Question about linux-explicit-synchronization

[Scott Anderson]

Hi,

I have a question about the usage of 
zwp_linux_explicit_synchronization_unstable_v1.

In the Weston implementation, it's simply a case of the compositor 
getting the fence from the client, using eglWaitSyncKHR (or equivilent) 
on it, and passing back a release fence from OUT_FENCE_FD.

However, is the protocol intended to allow a compositor which takes a 
more active role in monitoring the fences? For example, a compositor 
could check the state of the fence at drawing time and decide to use the 
client's previous buffer if the fence hasn't been signalled.

Is it worth a compositor being implemented like this? As far as I 
understand, doing this would stop any particuarly slow clients from 
stalling the compositor's drawing too much and possibly missing vblank. 
But I'm not an expert on fences or how eglWaitSyncKHR affects anything 
at a driver level.

Thanks,
Scott