Interactions between the host program and hardware kernels rely on creating
buffers and transferring data to and from the memory in the device. This process makes use of
functions like
clCreateBuffer
and clEnqueueMigrateMemObjects.
There are two methods for allocating memory buffers, and transferring data:
In the case where XRT allocates the buffer, use enqueueMapBuffer to capture the buffer handle. In the second case, allocate the
buffer directly with CL_MEM_USE_HOST_PTR, so you do not need
to capture the handle.
There are a number of coding practices you can adopt to maximize performance
and fine-grain control. The OpenCL API supports additional
commands for reading and writing buffers. For example, you can use
clEnqueueWriteBuffer
and
clEnqueueReadBuffer
commands in place of
clEnqueueMigrateMemObjects. However, some of these
commands have different effects that must be understood when using them. For example,
clEnqueueReadBufferRect
can read a rectangular region of a buffer object to
the host application, but it does not transfer the data from the device global memory to the
host. You must first use clEnqueueReadBuffer to transfer the
data from the device global memory, and then use clEnqueueReadBufferRect to read the desired rectangular portion into the host
application.