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

[Hans Verkuil]

Hi Ville,

I finally found some time to dig deeper into when a CEC device should be registered.

Since it's been a long while since we discussed this let me just recap the situation
and then propose three solutions:
CEC is implemented for DP-to-HDMI branch devices through DPCD CEC registers. When
HPD is high we can read these registers and check if CEC is supported or not.

If an external USB-C/DP/mini-DP to HDMI adapter is used, then when the HPD goes low
you lose access to the DPCD registers since that is (I think) powered by the HPD.

If an integrated branch device is used (such as for the HDMI connector on the NUC)
the DPCD registers will remain available even if the display is disconnected.

The problem is with external adapters since if the HPD goes low you do not know
if the adapter has been disconnected, or if the display just pulled the HPD low for a
short time (updating the EDID, HDCP changes). In the latter case you do not want to
unregister and reregister the cec device.

I see three options:

1) register a cec device when a connector is registered and keep it for the life time
of the connector. If HPD goes low, or the branch device doesn't support CEC, then just
set the physical address of the CEC adapter to f.f.f.f.

This is simple, but the disadvantage is that there is a CEC device around, even if the
branch device doesn't support CEC.

2) register a cec device when HPD goes high and if a branch device that supports CEC is
detected. Unregister the cec device when the HPD goes low and stays low for more than
a second. This prevents a cec device from disappearing whenever the display toggles
the HPD. It does require a delayed workqueue to handle this delay.

It would be nice if there is a way to avoid a delayed workqueue, but I didn't see
anything in the i915 code.

3) A hybrid option where the cec device is only registered when a CEC capable branch
device is detected, but then we keep it for the remaining lifetime of the connector.
This avoids the delayed workqueue, and avoids creating cec devices if the branch
device doesn't support CEC. But once it is created it won't be removed until the
connector is also unregistered.

I'm leaning towards the second or third option.

Opinions? Other ideas?

Regards,

	Hans