[Mesa-dev] [PATCH 6/6] intel/isl: Add a detailed comment about multisampling with HiZ

[Chad Versace]

On Fri 02 Sep 2016, Jason Ekstrand wrote:
> ---
>  src/intel/isl/isl.c | 60 +++++++++++++++++++++++++++++++++++++++++++++++++++--
>  1 file changed, 58 insertions(+), 2 deletions(-)
> 
> diff --git a/src/intel/isl/isl.c b/src/intel/isl/isl.c
> index 33e83b1..cb9eb23 100644
> --- a/src/intel/isl/isl.c
> +++ b/src/intel/isl/isl.c
> @@ -1284,6 +1284,63 @@ isl_surf_get_hiz_surf(const struct isl_device *dev,
>     assert(surf->msaa_layout == ISL_MSAA_LAYOUT_NONE ||
>            surf->msaa_layout == ISL_MSAA_LAYOUT_INTERLEAVED);
>  
> +   /* From the Broadwell PRM Vol. 7, "Hierarchical Depth Buffer":
> +    *
> +    *    "The Surface Type, Height, Width, Depth, Minimum Array Element, Render
> +    *    Target View Extent, and Depth Coordinate Offset X/Y of the
> +    *    hierarchical depth buffer are inherited from the depth buffer. The
> +    *    height and width of the hierarchical depth buffer that must be
> +    *    allocated are computed by the following formulas, where HZ is the
> +    *    hierarchical depth buffer and Z is the depth buffer. The Z_Height,
> +    *    Z_Width, and Z_Depth values given in these formulas are those present
> +    *    in 3DSTATE_DEPTH_BUFFER incremented by one.
> +    *
> +    *    "The value of Z_Height and Z_Width must each be multiplied by 2 before
> +    *    being applied to the table below if Number of Multisamples is set to
> +    *    NUMSAMPLES_4. The value of Z_Height must be multiplied by 2 and
> +    *    Z_Width must be multiplied by 4 before being applied to the table
> +    *    below if Number of Multisamples is set to NUMSAMPLES_8."
> +    *
> +    * In the Sky Lake PRM, the second paragraph is replaced with this:
> +    *
> +    *    "The Z_Height and Z_Width values must equal those present in
> +    *    3DSTATE_DEPTH_BUFFER incremented by one."
> +    *
> +    * In other words, on Sandy Bridge through Broadwell, each 128-bit HiX

Typo  s/HiX/HiZ/

> +    * block corresponds to a region of 8x4 samples in the primary depth
> +    * surface.  On Sky Lake, on the other hand, each HiZ block corresponds to
> +    * a region of 8x4 pixels in the primary depth surface regarldess of the

Typo                                                         ^^^^^^^^^^

> +    * number of samples.  The dimensions of a HiZ block in both pixes and

Typo                                                               ^^^^^

> +    * samples are given in the table below:
> +    *
> +    *                    | SNB - BDW |     SKL+
> +    *              ------------------+-------------
> +    *                1x  |  8 x 4 sa |   8 x 4 sa
> +    *               MSAA |  8 x 4 px |   8 x 4 px
> +    *              ------------------+-------------
> +    *                2x  |  8 x 4 sa |  16 x 4 sa
> +    *               MSAA |  4 x 4 px |   8 x 4 px
> +    *              ------------------+-------------
> +    *                4x  |  8 x 4 sa |  16 x 8 sa
> +    *               MSAA |  4 x 2 px |   8 x 4 px
> +    *              ------------------+-------------
> +    *                8x  |  8 x 4 sa |  32 x 8 sa
> +    *               MSAA |  2 x 2 px |   8 x 4 px
> +    *              ------------------+-------------
> +    *               16x  |    N/A    | 32 x 16 sa
> +    *               MSAA |    N/A    |  8 x  4 px
> +    *              ------------------+-------------

THANK YOU THANK YOU THANK YOU.
You win all the cat gifs [https://s-media-cache-ak0.pinimg.com/originals/34/71/dc/3471dc5d6443b2af5476ad3dbbca9d7c.jpg]

> +    *
> +    * There are a number of different ways that this discrepency could be
> +    * handled.  The way we have chosen is to simply make MSAA HiZ have the
> +    * same number of samples as the parent surface pre-Sky Lake and always be
> +    * single-sampled on Sky Lake and above.  Since the block sizes of
> +    * compressed formats are given in samples, this neatly handles everything
> +    * without the need for additional HiZ formats with different block sizes
> +    * on SKL+.
> +    */

This makes perfect sense.

> +   const unsigned samples = ISL_DEV_GEN(dev) >= 9 ? 1 : surf->samples;
> +
>     isl_surf_init(dev, hiz_surf,
>                   .dim = ISL_SURF_DIM_2D,
>                   .format = ISL_FORMAT_HIZ,
> @@ -1292,8 +1349,7 @@ isl_surf_get_hiz_surf(const struct isl_device *dev,
>                   .depth = 1,
>                   .levels = surf->levels,
>                   .array_len = surf->logical_level0_px.array_len,
> -                 /* On SKL+, HiZ is always single-sampled */
> -                 .samples = ISL_DEV_GEN(dev) >= 9 ? 1 : surf->samples,
> +                 .samples = samples,
>                   .usage = ISL_SURF_USAGE_HIZ_BIT,
>                   .tiling_flags = ISL_TILING_HIZ_BIT);
>  }
> -- 

Reviewed-by: Chad Versace <chadversary at chromium.org>