mesa, intel driver, and synchronous flipping

garrone pgarrone at
Fri Dec 19 16:29:13 PST 2008

On Wed, Dec 17, 2008 at 10:59:08AM +0100, Michel Dänzer wrote:
> On Wed, 2008-12-17 at 18:11 +1100, garrone wrote:
> > The mesa direct rendering with the intel driver appears to rely on
> > asychronous flipping. However the hardware, according to the 965
> > documentation I have downloaded from the intellinuxgraphics site, is
> > capable of synchronous flips. 
> > 
> > (The kernel file i915_dma.c ors in the "ASYNC_FLIP" constant in the function
> > i915_dispatch_flip, invoked eventually by a call to glXSwapBuffers.
> > For a description of the hardware capability, 
> > refer to volume 1 of the 965 graphics pdf files, searching for the
> > command MI_DISPLAY_FLIP)
> > 
> > The application I am targeting has dual opengl windows, 
> > each one running on a separate display, and covering the whole of that
> > display, from a single Intel graphics chipset.
> This was working with synchronous flips at some point at least on
> i915/i945 hardware, but I think the functionality has been ripped out
> again because GEM didn't mix well with it.

Thanks. I'm gradually following the pathway through the source. My
original post was mistaken, because the function
i915_dispatch_flip is in fact never called as part of the vertical blank
cycle, on my setup anyway. So in fact no flipping, synchronous or asynchronous
occurs, rather there is a buffer copy operation and then the opengl front 
buffer is scanned directly, in effect. And this works, for a single window on
a single display.

If you have multiple displays though, there is a problem. I wonder why
GEM doesn't mix well with page flipping or triplebuffer. I suppose it
allows graphics memory to be backed by swap.

Peter Garrone

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