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
---
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
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