Files/ComputerAssignments/CA2/code/docs/PERFETTO_ANSWERS.md

Q1. From the moment a sensor read request is made until data is obtained, what happens at the OS level?

Answer:
The Android sensor framework operates through the following layers:

1. App registers listener → creates a SensorEventQueue via SensorManager.
2. SensorService (system service) receives the request and calls into the HAL (Hardware Abstraction Layer) via activate() and batch().
3. HAL communicates with the kernel driver (e.g., iio subsystem for many sensors) to set sampling rate and enable the sensor.
4. Kernel driver configures the sensor hardware (often an I2C/SPI device) and starts interrupt‑driven sampling.
5. Hardware generates data at the requested rate, triggering an interrupt.
6. Kernel driver reads data via I2C/SPI, copies to a shared memory region, and wakes up the SensorService.
7. SensorService retrieves data from shared memory and enqueues it into the app’s SensorEventQueue.
8. App’s onSensorChanged is called on the thread that registered the listener (or a dedicated HandlerThread).

Perfetto evidence: Look for sensor related slices:

SELECT ts, dur, name FROM slice WHERE name GLOB '*Sensor*' LIMIT 10;