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

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

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.

-- 
Ville Syrjälä
Intel OTC
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