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

[Jason Ekstrand]

On Mon, Aug 21, 2017 at 9:25 AM, Ben Widawsky <ben at bwidawsk.net> wrote:

> On 17-08-18 11:34:40, 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 at jlekstrand.net>
>> Cc: Ben Widawsky <ben at bwidawsk.net>
>> Cc: Ville Syrjälä <ville.syrjala at 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.
>>
>
> I guess if you're future proofing, you might want to bake in the language
> for
> planar formats.
>

What language are you looking for.  It's simply not allowed.  Maybe I
should say something like "the image format must non-planar and compatible
with CCS_E..."


> + *
>> + * 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)
>>
>
> Can we just rename this to RSVD, or does Mesa build already require this?
>

Nothing requires it today.  I'd be happy to just delete it TBH.

--Jason
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