Re: [PATCH] i915, drm/fourcc: Improve the CCS modifier documentation

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Fri, Aug 18, 2017 at 11:34:40AM -0700, Jason Ekstrand wrote:
> This updates the documentation on the intel CCS modifiers for a couple
> of reasons:
> 
>  1) The old documentation required that the CCS modifier only be used
>     with 8888 formats.  While i915 currently only supports CCS scanout
>     with 8888 formats (and advertises such through the blob format), CCS
>     can be used with many other formats.  Mesa, in particular, can
>     handle CCS on the full range of formats supported by the hardware.
>     For image sharing entirely within userspace, we don't want this
>     restriction.
> 
>  2) The old documentation specified the scaling factor in terms of
>     pixels which breaks down when you start using formats which are not
>     32-bit.  By specifying it in terms of cache lines and tiles, we can
>     properly specify the scale-down relationship with no format size
>     assumptions.
> 
>  3) The new comment provides more detail about the "real" layout of CCS
>     on Sky Lake and also points out that the reason why Y tiling is
>     important is because it affects row pitch calculations.
> 
>  4) We shouldn't be documenting the Yf CCS modifier yet.  Userspace is
>     incapable of generating it right now and we don't fully know how it
>     works yet.  Trying to fully describe it is premature.
> 
> Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
> Cc: Ben Widawsky <ben@bwidawsk.net>
> Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
> ---
>  include/uapi/drm/drm_fourcc.h | 35 ++++++++++++++++++++++-------------
>  1 file changed, 22 insertions(+), 13 deletions(-)
> 
> diff --git a/include/uapi/drm/drm_fourcc.h b/include/uapi/drm/drm_fourcc.h
> index 3ad838d..9670da4 100644
> --- a/include/uapi/drm/drm_fourcc.h
> +++ b/include/uapi/drm/drm_fourcc.h
> @@ -266,21 +266,30 @@ extern "C" {
>  /*
>   * Intel color control surface (CCS) for render compression
>   *
> - * The framebuffer format must be one of the 8:8:8:8 RGB formats.
> - * The main surface will be plane index 0 and must be Y/Yf-tiled,
> - * the CCS will be plane index 1.
> - *
> - * Each CCS tile matches a 1024x512 pixel area of the main surface.
> - * To match certain aspects of the 3D hardware the CCS is
> - * considered to be made up of normal 128Bx32 Y tiles, Thus
> - * the CCS pitch must be specified in multiples of 128 bytes.
> - *
> - * In reality the CCS tile appears to be a 64Bx64 Y tile, composed
> - * of QWORD (8 bytes) chunks instead of OWORD (16 bytes) chunks.
> - * But that fact is not relevant unless the memory is accessed
> - * directly.
> + * The image format must be compatible with CCS_E (lossless render
> + * compression) as specified in the PRM for the relevant hardware.
> + * The main surface will be plane index 0 and must be Y-tiled,
> + * the CCS will be plane index 1 and is also Y-tiled.
> + *
> + * Each 64B cache line in the CCS (a region of 16B x 4 rows when
> + * Y-tiled) corresponds to a region of 32x16 cache lines in the main
> + * surface.  (As a corollary, each CCS tile corresponds to an area of
> + * 32x16 tiles in the main surface.)  This relationship holds regardless
> + * of the size of the number of bits per pixel of the image format.
> + *
> + * In reality, the cache lines in the CCS tile are proportioned in an
> + * 8B x 8 row configuration with each byte being 2x2 2-bit CCS entries.
> + * However, CCS is documented as Y-tiled with the scaling relationship
> + * described in terms of cache lines as above so we consider it to be
> + * Y-tiled for the purpose of specifying this modifier.  This means that
> + * the row pitch for the CCS assumes 128B/tile.
>   */
>  #define I915_FORMAT_MOD_Y_TILED_CCS	fourcc_mod_code(INTEL, 4)
> +
> +/* Reserved for the Yf version of the TILED_CCS modifier.
> + *
> + * Exact definition TBD.
> + */

IIRC the same explanation can be used for both Y and Yf. The CCS
surface is still using the wonky Y tile layout even if the main
surface is Yf.

>  #define I915_FORMAT_MOD_Yf_TILED_CCS	fourcc_mod_code(INTEL, 5)
>  
>  /*
> -- 
> 2.5.0.400.gff86faf

-- 
Ville Syrjälä
Intel OTC
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