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

[Neil Roberts]

> 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.

The resolve needs to be done for single-sampled render targets but for
multisample targets we can safely sample from the MCS buffer. The MCS
(or CCS or whatever it should be called in this case) is still useful
even if we have to resolve because it saves memory bandwidth during
rendering. Eg, the initial clear writes less data because it only writes
to the smaller MCS buffer and not the larger color buffer. Any blending
operations during rendering can also read from the CCS instead of the
color buffer. I guess the assumption is that the bandwidth savings
during rendering outweigh the extra cost added by having to resolve it.

To be clear, this patch doesn't add the resolve step needed before
texturing, nor does anything in the series. The resolve step is already
done for all gens in intel_update_state. Look at the bit with the
comment “Resolve depth buffer and render cache of each enabled texture”.
It calls intel_miptree_resolve_color which resolves the CCS buffer for
each singled-sampled surface that is going to be used as a texture. The
only change this patch makes it to not not even set the aux buffer in
the texture surface state.

- Neil