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OpenCL C++ API for Pipes doesn't provide read/write methods?

okramer
New Contributor I
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I'm looking for guidance here, as I'm confused on whether to use OpenCL C or C++ API. I'm learning to use host pipes. The examples use the C++ API for establishing queues and contexts, etc., but then the example pipes code is all in C. There is a C++ API for pipes (in cl2.hpp), namely, cl::Pipes. However, I don't see any method for reading or writing to a cl::Pipe object. How is this accomplished? Here is the relevant host code:

// Create host-to-FPGA pipe and bind to kernel, then launch: std::vector<cl::Device> DeviceList; cl::Context context; getContextAndDevices(mode, context, DeviceList); // Fetch items using OpenCL C++ API cl::Pipe p_out (context, sizeof(float), vectorSize, &err); host_reader_kernel.setArg(0, &p_out); err=queue1.enqueueTask(host_reader_kernel);   // Write data to the FPGA via pipe. THIS DOES NOT COMPILE because "write_pipe" not declared: float data=42.0; cl::write_pipe(p_out, data);

In the kernel code, there is `write_pipe()` so I took a guess that host-side would be the same API. It is supposed to be C++ though so would have expected write_pipe() to be a method on cl::Pipe. Are C++ Pipes not supported? Do I have to do everything (contexts, queues, etc) in OpenCL's C API?

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MEIYAN_L_Intel
Employee
727 Views

Hi,

The cl2.hpp is a wrapper API as shown in https://www.intel.com/content/dam/www/programmable/us/en/pdfs/literature/hb/opencl-sdk/aocl_getting_started.pdf table 1. However it is not fully supported C++ and tied to OpenCL specification based on the version.

 

To read and write between the kernel and host, you may need to refer https://www.intel.com/content/dam/www/programmable/us/en/pdfs/literature/hb/opencl-sdk/aocl_programming_guide.pdf in chapter 5.5.6.

 

Thanks

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okramer
New Contributor I
727 Views

To summarize the answer: I found that the C++ wrappers (cl::Context, cl:Kernel, etc.) all have the parens operator () to access the underlying C struct (cl_context, cl_kernel, etc.). So it is a trivial matter to mix the C and C++ APIs.

 

For host-to-dev comms, there are clWritePipeIntelFPGA and clReadPipeIntelFPGA functions on the host side defined in CL/cl_ext_intelfpga.h that must be included in the host code. It seems Intel should have implemented Pipe::write() and Pipe::read() in the wrapper code to call the C Intel functions above.

 

I also had to declare the following before including any CL headers:

#define CL_HPP_TARGET_OPENCL_VERSION=200 #define CL_TARGET_OPENCL_VERSION=200

I have a good compile now, though getting "ERROR: Enqueue Task (-52)" while running emulation.

 

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MEIYAN_L_Intel
Employee
727 Views

Hi,

 

Do you run compilation successfully?

 

Thanks

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okramer
New Contributor I
727 Views

Yes. Now working on a hang associated with host write - kernel read pipe operation.

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MEIYAN_L_Intel
Employee
727 Views

Hi,

The latest problem seem like same as here: https://forums.intel.com/s/question/0D50P00004XbSe1/host-pipes-not-working-as-documented-in-programming-guide.

I will reply on the forum stated above.

Thanks

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