creating an image from a buffer and a non-NULL host_ptr

[bashbaug]

In the OpenCL teleconference on May 3rd we were discussing whether it is possible to create an image from a buffer and to pass a non-NULL host_ptr while creating the image - see related comment here: #777 (comment)

I believe this case is disallowed by the current spec (v3.0.10) although it is not directly disallowed. Specifically, for clCreateImage we have the following error conditions:

CL_INVALID_HOST_PTR if [...] host_ptr is not NULL but CL_MEM_COPY_HOST_PTR or CL_MEM_USE_HOST_PTR are not set in flags.

CL_INVALID_VALUE if an image is being created from another memory object (buffer or image) under one of the following circumstances: [...] 3) flags specifies CL_MEM_USE_HOST_PTR or CL_MEM_ALLOC_HOST_PTR or CL_MEM_COPY_HOST_PTR.

Because a non-NULL host_ptr requires either CL_MEM_COPY_HOST_PTR or CL_MEM_USE_HOST_PTR, and because creating an image from another memory object specifically disallows CL_MEM_USE_HOST_PTR and CL_MEM_COPY_HOST_PTR, it is impossible to create an image from a buffer and have a non-NULL host_ptr.

Should we find a way to document this more clearly?