[PATCHv5 0/3] drm/i915: add DisplayPort CEC-Tunneling-over-AUX support

[Hans Verkuil]

This patch series adds support for the DisplayPort CEC-Tunneling-over-AUX
feature. This patch series is based on the 4.14 mainline release but applies
as well to drm-next.

This patch series has been tested with my NUC7i5BNK, a Samsung USB-C to 
HDMI adapter and a Club 3D DisplayPort MST Hub + modified UpTab DP-to-HDMI
adapter (where the CEC pin is wired up).

Please note this comment at the start of drm_dp_cec.c:

----------------------------------------------------------------------
Unfortunately it turns out that we have a chicken-and-egg situation
here. Quite a few active (mini-)DP-to-HDMI or USB-C-to-HDMI adapters
have a converter chip that supports CEC-Tunneling-over-AUX (usually the
Parade PS176 or MegaChips MCDP2900), but they do not wire up the CEC pin,
thus making CEC useless.

Sadly there is no way for this driver to know this. What happens is
that a /dev/cecX device is created that is isolated and unable to see
any of the other CEC devices. Quite literally the CEC wire is cut
(or in this case, never connected in the first place).

I suspect that the reason so few adapters support this is that this
tunneling protocol was never supported by any OS. So there was no
easy way of testing it, and no incentive to correctly wire up the
CEC pin.

Hopefully by creating this driver it will be easier for vendors to
finally fix their adapters and test the CEC functionality.

I keep a list of known working adapters here:

https://hverkuil.home.xs4all.nl/cec-status.txt

Please mail me (hverkuil at xs4all.nl) if you find an adapter that works
and is not yet listed there.

Note that the current implementation does not support CEC over an MST hub.
As far as I can see there is no mechanism defined in the DisplayPort
standard to transport CEC interrupts over an MST device. It might be
possible to do this through polling, but I have not been able to get that
to work.
----------------------------------------------------------------------

I really hope that this work will provide an incentive for vendors to
finally connect the CEC pin. It's a shame that there are so few adapters
that work (I found only two USB-C to HDMI adapters that work, and no
(mini-)DP to HDMI adapters at all).

Hopefully if this gets merged there will be an incentive for vendors
to make adapters where this actually works. It is a very nice feature
for HTPC boxes.

The main reason why this v5 is delayed by 2 months is due to the fact
that I needed some dedicated time to investigate what happens when an
MST hub is in use. It turns out that this is not working. There is no
mechanism defined in the DisplayPort standard to transport the CEC
interrupt back up the MST chain. I was actually able to send a CEC
message but the interrupt that tells when the transmit finished is
unavailable.

I attempted to implement this via polling, but I got weird errors
and was not able to read the DP_DEVICE_SERVICE_IRQ_VECTOR_ESI1
register. I decided to give up on this for now and just disable CEC
for DP-to-HDMI adapters after an MST hub. I plan to revisit this
later since it would be neat to make this work as well. Although it
might not be possible at all.

If anyone is interested, work-in-progress for this is here:

https://git.linuxtv.org/hverkuil/media_tree.git/log/?h=dp-cec-mst

Note that I removed the Tested-by tag from Carlos Santa due to the
almost complete rework of the third patch. Carlos, can you test this again?

Regards,

        Hans

Changes since v4:

- Updated comment at the start of drm_dp_cec.c
- Add edid pointer to drm_dp_cec_configure_adapter
- Reworked the last patch (adding CEC to i915) based on Ville's comments
  and my MST testing:
	- register/unregister CEC in intel_dp_connector_register/unregister
	- add comment and check if connector is registered in long_pulse
	- unregister CEC if an MST 'connector' is detected.