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doc/manual: driver writing tutorial

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Sebastien Bourdeauducq 2014-10-27 20:38:02 +08:00
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@ -8,6 +8,7 @@ Contents:
installing
getting_started
writing_a_driver
core_language_reference
core_drivers_reference
management_reference

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Writing a driver
================
These instructions cover writing a simple driver for a "slow" device, that uses the controller paradigm.
The controller
--------------
A controller is a piece of software that receives commands from a client over the network (or the ``localhost`` interface), drives a device, and returns information about the device to the client. The mechanism used is remote procedure calls (RPCs) using :class:`artiq.management.pc_rpc`, which makes the network layers transparent for the driver's user.
The controller we will develop is for a "device" that is very easy to work with: the console from which the controller is run. The operation that the driver will implement is writing a message to that console.
For using RPC, the functions that a driver provides must be the methods of a single object. We will thus define a class that provides our message-printing method: ::
class Hello:
def message(self, msg):
print("message: " + msg)
To turn it into a server, we use :class:`artiq.management.pc_rpc`. Import the function we will use: ::
from artiq.management.pc_rpc import simple_server_loop
and add a ``main`` function that is run when the program is executed: ::
def main():
simple_server_loop(Hello(), "::1", 7777)
if __name__ == "__main__":
main()
The parameters ``::1`` and 7777 are respectively the address to bind the server to (IPv6 localhost) and the TCP port to use. Then add a line: ::
#!/usr/bin/env python3
at the beginning of the file, save it to ``hello-controller`` and set its execution permissions: ::
$ chmod 755 hello-controller
Run it as: ::
$ ./hello-controller
and verify that you can connect to the TCP port: ::
$ telnet ::1 7777
Trying ::1...
Connected to ::1.
Escape character is '^]'.
:tip: Use the key combination Ctrl-AltGr-9 to get the ``telnet>`` prompt, and enter ``close`` to quit Telnet. Quit the controller with Ctrl-C.
The client
----------
Create a ``hello-client`` file with the following contents: ::
#!/usr/bin/env python3
from artiq.management.pc_rpc import Client
def main():
remote = Client("::1", 7777)
try:
remote.message("Hello World!")
finally:
remote.close_rpc()
if __name__ == "__main__":
main()
Run it as before, while the controller is running. You should see the message appearing on the controller's terminal: ::
$ ./hello-controller
message: Hello World!
Command-line arguments
----------------------
Use the Python ``argparse`` module to make the bind address and port configurable on the controller, and the server address, port and message configurable on the client.
We suggest naming the controller parameters ``--bind`` and ``--port`` so that those parameters stay consistent across controller, and use ``-s/--server`` and ``--port`` on the client.
The controller's code would contain something similar to this: ::
def _get_args():
parser = argparse.ArgumentParser(description="Hello world controller")
parser.add_argument("--bind", default="::1",
help="hostname or IP address to bind to")
parser.add_argument("--port", default=7777, type=int,
help="TCP port to listen to")
return parser.parse_args()
def main():
args = _get_args()
simple_server_loop(Hello(), args.bind, args.port)