Importing Buffer-Based Kernels - 2025.2 English

As explained in Data Accessing Mechanisms, the size of the input and output data blocks for buffer-based access depends on the specified buffer size. Vitis Model Composer supports the following buffer-based input and output data types as interfaces to the AI Engine kernel block.

input_buffer<Type>
output_buffer<Type>

Table 1. Buffer-Based Input and Output Data Types
<Type>	Complexity	Signedness
int8	Real	Signed
int16	Real	Signed
int32	Real	Signed
int64	Real	Signed
uint8	Real	Unsigned
uint16	Real	Unsigned
uint32	Real	Unsigned
uint64	Real	Unsigned
cint16	Complex	Signed
cint32	Complex	Signed
float	Real	N/A
cfloat	Complex	N/A
bfloat16	Real	N/A

As an example, to import a simple kernel with a buffer-based interface, the following simple.h header file defines the add_kernel function with two input buffers and one output buffer of type int16.

Simple.h

#ifndef __ADD_KERNEL_H__
#define __ADD_KERNEL_H__
 
#include <adf.h>
#define NUM_SAMPLES 4

using namespace adf;

void add_kernel(input_buffer<int16> & in1,input_buffer<int16> & in2, output_buffer<int16> & outw);
 
#endif

The kernel (simple.cc) is defined as follows. It processes a sum operation on in1 and in2 and produces output on outw.

#include "simple.h"
void add_kernel(input_buffer<int16> & in1,input_buffer<int16> & in2, output_buffer<int16> & outw)
{
    // Use scalar iterator to traverse data
    auto pIn1  = aie::begin(in1);
    auto pIn2  = aie::begin(in2);
    auto pOut = aie::begin(outw);

    for (unsigned i=0; i<NUM_SAMPLES; i++) {
        *pOut++ = *pIn1++ + *pIn2++;
    }
}

Tip: Although not required, the following recommendations are useful for reusability and faster compilation.

Define each kernel in its own source file.
Organize kernels by creating directories for header files and source files separately.
Ensure kernel source files include all relevant header files to allow for independent compilation.

To import the add_kernel function as a block in a Model Composer design, double-click the AIE Kernel block and update parameters as follows:

Kernel header file: kernels/include/simple.h
Kernel function: add_kernel
Kernel Init function: Leave empty
Kernel source file: kernels/source/simple.cc
Kernel search path: Leave empty
Preprocessor options: Leave empty

When you click the Import button in the Block Parameters dialog box, the tool parses the function signature in the header file. It also updates the AI Engine kernel block GUI interface as shown in the following figure.

Figure 1. AIE Kernel Block (Updated)

When you add the AIE Kernel block to the Simulink editor, input and output ports are not present. But, when you add the kernel parameters in the Block Parameters dialog box, the block updates with two input ports and one output port. The block name matches the imported kernel function.

After a successful import, the Function tab displays automatically, providing user-editable configuration parameters. You can quickly review the function definition of the imported kernel function and the port names with directions.

Figure 2. Function Tab

Enter appropriate values in the Function tab for Buffer size and Buffer margin (see the previous figure).

Setting the Buffer Margin Value

As explained in Data Accessing Mechanisms, buffer margin is the overlapping of input data samples. Model Composer accepts Buffer margin value in terms of the number of samples. Ensure that the values given in the Buffer margin fields are multiples of 32 bytes.

For example, if your input data type is int16 which is 2 bytes, the minimum accepted Buffer margin value is 16 samples (16*2). Other acceptable other values can be 32, 48, 64 samples and so on.

As another example, if your input is of type cint32, which is 8 bytes (4 real, 4 imaginary), then the minimum accepted buffer margin value is 4 samples. Other acceptable values can be 8, 12, 16 samples and so on.

The following table provides details on the parameters and description for each parameter.

Table 2. Buffer Port Parameters
Parameter Name	Criticality	Description
Buffer size	Mandatory	Buffer size is required for each port (argument) of the kernel function. The value represents the number of samples (elements). The buffer size must be a positive integer value. The buffer size must be a multiple of 16 bytes if no buffer margin is specified. If a margin is specified, the buffer size must be a multiple of 32 bytes.
Buffer margin	Mandatory	Buffer margin is required for each input port (argument) of the kernel function. The value represents the number of samples (elements). The buffer margin must be a non-negative integer. The buffer margin must be a multiple of 32 bytes.
Synchronicity	Mandatory	The Synchronicity value options available are `sync` and `async`. Port synchronicity is set to `sync` by default. You can optionally change it `async`.

After the successful import of kernel functions, the Import button label in the General tab changes to Update enabling further updates of block parameters. You can change the source code of the kernel function even after importing the block without requiring a re-import. However, if you change the function signature, or the parameters to the function, then you will need to click Update in the General tab to apply changes.

Figure 3. General Tab

Importing Buffer-Based Kernels - 2025.2 English - UG1483

Vitis Model Composer User Guide (UG1483)

Simple.h

Setting the Buffer Margin Value