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

[Jason Ekstrand]

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;
+
    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