[Mesa-dev] [Bug 27841] New: GLX/EGL extension to mark backbuffer purgeable after using glXCopySubBufferMESA

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=27841

           Summary: GLX/EGL extension to mark backbuffer purgeable after
                    using glXCopySubBufferMESA
           Product: Mesa
           Version: git
          Platform: Other
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: medium
         Component: GLX
        AssignedTo: mesa-dev at lists.freedesktop.org
        ReportedBy: bob at o-hand.com


Unlike the semantics of glXSwapBuffers which doesn't usually have to preserve
the contents of the backbuffer, glXCopySubBuffer can be called multiple times
though so the driver isn't ever be able to free the backbuffer under memory
pressure.

We use glXCopySubBufferMESA in Clutter when updating a small region of a
ClutterStage and would like to be able to tell the driver when the backbuffer
contents can be discarded.

Clutter is used by the Mutter compositor and there may be multiple OpenGL
applications on hidden/non-active workspaces and it would be ideal for the
corresponding backbuffers for those applications to be freed if the system is
under memory pressure.

Something like glXDiscardBackbufferMESA() could be one possability.

Alternatively a swap buffers API that could take a list of rectangles to blit
and immediately mark the backbuffer as purgeable might help.

(Apologies if I'm missing something and there is already a suitable mechanism.)

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