On 11/06/18 14:20, Hans Verkuil wrote: > 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? A quick update: I've been working on a next version of this patch series that combines options 2 and 3 and moves the logic out of the i915 driver into the core drm CEC code since all DP drivers will have the same problem. I hope to post the new series later today. Regards, Hans
