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92 lines
3.8 KiB
92 lines
3.8 KiB
Overview
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========
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This project is included in the internal Android source tree to support
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automated latency testing with Salad Fingers, a robot designed and built by
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Steve Pfetsch (spfetsch@). The three main components we use and modify are:
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- the android app (android/)
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- the WALT device firmware (arduino/)
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- the TCP-to-serial bridge (pywalt/)
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Salad Fingers uses a single Teensyduino running WALT firmware to test multiple
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devices without human intervention. The devices under test are connected to the
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same host as the Teensy. An end-to-end connection for a single device (only one
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device can use the WALT at a time) looks like this:
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Device ------ Host ------ Teensy
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(android/) (pywalt/) (arduino/)
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For the device to communicate with the host over TCP on a physical USB
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connection, a "reverse" port has to be set up with adb. For example:
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$ adb reverse tcp:50007 tcp:45454
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Any traffic the device sends to 127.0.0.1:50007 will come into the host on port
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45454 and vice versa. Port 50007 is defined in the app source, but the device
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port can be selected arbitrarily.
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The TCP-to-serial bridge runs on the host and connects the app's TCP pipe to
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the Teensy's serial pipe. However, there are two special commands the app can
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send to the bridge to synchronize the clocks between the device and the Teensy:
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"bridge sync" and "bridge update".
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This setup requires some modifications from the original source, which are
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explained in the next section, but behaves very similarly to a direct, Teensy-
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to-device USB connection.
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Modifications
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=============
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- Clock synchronization
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Despite the reliability and accuracy of NTP, device and host wall clocks can
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become significantly out of sync, especially if a device loses Wi-Fi
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connectivity. To avoid this problem and take advantage of the low-latency
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connection between the host and device, the clocks are synchronized based on
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the time difference between when the bridge zeroed the Teensy's clock and
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when the reply to the "bridge sync" command was sent to the device. This
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required parallel changes in pywalt/ and android/.
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- Automation intents
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The test scripts which control the robot (Salad Fingers) on which the Teensy
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is mounted require the ability to control certain aspects of the app running
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on a device. These are defined in a separate RobotAutomationEvent interface.
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This required changes to android/.
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- Reverse port support
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The WaltTcpConnection class was originally intended to communicate over a
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true network from a Chromebook to a dedicated bridge with a specific IP
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address. This address was changed to localhost in android/.
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- Strict networking and request ordering
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To prevent crashes due to network accesses performed on the main thread,
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which is disallowed in strict mode, all such accesses were moved to a
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dedicated network thread. As a side benefit, all requests sent to the bridge
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now wait for a response before the next request is sent. This complies with
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the serial nature of the device and guarantees correct ordering. This
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required changes to android/.
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- Hardware-specific firmware
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The Teensy on Salad Fingers uses sensors that are not bundled with the
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standard WALT hardware, so new thresholds for accelerometer shocks and photo-
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diode readings were required to achieve accurate results. This required
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changes to arduino/.
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Usage
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=====
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This project is not intended to be automatically built. Instead, as needed, the
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app will be built manually and checked in as a prebuilt that PTS will pick up.
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The firmware cannot be built or loaded automatically, as it requires a modded
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version of the Arduino project for Teensy. The required software can be loaded
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on a laptop to flash the Teensy mounted on Salad Fingers.
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See Also
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========
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Project metadata: vendor/google_meta/platform/external/walt/
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PTS integration: vendor/google_testing/pts/tests/salad_fingers/
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