[Mesa-dev] [PATCH 3/8] egl/gbm: Implement EGL_EXT_buffer_age

[Kristian Høgsberg]

On Fri, Dec 14, 2012 at 04:54:32PM -0800, Eric Anholt wrote:
> Kristian Høgsberg <krh at bitplanet.net> writes:
> 
> > ---
> >  src/egl/drivers/dri2/egl_dri2.h     |    1 +
> >  src/egl/drivers/dri2/platform_drm.c |   23 +++++++++++++++++++++++
> >  2 files changed, 24 insertions(+)
> >
> > diff --git a/src/egl/drivers/dri2/egl_dri2.h b/src/egl/drivers/dri2/egl_dri2.h
> > index be359d3..fa2a9f3 100644
> > --- a/src/egl/drivers/dri2/egl_dri2.h
> > +++ b/src/egl/drivers/dri2/egl_dri2.h
> > @@ -189,6 +189,7 @@ struct dri2_egl_surface
> >     struct {
> >        struct gbm_bo       *bo;
> >        int                  locked;
> > +      int                  age;
> >     } color_buffers[3], *back, *current;
> >  #ifndef HAVE_WAYLAND_PLATFORM
> >     __DRIbuffer           *dri_buffers[__DRI_BUFFER_COUNT];
> > diff --git a/src/egl/drivers/dri2/platform_drm.c b/src/egl/drivers/dri2/platform_drm.c
> > index 3e04a6c..1923033 100644
> > --- a/src/egl/drivers/dri2/platform_drm.c
> > +++ b/src/egl/drivers/dri2/platform_drm.c
> > @@ -324,11 +324,16 @@ dri2_swap_buffers(_EGLDriver *drv, _EGLDisplay *disp, _EGLSurface *draw)
> >  {
> >     struct dri2_egl_display *dri2_dpy = dri2_egl_display(disp);
> >     struct dri2_egl_surface *dri2_surf = dri2_egl_surface(draw);
> > +   int i;
> >  
> >     if (dri2_surf->base.Type == EGL_WINDOW_BIT) {
> >        if (dri2_surf->current)
> >  	 _eglError(EGL_BAD_SURFACE, "dri2_swap_buffers");
> > +      for (i = 0; i < ARRAY_SIZE(dri2_surf->color_buffers); i++)
> > +         if (dri2_surf->color_buffers[i].bo)
> > +            dri2_surf->color_buffers[i].age++;
> >        dri2_surf->current = dri2_surf->back;
> > +      dri2_surf->current->age = 1;
> >        dri2_surf->back = NULL;
> 
> OK, if I'm following this right, the ages of my buffers with this code are:

You're missing the if (dri2_surf->color_buffers[i].bo) condition on
incrementing age.  We start out with bo = NULL for color_buffer[0..2]
and allocate the bos on demand...

> entering my first swap
> buffers[0] = 0 <- front
> buffers[1] = 0 <- back
> 
> after first swap:
> buffers[0] = 1 <- back
> buffers[1] = 1 <- front

and so here back age will be 0 because we pick a new color_buffer from
the pool that doesn't yet have a bo and whose age was never incremented.

> after second swap:
> buffers[0] = 1 <- front
> buffers[1] = 2 <- back
> 
> But reading the spec, I think it's supposed to be:
> 
> entering my first swap
> buffers[0] = 0 <- front
> buffers[1] = 0 <- back
> 
> after first swap:
> buffers[0] = 0 <- back
> buffers[1] = 1 <- front
> 
> after second swap:
> buffers[0] = 1 <- front
> buffers[1] = 2 <- back
> 
> Note how after the first swap, my backbuffer should have an age of 0, for
> "unknown junk".

Yup, and that's what this patch gets.

Kristian