How to call the functions which is created in the top module

If I create a function in the top module, and I want to call this from test (or any other place), how can I achieve it ?

In reply to zz8318:
It depends where you are making the call from.

You can always call a task or function directly if it defined in a module instance above where you are calling it from.

module top;
  mid m();
  function void hello;
    $display("hello");
  endfunction
endmodule
module mid;
  bot b();
endmodule
module bot;
  initial hello();
endmodule

To avoid collisions with other routines having the same name in different modules, you can use an upwards reference by naming the module

module bigtop;
  top t();
  function void hello;
    $display("hi");
  endfunction
endmodule
module top;
  mid m();
  function void hello;
    $display("hello");
  endfunction
endmodule
module mid;
  bot b();
endmodule
module bot;
  initial top.hello();
endmodule

You can also use $root.bigtop.t.hello as an absolute reference instead of an upwards reference.

If you are inside a package, you cannot refer to anything outside the package except another package that has been imported.