Update on the CEC API

Florian Fainelli f.fainelli at gmail.com
Mon Oct 8 08:06:20 PDT 2012

Hi Hans,

On Thursday 27 September 2012 16:33:30 Hans Verkuil wrote:
> Hi all,
> During the Linux Plumbers Conference we (i.e. V4L2 and DRM developers) had a
> discussion on how to handle the CEC protocol that's part of the HDMI 
> The decision was made to create a CEC bus with CEC clients, each represented
> by a /dev/cecX device. So this is independent of the V4L or DRM APIs. 
> In addition interested subsystems (alsa for the Audio Return Channel, and
> possibly networking as well for ethernet over HDMI) can intercept/send CEC
> messages as well if needed. Particularly for the CEC additions made in
> HDMI 1.4a it is no longer possible to handle the CEC protocol completely in
> userspace, but part of the intelligence has to be moved to kernelspace.

What kind of "intelligence" are you talking about? I see nothing in HDMI 1.4a 
or earlier that requires doing stuff in kernelspace besides managing control to 
the hardware, but I might be missing something.

In my opinion ARC is just a control mechanism, and can be dealt with in user-
space, since you really want to just have hints about when ARC is 
enabled/disabled to take appropriate actions on the audio outputs or your 

> I've started working on this API but I am still at the stage of playing
> around with it and thinking about the best way this functionality should
> be exposed. At least I managed to get the first CEC packets transferred
> today :-)
> It will probably be a few weeks before I post something, but in the meantime
> if you want to use CEC and have certain requirements that need to be met,
> please let me know. If only so that I can be certain I haven't forgotten
> anything.

Here is my wish-list, if I may:
- allow for a CEC adapter to be in "detached" / "attached" mode, particularly 
useful if the hardware doing CEC can process a basic set of messages to act a 
a global wake-up source for the system

- allow for a CEC adapter to define several receive modes: unicast and 
"promiscuous", which is useful for dumping the CEC bus messages

- make the CEC adapter API asynchronous for the data path, so it is easy for a 
driver to report completion of a successfully transmitted/received CEC frame

Thank you!

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