Setting Kernel Arguments - 2021.2 English

Vitis Unified Software Platform Documentation: Application Acceleration Development (UG1393)

Document ID
English (United States)
Release Date
2021.2 English

In the Vitis software platform, two types of arguments can be set for kernel objects:

  1. Scalar arguments are used for small data transfer, such as constant or configuration type data. These are write-only arguments from the host application perspective, meaning they are inputs to the kernel.
  2. Memory buffer arguments are used for large data transfer. The value is a pointer to a memory object created with the context associated with the program and kernel objects. These can be inputs to, or outputs from the kernel.

Kernel arguments can be set using the clSetKernelArg command, as shown in the following example for setting kernel arguments for two scalar and two buffer arguments.

// Create memory buffers
cl_mem dev_buf1 = clCreateBuffer(context, CL_MEM_WRITE_ONLY | CL_MEM_USE_HOST_PTR, size, &host_mem_ptr1, NULL);
cl_mem dev_buf2 = clCreateBuffer(context, CL_MEM_READ_ONLY | CL_MEM_USE_HOST_PTR, size, &host_mem_ptr2, NULL);

int err = 0;
// Setup scalar arguments
cl_uint scalar_arg_image_width = 3840;
err |= clSetKernelArg(kernel, 0, sizeof(cl_uint), &scalar_arg_image_width); 
cl_uint scalar_arg_image_height = 2160; 
err |= clSetKernelArg(kernel, 1, sizeof(cl_uint), &scalar_arg_image_height); 
// Setup buffer arguments
err |= clSetKernelArg(kernel, 2, sizeof(cl_mem), &dev_buf1);
err |= clSetKernelArg(kernel, 3, sizeof(cl_mem), &dev_buf2);
Important: Although OpenCL allows setting kernel arguments any time before enqueuing the kernel, you should set kernel arguments as early as possible. XRT will error out if you try to migrate a buffer before XRT knows where to put it on the device. Therefore, set the kernel arguments before performing any enqueue operation (for example, clEnqueueMigrateMemObjects) on any buffer.

For all kernel buffer arguments you must allocate the buffer on the device global memories. However, sometimes the content of the buffer is not required before the start of the kernel execution. For example, the output buffer content will only be populated during the kernel execution, and hence it is not important prior to kernel execution. In this case, you should specify clEnqueueMigrateMemObject with the CL_MIGRATE_MEM_OBJECT_CONTENT_UNDEFINED flag so that migration of the buffer will not involve the DMA operation between the host and the device, thus improving performance.