[Mesa-dev] [PATCH] intel/isl: Tighten up restrictions for CCS on gen7

[Jason Ekstrand]

On Mon, Jul 31, 2017 at 12:11 PM, Nanley Chery <nanleychery at gmail.com>
wrote:

> On Sat, Jul 22, 2017 at 04:54:24PM -0700, Jason Ekstrand wrote:
> > It may technically be possible to enable some sort of fast-clear support
> > for at least the base slice of a 2D array texture on gen7.  However,
> > it's not documented to work, we've never tried to do it in GL, and we
> > have no idea what the hardware does if you turn on CCS_D with arrayed
> > rendering.  Let's just play it safe and disallow it for now.  If someone
> > really cares that much about gen7 performance, they can come along and
> > try to get it working later.
> > ---
> >  src/intel/isl/isl.c | 34 ++++++++++++++++++++++++----------
> >  1 file changed, 24 insertions(+), 10 deletions(-)
> >
> > diff --git a/src/intel/isl/isl.c b/src/intel/isl/isl.c
> > index 9cf5821..5465496 100644
> > --- a/src/intel/isl/isl.c
> > +++ b/src/intel/isl/isl.c
> > @@ -1749,9 +1749,30 @@ isl_surf_get_ccs_surf(const struct isl_device
> *dev,
> >     if (surf->usage & ISL_SURF_USAGE_DISABLE_AUX_BIT)
> >        return false;
> >
> > +   /* The PRM doesn't say this explicitly, but fast-clears don't appear
> to
> > +    * work for 3D textures until gen9 where the layout of 3D textures
> changes
> > +    * to match 2D array textures.
> > +    */
> >     if (ISL_DEV_GEN(dev) <= 8 && surf->dim != ISL_SURF_DIM_2D)
> >        return false;
> >
> > +   /* From the HSW PRM Volume 7: 3D-Media-GPGPU, page 652 (Color Clear
> of
> > +    * Non-MultiSampler Render Target Restrictions):
> > +    *
> > +    *    "Support is for non-mip-mapped and non-array surface types
> only."
> > +    *
> > +    * This restriction is lifted on gen8+.  Technically, it may be
> possible to
> > +    * create a CCS for an arrayed or mipmapped image and only enable
> CCS_D
> > +    * when rendering to the base slice.  However, there is no
> documentation
> > +    * tell us what the hardware would do in that case or what it does
> if you
> > +    * walk off the bases slice.  (Does it ignore CCS or does it start
> > +    * scribbling over random memory?)  We play it safe and just follow
> the
> > +    * docs and don't allow CCS_D for arrayed or mip-mapped surfaces.
> > +    */
> > +   if (ISL_DEV_GEN(dev) <= 7 &&
> > +       (surf->levels > 1 || surf->logical_level0_px.array_len > 1))
> > +      return false;
> > +
>
> Why are mipmapped surfaces unsafe? A user is restricted to rendering
> into one miplevel at a time.
>

Same two reasons:

 1) The docs say it doesn't work.
 2) The miptree layout may be complene nonsense when it comes to the CCS.
On gen8+, the surface is re-laid-out for the auxiliary surface but, on
gen7, they try to do a direct translation of x/y coordinates in the main
surface to x/y in the CCS.

--Jason


> -Nanley
>
> >     if (isl_format_is_compressed(surf->format))
> >        return false;
> >
> > @@ -1789,21 +1810,14 @@ isl_surf_get_ccs_surf(const struct isl_device
> *dev,
> >        return false;
> >     }
> >
> > -   /* Multi-LOD and multi-layer CCS isn't supported on gen7. */
> > -   const uint8_t levels = ISL_DEV_GEN(dev) <= 7 ? 1 : surf->levels;
> > -   const uint32_t array_len = ISL_DEV_GEN(dev) <= 7 ?
> > -                              1 : surf->logical_level0_px.array_len;
> > -   const uint32_t depth = ISL_DEV_GEN(dev) <= 7 ?
> > -                          1 : surf->logical_level0_px.depth;
> > -
> >     return isl_surf_init(dev, ccs_surf,
> >                          .dim = surf->dim,
> >                          .format = ccs_format,
> >                          .width = surf->logical_level0_px.width,
> >                          .height = surf->logical_level0_px.height,
> > -                        .depth = depth,
> > -                        .levels = levels,
> > -                        .array_len = array_len,
> > +                        .depth = surf->logical_level0_px.depth,
> > +                        .levels = surf->levels,
> > +                        .array_len = surf->logical_level0_px.array_len,
> >                          .samples = 1,
> >                          .row_pitch = row_pitch,
> >                          .usage = ISL_SURF_USAGE_CCS_BIT,
> > --
> > 2.5.0.400.gff86faf
> >
> > _______________________________________________
> > mesa-dev mailing list
> > mesa-dev at lists.freedesktop.org
> > https://lists.freedesktop.org/mailman/listinfo/mesa-dev
>
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