Importing & mapping an NV12 ID3D11Texture2D to CUDA

Hi, I’m trying to import and map an ID3D11Texture2D with an NV12 pixel format into CUDA using the CUDA external resource interop functions.

I call cudaExternalMemoryGetMappedBuffer twice, first time to to map the luminance channel and the second call I try to map the chroma channel by setting cudaExternalMemoryBufferDesc::offset property but this second call fails. What is the offset value suppose to be, offset of the buffer in bytes? is this the correct way to go about doing this?