[Mesa-dev] [PATCH 01/15] i965/msaa: Expand odd-sized MSAA surfaces to account for interleaving pattern.

[Eric Anholt]

On Fri, 11 May 2012 11:03:44 -0700, Paul Berry <stereotype441 at gmail.com> wrote:
> Gen6 MSAA buffers (and Gen7 MSAA depth/stencil buffers) interleave
> MSAA samples in a complex pattern that repeats every 2x2 pixel block.
> Therefore, when allocating an MSAA buffer, we need to make sure to
> allocate an integer number of 2x2 blocks; if we don't, then some of
> the samples in the last row and column will be cut off.
> 
> Fixes piglit tests "EXT_framebuffer_multisample/unaligned-blit {2,4}
> color msaa" on i965/Gen6.

Given that you had to go through this two with us, I think the
following comment would help here:

/* In the Sandy Bridge PRM, volume 4, part 1, page 31, it says:
 *
 *     "Any of the other messages (sample*, LOD, load4) used with a
 *      (4x) multisampled surface will in-effect sample a surface with
 *      double the height and width as that indicated in the surface
 *      state. Each pixel position on the original-sized surface is
 *      replaced with a 2x2 of samples with the following arrangement:
 *
 *         sample 0 sample 2
 *         sample 1 sample 3"
 *
 * You would read this as saying that the layout in memory of a 2x2
 * pixel area for (x,y,sample) is:
 *
 *      (0,0,0) (0,0,2)   (1,0,0) (1,0,2)
 *      (0,0,1) (0,0,3)   (1,0,1) (1,0,3)
 *
 *      (0,1,0) (0,1,2)   (1,1,0) (1,1,2)
 *      (0,1,1) (0,1,3)   (1,1,1) (1,1,3)
 *
 * That is a lie.  The actual layout is:
 *
 *      (0,0,0) (1,0,0)   (0,0,2) (1,0,2)
 *      (0,1,0) (1,1,0)   (0,1,2) (1,1,2)
 *
 *      (0,0,1) (1,0,1)   (0,0,3) (1,0,3)
 *      (0,1,1) (1,1,1)   (0,1,3) (1,1,3)
 *
 * As a result, when calculating the size of our 4-sample buffer for
 * an odd width or height, we have to align before scaling up because
 * sample 3 is in that bottom right 2x2 block.
 */

I may have swapped pixels (0,1) and (1,0)'s position in the "actual
layout" blocks, though.
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