[Mesa-dev] [RFC 1/6] dri: Support 64 bit rgba masks

[Adam Jackson]

On Wed, 2019-01-16 at 17:58 +0000, Strasser, Kevin wrote:
> Adam Jackson wrote:
> 
> > Probably what I'd do is only define those masks to non-zero for <=32bpp 
> > formats,
> 
> Not sure if I understood this statement. Do you mean if we keep the _HI and _LO
> attributes, only make them valid for formats that are <=64bpp, like fp16?

I was thinking:

a) Leave __DRI_ATTRIB_XXX_MASK in place, and simply set it to zero for
configs that don't fit in 32bpp

b) New attribute(s) for __DRI_ATTRIB_XXX_SHIFT, whose value is the
right-hand side of the << operator you'd use to find that channel

c) Fix dri2_add_config() and friends to work in terms of shifts not
masks

This nicely avoids worrying about just how wide a pixel will ever get
(ie how many _HI attribs you'd need for fp32, fp64, etc), because the
left-shift is a whole 32-bit integer, and we're almost certainly not
worried about needing a billion bits per channel. The only other
flexibility masks would give you is allowing discontiguous bit ranges
for color channels, but that's not a feature of any hardware we do or
will care about (and I'm not sure was ever really a thing, tbh).

Setting the MASK attribs to zero appears to magically DTRT when loading
a newer driver (with fp16 format support) on an older libEGL or
xserver; since the masks of the winsys' visuals will be non-zero, fp
formats will never match.

- ajax