On 11/07/17 15:30, Hans Verkuil wrote:
> From: Hans Verkuil <hans.verk...@cisco.com>
> This patch series adds support for the DisplayPort CEC-Tunneling-over-AUX
> feature. This patch series is based on the latest mainline kernel (as of 
> today)
> which has all the needed cec and drm 4.13 patches merged.
> This patch series has been tested with my NUC7i5BNK and a Samsung USB-C to 
> HDMI adapter.


> 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), 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.

I got confirmation of this suspicion. This is indeed the reason why so
few adapters hook this up. Having native linux support for this feature
will definitely help the adoption of CEC over DP.



> 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 (hverk...@xs4all.nl) if you find an adapter that works
> and is not yet listed there.
> ----------------------------------------------------------------------
> 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).
> Daniel, I incorporated all your suggestions/comments from the RFC patch
> series from about 2 months ago.
> Regards,
>         Hans
> Hans Verkuil (3):
>   drm: add support for DisplayPort CEC-Tunneling-over-AUX
>   drm-kms-helpers.rst: document the DP CEC helpers
>   drm/i915: add DisplayPort CEC-Tunneling-over-AUX support
>  Documentation/gpu/drm-kms-helpers.rst |   9 +
>  drivers/gpu/drm/Kconfig               |  10 ++
>  drivers/gpu/drm/Makefile              |   1 +
>  drivers/gpu/drm/drm_dp_cec.c          | 308 
> ++++++++++++++++++++++++++++++++++
>  drivers/gpu/drm/i915/intel_dp.c       |  18 +-
>  include/drm/drm_dp_helper.h           |  24 +++
>  6 files changed, 366 insertions(+), 4 deletions(-)
>  create mode 100644 drivers/gpu/drm/drm_dp_cec.c

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