[Mesa-dev] [PATCH] i965: Add a better PRM citation for the IMS dimension mangling.

[Kenneth Graunke]

Paul originally had to reverse engineer these formulas based on the
description about how the sampler works.  The description here is not
the easiest to follow - especially given that it's from the Sandybridge
era, when the hardware only did 4x multisampling.

Jordan and I recently found another part of the documentation where they
simply state that IMS dimensions must be adjusted by a set of formulas.
Quoting this section provides an easy to follow explanation for the
code, including 2x/4x/8x/16x.

Signed-off-by: Kenneth Graunke <kenneth at whitecape.org>
---
 src/mesa/drivers/dri/i965/intel_mipmap_tree.c | 23 ++++++++++++++++++++++-
 1 file changed, 22 insertions(+), 1 deletion(-)

diff --git a/src/mesa/drivers/dri/i965/intel_mipmap_tree.c b/src/mesa/drivers/dri/i965/intel_mipmap_tree.c
index 26e137d..9e79f6e 100644
--- a/src/mesa/drivers/dri/i965/intel_mipmap_tree.c
+++ b/src/mesa/drivers/dri/i965/intel_mipmap_tree.c
@@ -290,7 +290,28 @@ intel_miptree_create_layout(struct brw_context *brw,
       /* Adjust width/height/depth for MSAA */
       mt->msaa_layout = compute_msaa_layout(brw, format, mt->target);
       if (mt->msaa_layout == INTEL_MSAA_LAYOUT_IMS) {
-         /* In the Sandy Bridge PRM, volume 4, part 1, page 31, it says:
+         /* From the Ivybridge PRM, Volume 1, Part 1, page 108:
+          * "If the surface is multisampled and it is a depth or stencil
+          *  surface or Multisampled Surface StorageFormat in SURFACE_STATE is
+          *  MSFMT_DEPTH_STENCIL, WL and HL must be adjusted as follows before
+          *  proceeding:
+          *
+          *  +----------------------------------------------------------------+
+          *  | Num Multisamples |        W_l =         |        H_l =         |
+          *  +----------------------------------------------------------------+
+          *  |         2        | ceiling(W_l / 2) * 4 | H_l (no adjustment)  |
+          *  |         4        | ceiling(W_l / 2) * 4 | ceiling(H_l / 2) * 4 |
+          *  |         8        | ceiling(W_l / 2) * 8 | ceiling(H_l / 2) * 4 |
+          *  |        16        | ceiling(W_l / 2) * 8 | ceiling(H_l / 2) * 8 |
+          *  +----------------------------------------------------------------+
+          * "
+          *
+          * Note that MSFMT_DEPTH_STENCIL just means the IMS (interleaved)
+          * format rather than UMS/CMS (array slices).  The Sandybridge PRM,
+          * Volume 1, Part 1, Page 111 has the same formula for 4x MSAA.
+          *
+          * Another more complicated explanation for these adjustments comes
+          * from the Sandybridge PRM, volume 4, part 1, page 31:
           *
           *     "Any of the other messages (sample*, LOD, load4) used with a
           *      (4x) multisampled surface will in-effect sample a surface with
-- 
2.2.2