Question Compute Shader not work on older GPU with DX11

Hi, I created Compute Shader in unity 2019.4 (DX11) and works well on RTX 2070S. But when I try run it in same version of Unity or even build on my older laptop with GT 650M (2GB) I have black screen. I started deleting some parts of code and I find out it does not read input properly. when I use this code it works and output red color.

Code (CSharp):

1.     #pragma kernel SomeKernel
2.    
3.      RWTexture2D<float4> Input;
4.      RWTexture2D<float4> Result;
5.    
6.      [numthreads(8, 8, 1)]
7.      void SomeKernel(uint3 id : SV_DispatchThreadID)
8.      {
9.          Result[id.xy] = float4(1,0,0,1); // working
10.      }

But when I use this, it output black on GT 650M but on RTX 2070S it works well.

Code (CSharp):

1.     #pragma kernel SomeKernel
2.    
3.      RWTexture2D<float4> Input;
4.      RWTexture2D<float4> Result;
5.    
6.      [numthreads(8, 8, 1)]
7.      void SomeKernel(uint3 id : SV_DispatchThreadID)
8.      {
9.          Result[id.xy] = Input[id.xy]; // not working on GT 650M but RTX2070S yes
10.      }

it's weird, because if there was a problem with the syntax, even a red result wouldn't work. But it have maybe something to do with num threads? I tried [numthreads(8, 8, 1)] and [numthreads(32, 32, 1)] with same result, new card work, old not.

Old card have hardware support for DX11.1 with SW DX12 and everything you need. https://www.techpowerup.com/gpu-specs/nvidia-geforce-gt-650m-2-gb.b1623


Any idea where can be problem or difference? Thanks.

 

Same issue here on 870m laptop. My code works fine on 1080 and 2080 ti

Did you ever find a workaround for this?

 

The short version is the GT 650M and GTX 870M are the same GPU architecture, Nvidia's Kepler from 2012. The GT 650M is a GK107, and the GTX 870M is a GK104, aka a rebadged GT 680MX, which was also sold as the GTX 780M. In Nvidia's chip numbering schemes, higher numbers are generally slower. Even the later GT 920M is a GK208, which was the basically just desktop GT 630 still being sold 4 years later.

Kepler isn't actually fully DX11 compliant, so some stuff is randomly broken. There's not really anything you can do to fix that.

 

Does it work if you use a Texture2D instead of a RWTexture2D for the Input?