[Mesa-dev] [PATCH 2/5] i965/gen8+: Don't upload the MCS buffer for single-sampled textures

[Pohjolainen, Topi]

On Thu, Nov 26, 2015 at 11:18:34AM +0200, Pohjolainen, Topi wrote:
> On Wed, Nov 25, 2015 at 11:01:18AM -0800, Ben Widawsky wrote:
> > On Wed, Nov 25, 2015 at 06:36:36PM +0100, Neil Roberts wrote:
> > > For single-sampled textures the MCS buffer is only used to implement
> > > fast clears. However the surface always needs to be resolved before
> > > being used as a texture anyway so the the MCS buffer doesn't actually
> > > achieve anything. This is important for Gen9 because in that case SRGB
> > 
> > I admit a good deal of ignorance, but why do we have to do a resolve before we
> > sample from it? I thought the whole point of the MCS was that we can sample from
> > it without a resolve.
> 
> This is my understanding also, I can't see much benefit from the fast clear
> if it would need to be anyway resolved before reading it using GPU (reading
> using CPU is then another story of course).

I know we have this piece of text in bspec:

"If the MCS is enabled on a non-multisampled render target, the render
target must be resolved before being used for other purposes (display,
texture, CPU lock)."

But on the other hand the fact that surface state supports definition of fast
cleared MCS buffer even for sampler engine suggest that "texture" in that
sentence refers to something else than sampling. Not to mention that we
already sample fast cleared color buffers with current Mesa driver and seem
to have no problems with it.