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

[Jason Ekstrand <jason@jlekstrand.net>]

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

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