[PATCHv2 0/3] drm/i915: add DisplayPort CEC-Tunneling-over-AUX support

[Hans Verkuil]

From: Hans Verkuil <hans.verkuil at cisco.com>

This patch series adds support for the DisplayPort CEC-Tunneling-over-AUX
feature. This patch series is based on 4.13-rc4 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.

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.
----------------------------------------------------------------------

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).

Note that a colleague who actually knows his way around a soldering iron
modified an UpTab DisplayPort-to-HDMI adapter for me, hooking up the CEC
pin. And after that change it worked. I also received confirmation that
this really is a chicken-and-egg situation: it is because there is no CEC
support for this feature in any OS that they do not hook up the CEC pin.

So 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.

Changes since v1:

- Incorporated Sean's review comments in patch 1/3.

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          | 302 ++++++++++++++++++++++++++++++++++
 drivers/gpu/drm/i915/intel_dp.c       |  18 +-
 include/drm/drm_dp_helper.h           |  24 +++
 6 files changed, 360 insertions(+), 4 deletions(-)
 create mode 100644 drivers/gpu/drm/drm_dp_cec.c

-- 
2.13.2