Enhancements for Render composite request

Barry Scott barry.scott at onelan.co.uk
Tue Sep 1 10:08:55 PDT 2009

Keith Packard wrote:
>  2) No vblank synchronization. Anyone wanting to double buffer 2D apps
>     has no way of avoiding tearing. I'd like this inside the X server
>     to make updates under a RandR transform sync to vblank.
> It seems like operation 2) should be an option on the picture object;
> set a sync mode on the picture and all operations would be covered by
> that mode. It would be 'best effort', so that drivers not supporting the
> sync mode would simply skip it. The question is how fancy this option
> should be; in the simple case, we'd make it just avoid tearing, more
> complex cases could involve having sequential operations to the same
> picture wait for a specific frame number. I'd love to have comments on
> precisely which 'swap modes' would be useful here.
This sounds like you end up in the same problems that the XV tear free 
support has.
Namely that because the Intel driver can only have the GPU do one thing 
at a time
the frame rate goes to hell if more then one app writes to the screen. 
The worst case
would be to split the screen in two with left and right windows. If both 
the left and
right apps ask for a tear free render the frame rate will halve.


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