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Overview
========

This project is included in the internal Android source tree to support
automated latency testing with Salad Fingers, a robot designed and built by
Steve Pfetsch (spfetsch@). The three main components we use and modify are:

  - the android app (android/)
  - the WALT device firmware (arduino/)
  - the TCP-to-serial bridge (pywalt/)

Salad Fingers uses a single Teensyduino running WALT firmware to test multiple
devices without human intervention. The devices under test are connected to the
same host as the Teensy. An end-to-end connection for a single device (only one
device can use the WALT at a time) looks like this:

    Device ------ Host ------ Teensy
  (android/)    (pywalt/)   (arduino/)

For the device to communicate with the host over TCP on a physical USB
connection, a "reverse" port has to be set up with adb. For example:

  $ adb reverse tcp:50007 tcp:45454

Any traffic the device sends to 127.0.0.1:50007 will come into the host on port
45454 and vice versa. Port 50007 is defined in the app source, but the device
port can be selected arbitrarily.

The TCP-to-serial bridge runs on the host and connects the app's TCP pipe to
the Teensy's serial pipe. However, there are two special commands the app can
send to the bridge to synchronize the clocks between the device and the Teensy:
"bridge sync" and "bridge update".

This setup requires some modifications from the original source, which are
explained in the next section, but behaves very similarly to a direct, Teensy-
to-device USB connection.


Modifications
=============

- Clock synchronization
  Despite the reliability and accuracy of NTP, device and host wall clocks can
  become significantly out of sync, especially if a device loses Wi-Fi
  connectivity. To avoid this problem and take advantage of the low-latency
  connection between the host and device, the clocks are synchronized based on
  the time difference between when the bridge zeroed the Teensy's clock and
  when the reply to the "bridge sync" command was sent to the device. This
  required parallel changes in pywalt/ and android/.

- Automation intents
  The test scripts which control the robot (Salad Fingers) on which the Teensy
  is mounted require the ability to control certain aspects of the app running
  on a device. These are defined in a separate RobotAutomationEvent interface.
  This required changes to android/.

- Reverse port support
  The WaltTcpConnection class was originally intended to communicate over a
  true network from a Chromebook to a dedicated bridge with a specific IP
  address. This address was changed to localhost in android/.

- Strict networking and request ordering
  To prevent crashes due to network accesses performed on the main thread,
  which is disallowed in strict mode, all such accesses were moved to a
  dedicated network thread. As a side benefit, all requests sent to the bridge
  now wait for a response before the next request is sent. This complies with
  the serial nature of the device and guarantees correct ordering. This
  required changes to android/.

- Hardware-specific firmware
  The Teensy on Salad Fingers uses sensors that are not bundled with the
  standard WALT hardware, so new thresholds for accelerometer shocks and photo-
  diode readings were required to achieve accurate results. This required
  changes to arduino/.


Usage
=====

This project is not intended to be automatically built. Instead, as needed, the
app will be built manually and checked in as a prebuilt that PTS will pick up.
The firmware cannot be built or loaded automatically, as it requires a modded
version of the Arduino project for Teensy. The required software can be loaded
on a laptop to flash the Teensy mounted on Salad Fingers.


See Also
========

Project metadata: vendor/google_meta/platform/external/walt/
PTS integration:  vendor/google_testing/pts/tests/salad_fingers/