x274
[    0.917259] [CRTC:26] vblank wait timed out

which was fixed after applying Philipp Zabel's "drm/imx: don't wait for vblank and stop calling cleanup_planes in commit_tail" patch series.
However that didn't solve the output issue.

I'm out of things to try to diagnose this, does anyone perhaps have an idea or seen this issue
before?

Regards,
Wladimir van der Laan
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