[Bug 109985] [SNB] INTEL_DEBUG=nohiz is likely broken for depth/stencil buffers

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=109985

--- Comment #14 from Nanley Chery <nanleychery at gmail.com> ---
(In reply to Nanley Chery from comment #13)
> (In reply to asimiklit from comment #12)
> > (In reply to Nanley Chery from comment #0)
> > > For SNB, INTEL_DEBUG=nohiz seems to be broken in two places:
> > > 
> > > 1. In miptree_create() in intel_mipmap_tree.c. Trying to create a
> > > depth/stencil buffer will cause a depth miptree to be created without
> > > stencil.
> > 
> > As far as found this code disallows us to create combined depth/stencil on
> > SNB:
> >    mesa_format mt_fmt = format;
> >    if (!_mesa_is_format_color_format(format) && devinfo->gen >= 6) {
> >       /* Fix up the Z miptree format for how we're splitting out separate
> >        * stencil. Gen7 expects there to be no stencil bits in its depth
> > buffer.
> >        */
> >       mt_fmt = intel_depth_format_for_depthstencil_format(format);
> >    }
> > 
> >    Note: This code just replaces MESA_FORMAT_Z24_UNORM_S8_UINT by
> > MESA_FORMAT_Z24_UNORM_X8_UINT
> >          but in this case 'mt_surf_usage' does not return a
> > ISL_SURF_USAGE_STENCIL_BIT and that leads to
> >          an assertion: assert(src_surf->surf->usage &
> > ISL_SURF_USAGE_STENCIL_BIT); in a blorp_blit function.
> >          PS: Actually I didn't check if this assertion actually matter.
> > 
> > But just avoiding of this code doesn't make us happy too (
> > Because we fail a choosing of a surf tiling method and
> > "isl_surf_choose_tiling" always returns false.
> > 
> > Now I am investigates a SNB PRM for the valid combinations of tiling methods
> > with combined depth/stencil.
> > 
> > Any advices or suggestions are welcome :)
> > 
> 
> The cause of this problem is that we have two non-equivalent conditions for
> selecting the main miptree format and creating a separate stencil miptree:
> The condition you mentioned above and the call to needs_separate_stencil().
> One solution is to refactor and reuse needs_separate_stencil() for the
> format selection.
> 

Sorry, I only addressed the problem with avoiding the format selection code.

The tiling issue you've pointed out is a bug in isl_gen6_filter_tiling(). For
depth-stencil buffers on SNB, we have a Y-tiled surface with the elements laid
out as described in the "3D Depth Buffer Surfaces" section of the PRM.

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