Communicating between tiles

Usually applications running on separate tiles will need to communicate between tiles and synchronise their activity. Such communication is passed via XLINKS and switches using the xCONNECT architecture. A mapping file can also declare channels between the tiles it places entry-points on.

Declaring a channel in a mapfile

More concretely, the XCORE-200-EXPLORER has two tiles and a switch for communication between them contained within the package. The tools will automatically configure a channel via the switch using mapfile.xc as below:

mapfile.xc

#include <platform.h>

typedef chanend chanend_t;

extern "C" {
  void main_tile0(chanend_t);
  void main_tile1(chanend_t);
}

int main(void)
{
  chan c;

  par {
    on tile[0]: main_tile0(c);
    on tile[1]: main_tile1(c);
  }

  return 0;
}

In this example, the tile level entrypoint functions each accept a chanend. In the forward-declarations these functions take a chanend argument; this is compatible with a lib_xcore chanend_t at link-time. The chan keyword is used in main to declare a channel. When a channel is passed to tile-level main, an end is automatically assigned to each entrypoint which uses it. In the task-level entrypoints, these chanends are used just like local ones.

Using the channel

In the tile-level entrypoints, the chanends are used as per Programming an XCore tile with C and lib_xcore. This time in main.c, we have a more interesting pair of tile functions. Each is passed a chanend_t c, and they use it to communicate between tiles and synchronise their activities.

main.c

#include <stdio.h>
#include <xcore/channel.h>

#define ITERATIONS 10

void main_tile0(chanend_t c)
{
  int result = 0;
  
  printf("Tile 0: Result %d\n", result);

  chan_out_word(c, ITERATIONS);
  result = chan_in_word(c);

  printf("Tile 0: Result %d\n", result);
}

void main_tile1(chanend_t c)
{
  int iterations = chan_in_word(c);

  int accumulation = 0;

  for (int i = 0; i < iterations; i++)
  {
    accumulation += i;
    printf("Tile 1: Iteration %d Accumulation: %d\n", i, accumulation);
  }

  chan_out_word(c, accumulation);
}

Building and executing this multi-tile application produces the expected result:

$ xcc -target=XCORE-200-EXPLORER mapfile.xc main.c
$ xrun --io a.xe
Tile 0: Result 0
Tile 1: Iteration 0 Accumulation: 0
Tile 1: Iteration 1 Accumulation: 1
Tile 1: Iteration 2 Accumulation: 3
Tile 1: Iteration 3 Accumulation: 6
Tile 1: Iteration 4 Accumulation: 10
Tile 1: Iteration 5 Accumulation: 15
Tile 1: Iteration 6 Accumulation: 21
Tile 1: Iteration 7 Accumulation: 28
Tile 1: Iteration 8 Accumulation: 36
Tile 1: Iteration 9 Accumulation: 45
Tile 0: Result 45

Summary

You have now written a multi-tile application which, through the declarations in the mapfile, configures a path through the XLINKS and switch and provides pre-allocated chanends to the C functions.