[Mesa-dev] [PATCH 05/29] intel/isl: Add a helper for inverting swizzles

[Pohjolainen, Topi]

On Mon, Feb 26, 2018 at 08:42:42AM -0800, Jason Ekstrand wrote:
> On Mon, Feb 26, 2018 at 6:19 AM, Pohjolainen, Topi <
> topi.pohjolainen at gmail.com> wrote:
> 
> > On Fri, Jan 26, 2018 at 05:59:34PM -0800, Jason Ekstrand wrote:
> > > ---
> > >  src/intel/isl/isl.c | 30 ++++++++++++++++++++++++++++++
> > >  src/intel/isl/isl.h |  2 ++
> > >  2 files changed, 32 insertions(+)
> > >
> > > diff --git a/src/intel/isl/isl.c b/src/intel/isl/isl.c
> > > index a2d3ae6..420d387 100644
> > > --- a/src/intel/isl/isl.c
> > > +++ b/src/intel/isl/isl.c
> > > @@ -2379,3 +2379,33 @@ isl_swizzle_compose(struct isl_swizzle first,
> > struct isl_swizzle second)
> > >        .a = swizzle_select(first.a, second),
> > >     };
> > >  }
> > > +
> > > +/**
> > > + * Returns a swizzle that is the pseudo-inverse of this swizzle.
> > > + */
> > > +struct isl_swizzle
> > > +isl_swizzle_invert(struct isl_swizzle swizzle)
> > > +{
> > > +   /* Default to zero for channels which do not show up in the swizzle
> > */
> > > +   enum isl_channel_select chans[4] = {
> > > +      ISL_CHANNEL_SELECT_ZERO,
> > > +      ISL_CHANNEL_SELECT_ZERO,
> > > +      ISL_CHANNEL_SELECT_ZERO,
> > > +      ISL_CHANNEL_SELECT_ZERO,
> > > +   };
> > > +
> > > +   /* We go in ABGR order so that, if there are any duplicates, the
> > first one
> > > +    * is taken if you look at it in RGBA order.  This is what Haswell
> > hardware
> > > +    * does for render target swizzles.
> > > +    */
> > > +   if ((unsigned)(swizzle.a - ISL_CHANNEL_SELECT_RED) < 4)
> > > +      chans[swizzle.a - ISL_CHANNEL_SELECT_RED] =
> > ISL_CHANNEL_SELECT_ALPHA;
> > > +   if ((unsigned)(swizzle.b - ISL_CHANNEL_SELECT_RED) < 4)
> > > +      chans[swizzle.b - ISL_CHANNEL_SELECT_RED] =
> > ISL_CHANNEL_SELECT_BLUE;
> > > +   if ((unsigned)(swizzle.g - ISL_CHANNEL_SELECT_RED) < 4)
> > > +      chans[swizzle.g - ISL_CHANNEL_SELECT_RED] =
> > ISL_CHANNEL_SELECT_GREEN;
> > > +   if ((unsigned)(swizzle.r - ISL_CHANNEL_SELECT_RED) < 4)
> > > +      chans[swizzle.r - ISL_CHANNEL_SELECT_RED] =
> > ISL_CHANNEL_SELECT_RED;
> > > +
> > > +   return (struct isl_swizzle) { chans[0], chans[1], chans[2], chans[3]
> > };
> >
> > If given
> >
> >     swizzle == { ISL_CHANNEL_SELECT_RED,
> >                  ISL_CHANNEL_SELECT_GREEN,
> >                  ISL_CHANNEL_SELECT_BLUE,
> >                  ISL_CHANNEL_SELECT_ALPHA },
> >
> > then
> >     chans[ISL_CHANNEL_SELECT_ALPHA - ISL_CHANNEL_SELECT_RED] == chans[3] ==
> >     ISL_CHANNEL_SELECT_ALPHA
> >
> > and so on, and the function returns the same swizzle as given?
> 
> 
> Yes, that is how the subtraction works.

I was expecting it to "invert" that, i.e., to return ABGR. But okay, if given
identity swizzle it returns identity.

In order to understand how it works I thought I read further the series to
find an example - there seems to be one in patch 12 and another in patch 16.
In case of 16 and destination format B4G4R4A4 the swizzle looks to be BGRA
(looking at anv_formats.c::main_formats).

In that case we get:

   chans[ALPHA - RED] = chans[3] = ALPHA
   chans[RED   - RED] = chans[0] = BLUE
   chans[GREEN - RED] = chans[1] = GREEN
   chans[BLUE  - RED] = chans[2] = RED

and as a swizzle BLUE, GREEN, RED, ALPHA. This is again the same as given.
What am I not understanding?