In the AI Engine kernel code (aie/kernels/hb24.cc), the interface is declared as:
void fir24_sym(input_window<cint16> * iwin, output_window<cint16> * owin, const int32(&coeffs)[12], int32(&coeffs_readback)[12]);
For the RTP array input, const is used for the array reference. From the graph, the RTP port can only be input or inout. The inout port in the graph can only be read by the PS program, it cannot be written by the PS program. Therefore, another port coeffs_readback is defined to read back the coefficient.
In the graph definition (aie/graph.h), the RTP declaration and connection are added as follows:
port<direction::in> coefficients;
port<direction::inout> coefficients_readback;
connect< parameter >(coefficients, async(fir24.in[1]));
connect< parameter >(async(fir24.inout[0]),coefficients_readback);
In aie/graph.cpp (for AI Engine simulator), the RTP update and read commands are:
gr.update(gr.coefficients, narrow_filter, 12);
gr.run(16); // start PL kernel & AIE kernel
gr.read(gr.coefficients_readback,coeffs_readback,12);
std::cout<<"Coefficients read back are:";
for(int i=0;i<12;i++)std::cout<<coeffs_readback[i]<<",\t";
std::cout<<coeffs_readback[i]<<",\t";
}
std::cout<<std::endl;
gr.wait(); // wait PL kernel & AIE kernel to complete
gr.read(gr.coefficients_readback,coeffs_readback,12);
std::cout<<"Coefficients read back are:";
for(int i=0;i<12;i++){
std::cout<<coeffs_readback[i]<<",\t";
}
std::cout<<std::endl;
gr.update(gr.coefficients, wide_filter, 12);
gr.run(16); // start PL kernel & AIE kernel
gr.read(gr.coefficients_readback,coeffs_readback,12);
std::cout<<"Coefficients read back are:";
for(int i=0;i<12;i++){
std::cout<<coeffs_readback[i]<<",\t";
}
std::cout<<std::endl;
Because the RTP read is asynchronous, it can not ensure that reads are happening in between kernels running. Only after graph wait() runs (which is a synchronization point), are the coefficients updated and can be read back.