[Intel-gfx] [RFC] drm/i915: Framebuffers need not be limited to 256MB on gen8+.

[Damien Lespiau]

On Thu, Feb 26, 2015 at 04:01:32PM +0000, Chris Wilson wrote:
> On Thu, Feb 26, 2015 at 09:01:52PM +0530, deepak.s at linux.intel.com wrote:
> > From: Deepak S <deepak.s at linux.intel.com>
> > 
> > The restriction of pinningFramebuffer to first 256MB is removed from gen8+.
> > Removing the restriction so that FB can be pinned in any space within
> > GTT/PPGTT. Also, for gen8+ no need to use pin_mappable for Framebuffer &
> > also we do not take fence as Framebuffer compression is not enabled.
> 
> Not quite, the reason why we restrict the framebuffer to mappable was
> that certain display servers depend upon being able to GTT mmap the
> framebuffer for direct access. Thus they would crash if we found that the
> framebuffer was already pinned in an unreachable location. That legacy
> code is currently still able to run on gen8+ and so you need to have
> userspace request an ABI break...

Also, if we do that, we need to make sure we can't bind the buffer at
the top of GGTT as we need to be able to prefetch 128 PTEs past the fb?
(on SKL 136, "The end of the surface cannot be within 136 PTEs of the
end of the graphics memory").

-- 
Damien