DRI2 Design Wiki Page

Keith Whitwell keith at tungstengraphics.com
Thu Oct 4 11:05:03 PDT 2007

Kristian Høgsberg wrote:
> On 10/4/07, Keith Packard <keithp at keithp.com> wrote:
>> On Thu, 2007-10-04 at 01:27 -0400, Kristian Høgsberg wrote:
>>> There is an issue with the design, though, related to how and when the
>>> DRI driver discovers that the front buffer has changed (typically
>>> resizing).
>> Why would the rendering application even need to know the size of the
>> front buffer? The swap should effectively be under the control of the
>> front buffer owner, not the rendering application.
> Ok, I phrased that wrong: what the DRI driver needs to look out for is
> when size of the rendering buffers change.  For a redirected window,
> this does involve resizing the front buffer, but that's not the case
> for a non-redirected window.  The important part, though, is that the
> drawable size changes and before submitting rendering, the DRI driver
> has to allocate new private backbuffers that are big enough to hold
> the contents.
>> As far as figuring out how big to make the rendering buffers, that's
>> outside the scope of DRM in my book. The GLX interface can watch for
>> ConfigureNotify events on the associated window and resize the back
>> buffers as appropriate.
> I guess you're proposing libGL should transparently listen for
> ConfigureNotify events?  I don't see how that can work, there is no
> guarantee that an OpenGL application handles events.  For example,
> glxgears without an event loop, just rendering.  If the rendering
> extends outside the window bounds and you increase the window size,
> the next frame should include those parts that were clipped by the
> window in previous frames.  X events aren't reliable for this kind of
> notification.
> And regardless, the issue isn't so much how to get the resize
> notification from the X server to the direct rendering client, but
> rather that the Gallium design doesn't expect these kinds of
> interruptions while rendering a frame.  So while libGL (or AIGLX) may
> be able to notice that the window size changed, what I'm missing is a
> mechanism to ask the DRI driver to reallocate its back buffers.

I think basically we just need a tweak to what we're already doing for 
private backbuffers to cope with the periodic rendering case you've 
highlighted.  So basically checking before the first draw and again 
before swapbuffers, rather than just before swapbuffers.

This doesn't address the question about contexts in potentially 
different processes sharing a backbuffer, but I'm not 100% convinced its 
possible, and if it is possible under glx, I'm not 100% convinced that 
its a sensible thing to support anyway...


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