[PATCH RFC wayland-protocols] Add secure output protocol

[Scott Anderson]

Hi,

As far as I understand, the different types and versions of protection a 
client would want is based on the resolution of the content, rather than 
anything about what the content actually is. Is there any particular 
reason a client would care if their content is being used on a higher 
HDCP version than is necessary? e.g. would a client with 720p content 
care about using HDCP 2.2?

If that's the case, I think it would make sense for the compositor to 
always try to negotitate the strongest level of protection that it can 
(or a lower level if set by some policy), and report to the client the 
largest resolution that it can support securely. With that, the client 
can then make the decision about what content it can provide.

<interface name="...">
   <event name="secure_resolution">
     <arg name="width", type="int">
     <arg name="height", type="int">
     <arg name="refresh", type="int">
   </event>
</interface>

This would remove a lot of the back-and-forth between the client and the 
compositor, where the client says what content it has, and the 
compositor saying if it can securely display it.

This event could also be re-emitted when the protection status changes. 
There could also be the special case of 0x0, where the compositor failed 
to negotiate any secure connection, and no resolution is secure.

A compositor may also choose to emit this signal based on what output 
the client is set to display on, but that would probably be left up to 
the compositor policy. It's possible that wl_output could be integrated 
into this somehow, but I haven't thought too much about how yet.

Scott