Parameter Update/Read Using Graph APIs - 2021.2 English

Versal ACAP AI Engine Programming Environment User Guide (UG1076)

Document ID
UG1076
ft:locale
English (United States)
Release Date
2021-12-17
Version
2021.2 English

In default compilation mode, the main application is compiled as a separate control thread which needs to be executed on the PS in parallel with the graph executing on the AI Engine array. The main application can use update and read APIs to access run-time parameters declared within the graphs at any level. This section describes these APIs using examples.

Synchronous Update/Read

The following code shows the main application of the simple_param graph described in Specifying Run-Time Data Parameters.

#include "param.h"
parameterGraph mygraph;

int main(void) {
  mygraph.init();
  mygraph.run(2);
  
  mygraph.update(mygraph.select_value, 23);
  mygraph.update(mygraph.select_value, 45);

  mygraph.end();
  return 0;
}

In this example, the graph mygraph is initialized first and then run for two iterations. It has a triggered input parameter port select_value that must be updated with a new value for each invocation of the receiving kernel. The first argument of the update API identifies the port to be updated and the second argument provides the value. Several other forms of update APIs are supported based on the direction of the port, its data type, and whether it is a scalar or array parameter, see Adaptive Data Flow Graph Specification Reference.

If the program is compiled with a fixed number of test iterations, then for triggered parameters the number of update API calls in the main program must match the number of test iterations, otherwise the simulation could be waiting for additional updates. For asynchronous parameters, the updates are done asynchronously with the graph execution and the kernel uses the old value if the update was not made.

If additionally, the previous graph was compiled with a synchronous inout parameter, then the update and read calls must be interleaved as shown in the following example.

#include "param.h"
parameterGraph mygraph;

int main(void) {
  int result0, result1;
  mygraph.init();
  mygraph.run(2);

  mygraph.update(mygraph.select_value, 23);
  mygraph.read(mygraph.result_out, result0);
  mygraph.update(mygraph.select_value, 45);
  mygraph.read(mygraph.result_out, result1);

  mygraph.end();
  return 0;
}

In this example, it is assumed that the graph produces a scalar result every iteration through the inout port result_out. The read API is used to read out the value of this port synchronously after each iteration. The first argument of the read API is the graph inout port to be read back and the second argument is the location where the value will be stored (passed by reference).

The synchronous protocol ensures that the read operation will wait for the value to be produced by the graph before sampling it and the graph will wait for the value to be read before proceeding to the next iteration. This is why it is important to interleave the update and read operations.

Asynchronous Update/Read

When an input parameter is specified with asynchronous protocol, the kernel execution waits for the first update to happen for parameter initialization. However, an arbitrary number of kernel invocations can take place before the next update. This is usually the intent of the asynchronous update during application deployment. However, for debugging, wait API can be used to finish a predetermined set of iterations before the next update as shown in the following example.

#include "param.h"
asyncGraph mygraph;

int main(void) {
  int result0, result1;
  mygraph.init();

  mygraph.update(mygraph.select_value, 23);
  mygraph.run(5);
  mygraph.wait();
  mygraph.update(mygraph.select_value, 45);
  mygraph.run(15);
  mygraph.end();
  return 0;
}

In the previous example, after the initial update, five iterations are run to completion followed by another update, then followed by another set of 15 iterations. If the graph has asynchronous inout ports, that data can also be read back immediately after the wait (or end).

Another template for asynchronous updates is to use timeouts in wait API as shown in the following example.

#include "param.h"
asyncGraph mygraph;

int main(void) {
  int result0, result1;
  mygraph.init();
  mygraph.run();
  mygraph.update(mygraph.select_value, 23);
  mygraph.wait(10000);
  mygraph.update(mygraph.select_value, 45);
  mygraph.resume();
  mygraph.end(15000);
  return 0;
}

In this example, the graph is set up to run forever. However, after the run API is called, it still blocks for the first update to happen for parameter initialization. Then, it runs for 10,000 cycles (approximately) before allowing the control thread to make another update. The new update takes effect at the next kernel invocation boundary. Then the graph is allowed to run for another 15,000 cycles before terminating.