[RFC PATCH 7/7] drm/i915: add DisplayPort CEC-Tunneling-over-AUX support

[Hans Verkuil]

On 05/30/2017 04:19 PM, Clint Taylor wrote:
> 
> 
> On 05/30/2017 12:11 AM, Jani Nikula wrote:
>> On Tue, 30 May 2017, Hans Verkuil <hverkuil at xs4all.nl> wrote:
>>> On 05/29/2017 09:00 PM, Daniel Vetter wrote:
>>>> On Fri, May 26, 2017 at 12:20:48PM +0200, Hans Verkuil wrote:
>>>>> On 05/26/2017 09:15 AM, Daniel Vetter wrote:
>>>>>> Did you look into also wiring this up for dp mst chains?
>>>>> Isn't this sufficient? I have no way of testing mst chains.
>>>>>
>>>>> I think I need some pointers from you, since I am a complete newbie when it
>>>>> comes to mst.
>>>> I don't really have more clue, but yeah if you don't have an mst thing (a
>>>> simple dp port multiplexer is what I use for testing here, then plug in a
>>>> converter dongle or cable into that) then probably better to not wire up
>>>> the code for it.
>>> I think my NUC already uses mst internally. But I was planning on buying a
>>> dp multiplexer to make sure there is nothing special I need to do for mst.
>>>
>>> The CEC Tunneling is all in the branch device, so if I understand things
>>> correctly it is not affected by mst.
>>>
>>> BTW, I did a bit more testing on my NUC7i5BNK: for the HDMI output they
>>> use a MegaChip MCDP2800 DP-to-HDMI converter which according to their
>>> datasheet is supposed to implement CEC Tunneling, but if they do it is not
>>> exposed as a capability. I'm not sure if it is a MegaChip firmware issue
>>> or something else. The BIOS is able to do some CEC, but whether they hook
>>> into the MegaChip or have the CEC pin connected to a GPIO or something and
>>> have their own controller is something I do not know.
>>>
>>> If anyone can clarify what Intel did on the NUC, then that would be very
>>> helpful.
>> It's called LSPCON, see i915/intel_lspcon.c, basically to support HDMI
>> 2.0. Currently we only use it in PCON mode, shows up as DP for us. It
>> could be used in LS mode, showing up as HDMI 1.4, but we don't support
>> that in i915.
>>
>> I don't know about the CEC on that.
> 
> My NUC6i7KYK has the MCDP2850 LSPCON and it does support CEC over Aux.
> The release notes for the NUC state that there is a BIOS configuration
> option for enabling support. My doesn't have the option but the LSPCON
> fully supports CEC.

What is the output of:

dd if=/dev/drm_dp_aux0 of=aux0 skip=12288 ibs=1 count=48
od -t x1 aux0

Assuming drm_dp_aux0 is the aux channel for the HDMI output on your NUC.

If the first byte is != 0x00, then it advertises CEC over Aux.

For me it says 0x00.

When you say "it does support CEC over Aux", does that mean you have actually
tested it somehow? The only working solution I have seen mentioned for the
NUC6i7KYK is a Pulse-Eight adapter.

With the NUC7i Intel made BIOS support for CEC, but it is not at all
clear to me if they used CEC tunneling or just hooked up the CEC pin to
some microcontroller.

The only working chipset I have seen is the Parade PS176.

Regards,

	Hans

> 
> -Clint
> 
>>
>>
>> BR,
>> Jani.
>>
>>> It would be so nice to get MegaChip CEC Tunneling working on the NUC, because
>>> then you have native CEC support without requiring any Pulse Eight adapter.
>>>
>>> And add a CEC-capable USB-C to HDMI adapter and you have it on the USB-C
>>> output as well.
>>>
>>> Regards,
>>>
>>> 	Hans
>