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

Hans Verkuil hverkuil at xs4all.nl
Fri Dec 1 07:23:53 UTC 2017


I really like to get this in for 4.16 so I can move forward with hooking
this up for nouveau/amd.



On 11/20/2017 12:42 PM, Hans Verkuil wrote:
> 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.
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