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

Hans Verkuil hverkuil at xs4all.nl
Mon Dec 11 08:57:26 UTC 2017

Ping again. Added a CC to Ville whom I inexplicably forgot to add when
I sent the v5 patch series.



On 01/12/17 08:23, Hans Verkuil wrote:
> Ping!
> I really like to get this in for 4.16 so I can move forward with hooking
> this up for nouveau/amd.
> Regards,
> 	Hans
> 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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