radeon EXA & fallbacks

[Roland Scheidegger]

Eric Anholt wrote:
> Yep, you've got subpixel antialiasing on your fonts, right?  This is 
> really hard to accelerate.  Basically you've got *two* ARGB8888
> values for source that need to get passed in to the blending along
> with the dest, which the card's BLEND_CNTL-type blending doesn't
> support.
Sounds pretty terrible. I don't think there is ANY hardware which could
support this directly (in the alpha blend stage), blending is just not 
defined that way. Though in this example the source seems to be 1x1 
sized so maybe it could be used as the fixed blend color instead (only 
on r200, not r100, however)? Not sure though, I've no idea what sort of 
blend operation exa wants to do there, if it's a dot product it still 
won't work :-).

> I'm pretty sure we could accelerate it in a single pass on Radeons
> using a temporary pixmap containing a copy of the destination as a
> source texture, but this is going to require help from EXA to do it
> cleanly. And I haven't verified, so I'm waving my hands and saying
> "the texture blenders are enough for the OP blending, right?"
This method should work with any newer hardware I guess.

Roland