[Mesa-dev] [Mesa3d-dev] ARB draw buffers + texenv program

[Keith Whitwell]

On Fri, 2010-12-17 at 00:49 -0800, Dave Airlie wrote:
> On Tue, Apr 27, 2010 at 7:10 AM, Dave Airlie <airlied at gmail.com> wrote:
> >>>> buffers.
> >>> But you'll have more shader instructions for writing to all these
> >>> outputs right? I think that could still make a difference, though it
> >>> might be more theoretical rather than in practice.
> >>>
> >>
> >> Right, I was talking specifically about memory bandwidth. But it would
> >> take a few extra instructions as well.  I think the major impact will
> >> be the additional memory bandwidth rather than the added instructions.
> >>
> >
> >
> > I also thought about this a bit more, and I'm guessing GPUs use this
> > to optimise cache usage. If they know they are rendering the same data
> > to all 1..n render targets they only need to store one copy in the dst
> > cache.
> 
> Okay I'm finally getting back to resurrecting this,
> 
> http://people.freedesktop.org/~airlied/scratch/0001-gallium-r300g-fix-frag-color-writing-attempt-2.patch
> 
> Basically I've added a TGSI property that states whether single writes
> are meant to go to single out or all of them.
> 
> I've still got to fixup softpipe and there's lots of debugging in
> there, but with that r300g passes the fbo-drawbuffers-fragcolor test.

Dave,

I think this is subtle enough that it needs some documentation.  

I trawled the revived thread to figure out the detail of what's here
(having forgotten all about it), and it looks fine but I think this
really needs a couple of good paragraphs in the documentation and a
from-first-principles description in the patch.

Also, WRITE_ALL is a bit ambiguous -- could it be something like
"BROADCAST_COLOR0_WRITES" or maybe "WRITE_ALL_CBUFS"?

Keith