Re: [PATCH 1/8] drm/i915: Add a comment exlaining CCS hsub/vsub

[Jason Ekstrand <jason@jlekstrand.net>]

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On Wed, Jan 10, 2018 at 9:48 AM, Ville Syrjälä <
ville.syrjala@linux.intel.com> wrote:

> On Wed, Jan 10, 2018 at 09:03:14AM -0800, Jason Ekstrand wrote:
> > On Fri, Dec 22, 2017 at 11:22 AM, Ville Syrjala <
> > ville.syrjala@linux.intel.com> wrote:
> >
> > > From: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > >
> > > Let's document why we claim hsub==8,vsub==16 for CCS even though the
> > > memory layout would suggest that we use 16x8 instead.
> > >
> > > Cc: Daniel Vetter <daniel@ffwll.ch>
> > > Cc: Ben Widawsky <ben@bwidawsk.net>
> > > Cc: Jason Ekstrand <jason@jlekstrand.net>
> > > Cc: Daniel Stone <daniels@collabora.com>
> > > Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > > ---
> > >  drivers/gpu/drm/i915/intel_display.c | 7 +++++++
> > >  1 file changed, 7 insertions(+)
> > >
> > > diff --git a/drivers/gpu/drm/i915/intel_display.c
> > > b/drivers/gpu/drm/i915/intel_display.c
> > > index 0cd355978ab4..83aec68537b4 100644
> > > --- a/drivers/gpu/drm/i915/intel_display.c
> > > +++ b/drivers/gpu/drm/i915/intel_display.c
> > > @@ -2387,6 +2387,13 @@ static unsigned int intel_fb_modifier_to_tiling(
> uint64_t
> > > fb_modifier)
> > >         }
> > >  }
> > >
> > > +/*
> > > + * 1 byte of CCS actually corresponds to 16x8 pixels on the main
> > > + * surface, and the memory layout for the CCS tile is 64x64 bytes.
> > > + * But since we're pretending the CCS tile is 128 bytes wide we
> > > + * adjust hsub/vsub here accordingly to 8x16 so that the
> > > + * bytes<->x/y conversions come out correct.
> > >
> >
> > I'm not particularly happy with this comment as I think it pushes the
> > mental model for these calculations in the wrong direction.  The PRM
> says:
> >
> > The Color Control Surface (CCS) contains the compression status of the
> > cache-line pairs. The
> > compression state of the cache-line pair is specified by 2 bits in the
> CCS.
> > Each CCS cache-line represents
> > an area on the main surface of 16 x16 sets of 128 byte Y-tiled
> > cache-line-pairs. CCS is always Y tiled.
> >
> > If you understand that a "cache line pair" in the main surface is a
> > horizontally adjacent cache line pair (cl1_addr = cl0_addr + 512) and you
> > just accept the statement about Y-tiling, this is the correct
> calculation.
> > Calculating these things in terms of pixels is occasionally useful but is
> > the wrong mental model.  The cache line statement above both accurately
> > describes the layout of the CCS (at the cache line granularity) and
> scales
> > to other pixel formats which are not 32-bit.
> >
> > I know that Ville and I have disagreed on this in the past but I don't
> > think adding comments about how we're "pretending the CCS tile is 128
> bytes
> > wide" is making anything more clear.
>
> I don't really see how talk about cachelines is going to help explain
> the hsub/vsub values we use here.
>
> But I don't really care that much. We could just leave them as magic
> numbers if no one can come up with a clear explanation for them.
>

How about a comment like this:

/*From the Sky Lake PRM:
 *
 *    The Color Control Surface (CCS) contains the compression status of
the cache-line pairs. The
 *    compression state of the cache-line pair is specified by 2 bits in
the CCS. Each CCS cache-line represents
 *    an area on the main surface of 16 x16 sets of 128 byte Y-tiled
cache-line-pairs. CCS is always Y tiled.
 *
 * Since cache line pairs refers to horizontally adjacent cache lines, each
cache line in the CCS
 * corresponds to an area of 32x16 cache lines on the main surface.  Since
each pixel is 4 bytes,
 * this gives us a ratio of one byte in the CCS for each 8x16 pixels in the
main surface.
 */

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