[Mesa-dev] [PATCH V3 15/19] i965: take the target into account for Gen7 MSAA modes

[Chris Forbes]

Gen7 has an erratum affecting the ld_mcs message, making it unsafe to
use when the surface doesn't have an associated MCS.

>From the Ivy Bridge PRM, Vol4 Part1 p77 ("MCS Enable"):

   "If this field is disabled and the sampling engine <ld_mcs>
   message is issued on this surface, the MCS surface may be
   accessed. Software must ensure that the surface is defined
   to avoid GTT errors."

To allow the shader to treat all surfaces uniformly, force UMS if the
surface is to be used as a multisample texture, even if CMS would have
been possible.

V3: - Quoted erratum text

Signed-off-by: Chris Forbes <chrisf at ijw.co.nz>
---
 src/mesa/drivers/dri/intel/intel_mipmap_tree.c | 22 +++++++++++++++++++---
 1 file changed, 19 insertions(+), 3 deletions(-)

diff --git a/src/mesa/drivers/dri/intel/intel_mipmap_tree.c b/src/mesa/drivers/dri/intel/intel_mipmap_tree.c
index 139bf0e..e22c7a0 100644
--- a/src/mesa/drivers/dri/intel/intel_mipmap_tree.c
+++ b/src/mesa/drivers/dri/intel/intel_mipmap_tree.c
@@ -72,7 +72,7 @@ target_to_target(GLenum target)
  * created, based on the chip generation and the surface type.
  */
 static enum intel_msaa_layout
-compute_msaa_layout(struct intel_context *intel, gl_format format)
+compute_msaa_layout(struct intel_context *intel, gl_format format, GLenum target)
 {
    /* Prior to Gen7, all MSAA surfaces used IMS layout. */
    if (intel->gen < 7)
@@ -101,7 +101,23 @@ compute_msaa_layout(struct intel_context *intel, gl_format format)
          assert(intel->gen == 7);
          return INTEL_MSAA_LAYOUT_UMS;
       } else {
-         return INTEL_MSAA_LAYOUT_CMS;
+         /* For now, if we're going to be texturing from this surface,
+          * force UMS, so that the shader doesn't have to do different things
+          * based on whether there's a multisample control surface needing sampled first.
+          * We can't just blindly read the MCS surface in all cases because:
+          *
+          * From the Ivy Bridge PRM, Vol4 Part1 p77 ("MCS Enable"):
+          *
+          *    If this field is disabled and the sampling engine <ld_mcs> message
+          *    is issued on this surface, the MCS surface may be accessed. Software
+          *    must ensure that the surface is defined to avoid GTT errors.
+          */
+         if (target == GL_TEXTURE_2D_MULTISAMPLE ||
+               target == GL_TEXTURE_2D_MULTISAMPLE_ARRAY) {
+            return INTEL_MSAA_LAYOUT_UMS;
+         } else {
+            return INTEL_MSAA_LAYOUT_CMS;
+         }
       }
    }
 }
@@ -150,7 +166,7 @@ intel_miptree_create_layout(struct intel_context *intel,
 
    if (num_samples > 1) {
       /* Adjust width/height/depth for MSAA */
-      mt->msaa_layout = compute_msaa_layout(intel, format);
+      mt->msaa_layout = compute_msaa_layout(intel, format, mt->target);
       if (mt->msaa_layout == INTEL_MSAA_LAYOUT_IMS) {
          /* In the Sandy Bridge PRM, volume 4, part 1, page 31, it says:
           *
-- 
1.8.1.2