[Mesa-dev] [PATCH] i965: Fix vertical alignment for multisampled buffers.

[Paul Berry]

>From the Sandy Bridge PRM, Vol 1 Part 1 7.18.3.4 (Alignment Unit
Size):

    j [vertical alignment] = 4 for any render target surface is
    multisampled (4x)

>From the Ivy Bridge PRM, Vol 4 Part 1 2.12.2.1 (SURFACE_STATE for most
messages), under the "Surface Vertical Alignment" heading:

    This field is intended to be set to VALIGN_4 if the surface was
    rendered as a depth buffer, for a multisampled (4x) render target,
    or for a multisampled (8x) render target, since these surfaces
    support only alignment of 4.

Back in 2012 when we added multisampling support to the i965 driver,
we forgot to update the logic for computing the vertical alignment, so
we were often using a vertical alignment of 2 for multisampled
buffers, leading to subtle rendering errors.

Note that the specs also require a vertical alignment of 4 for all
Y-tiled render target surfaces; I plan to address that in a separate
patch.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=53077
---
 src/mesa/drivers/dri/i965/brw_tex_layout.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/src/mesa/drivers/dri/i965/brw_tex_layout.c b/src/mesa/drivers/dri/i965/brw_tex_layout.c
index d912862..d05dbeb 100644
--- a/src/mesa/drivers/dri/i965/brw_tex_layout.c
+++ b/src/mesa/drivers/dri/i965/brw_tex_layout.c
@@ -86,7 +86,7 @@ intel_horizontal_texture_alignment_unit(struct brw_context *brw,
 
 static unsigned int
 intel_vertical_texture_alignment_unit(struct brw_context *brw,
-                                     gl_format format)
+                                      gl_format format, bool multisampled)
 {
    /**
     * From the "Alignment Unit Size" section of various specs, namely:
@@ -110,8 +110,6 @@ intel_vertical_texture_alignment_unit(struct brw_context *brw,
     *
     * On SNB+, non-special cases can be overridden by setting the SURFACE_STATE
     * "Surface Vertical Alignment" field to VALIGN_2 or VALIGN_4.
-    *
-    * We currently don't support multisampling.
     */
    if (_mesa_is_format_compressed(format))
       return 4;
@@ -119,6 +117,9 @@ intel_vertical_texture_alignment_unit(struct brw_context *brw,
    if (format == MESA_FORMAT_S8)
       return brw->gen >= 7 ? 8 : 4;
 
+   if (multisampled)
+      return 4;
+
    GLenum base_format = _mesa_get_format_base_format(format);
 
    if (brw->gen >= 6 &&
@@ -276,8 +277,10 @@ brw_miptree_layout_texture_3d(struct brw_context *brw,
 void
 brw_miptree_layout(struct brw_context *brw, struct intel_mipmap_tree *mt)
 {
+   bool multisampled = mt->num_samples > 1;
    mt->align_w = intel_horizontal_texture_alignment_unit(brw, mt->format);
-   mt->align_h = intel_vertical_texture_alignment_unit(brw, mt->format);
+   mt->align_h =
+      intel_vertical_texture_alignment_unit(brw, mt->format, multisampled);
 
    switch (mt->target) {
    case GL_TEXTURE_CUBE_MAP:
-- 
1.8.4.2