[Freedreno] MSAA

[Ilia Mirkin]

On Wed, Mar 9, 2016 at 12:04 AM, Bharat Bhushan <bb14521 at gmail.com> wrote:
>  MSSA support for Adreno A3x
> i think, due to the fact that its affects nearly the entire
> rasterization pipeline used in GPU’s. It’s also complicated because
> really understanding why it works requires at least a basic
> understanding of signal processing and image re sampling.

The GPU has hardware which does all this. It's a matter of properly
activating that hardware. I appear to have some very old patches at
https://github.com/imirkin/mesa/commits/fd which kinda-sorta start
doing this, but clearly I didn't get very far. Unfortunately esp on
A3xx if you get anything wrong, the GPU expresses its displeasure by
hanging. On the brighter side, we have documentation which basically
explains what all the various bits mean (courtesy of the db410c
board).

> i am Little bit confused about Per-sample shading,  what is it exactly ?
> let me know if there is any other meaning of per-sample shading.

Normally MSAA is done by running a single shader invocation per pixel
to determine the color, and then applying it for the whole coverage
mask. Per-sample shading allows you to compute a different color for
each sample. While A3XX allows for some form of per-sample shading, it
is very much not done the way that GL wants it, so it probably
wouldn't be worth supporting. A4XX's implementation I think is much
more GL-friendly.

  -ilia