External Bus and Bus Connected Peripherals Emulators

Overview

Zephyr supports a simple emulator framework to support testing of external peripheral drivers without requiring real hardware.

Emulators are used to emulate external hardware devices, to support testing of various subsystems. For example, it is possible to write an emulator for an I2C compass such that it appears on the I2C bus and can be used just like a real hardware device.

Emulators often implement special features for testing. For example a compass may support returning bogus data if the I2C bus speed is too high, or may return invalid measurements if calibration has not yet been completed. This allows for testing that high-level code can handle these situations correctly. Test coverage can therefore approach 100% if all failure conditions are emulated.

Concept

The diagram below shows application code / high-level tests at the top. This is the ultimate application we want to run.

Emulator architecture showing tests, emulators and drivers

Below that are peripheral drivers, such as the AT24 EEPROM driver. We can test peripheral drivers using an emulation driver connected via a emulated I2C controller/emulator which passes I2C traffic from the AT24 driver to the AT24 simulator.

Separately we can test the STM32 and NXP I2C drivers on real hardware using API tests. These require some sort of device attached to the bus, but with this, we can validate much of the driver functionality.

Putting the two together, we can test the application and peripheral code entirely on native_sim. Since we know that the I2C driver on the real hardware works, we should expect the application and peripheral drivers to work on the real hardware also.

Using the above framework we can test an entire application (e.g. Embedded Controller) on native_sim using emulators for all non-chip drivers.

With this approach we can:

  • Write individual tests for each driver (green), covering all failure modes, error conditions, etc.

  • Ensure 100% test coverage for drivers (green)

  • Write tests for combinations of drivers, such as GPIOs provided by an I2C GPIO expander driver talking over an I2C bus, with the GPIOs controlling a charger. All of this can work in the emulated environment or on real hardware.

  • Write a complex application that ties together all of these pieces and runs on native_sim. We can develop on a host, use source-level debugging, etc.

  • Transfer the application to any board which provides the required features (e.g. I2C, enough GPIOs), by adding Kconfig and devicetree fragments.

Creating a Device Driver Emulator

The emulator subsystem is modeled on the Device Driver Model. You create an emulator instance using one of the EMUL_DT_DEFINE() or EMUL_DT_INST_DEFINE() APIs.

Emulators for peripheral devices reuse the same devicetree node as the real device driver. This means that your emulator defines DT_DRV_COMPAT using the same compat value from the real driver.

/* From drivers/sensor/bm160/bm160.c */
#define DT_DRV_COMPAT bosch_bmi160

/* From drivers/sensor/bmi160/emul_bmi160.c */
#define DT_DRV_COMPAT bosch_bmi160

The EMUL_DT_DEFINE() function accepts two API types:

  1. bus_api - This points to the API for the upstream bus that the emulator connects to. The bus_api parameter is required. The supported emulated bus types include I2C, SPI, eSPI, and MSPI.

  2. _backend_api - This points to the device-class specific backend API for the emulator. The _backend_api parameter is optional.

The diagram below demonstrates the logical organization of the bus_api and _backend_api using the BC1.2 charging detector driver as the model device-class.

Device class example, demonstrating BC1.2 charging detectors.

The real code is shown in green, while the emulator code is shown in yellow.

The bus_api connects the BC1.2 emulators to the native_sim I2C controller. The real BC1.2 drivers are unchanged and operate exactly as if there was a physical I2C controller present in the system. The native_sim I2C controller uses the bus_api to initiate register reads and writes to the emulator.

The _backend_api provides a mechanism for tests to manipulate the emulator out of band. Each device class defines it’s own API functions. The backend API functions focus on high-level behavior and do not provide hooks for specific emulators.

In the case of the BC1.2 charging detector the backend API provides functions to simulate connecting and disconnecting a charger to the emulated BC1.2 device. Each emulator is responsible for updating the correct vendor specific registers and potentially signalling an interrupt.

Example test flow:

  1. Test registers BC1.2 detection callback using the Zephyr BC1.2 driver API.

  2. Test connects a charger using the BC1.2 emulator backend.

  3. Test verifies B1.2 detection callback invoked with correct charger type.

  4. Test disconnects a charger using the BC1.2 emulator backend.

With this architecture, the same test can be used will all supported drivers in the same driver class.

Available Emulators

Zephyr includes the following emulators:

  • I2C emulator driver, allowing drivers to be connected to an emulator so that tests can be performed without access to the real hardware

  • SPI emulator driver, which does the same for SPI

  • eSPI emulator driver, which does the same for eSPI. The emulator is being developed to support more functionalities.

  • MSPI emulator driver, allowing drivers to be connected to an emulator so that tests can be performed without access to the real hardware.

I2C Emulation features

In the binding of the I2C emulated bus, there’s a custom property for address based forwarding. Given the following devicetree node:

i2c0: i2c@100 {
  status = "okay";
  compatible = "zephyr,i2c-emul-controller";
  clock-frequency = <I2C_BITRATE_STANDARD>;
  #address-cells = <1>;
  #size-cells = <0>;
  #forward-cells = <1>;
  reg = <0x100 4>;
  forwards = <&i2c1 0x20>;
};

The final property, forwards indicates that any read/write requests sent to address 0x20 should be sent to i2c1 with the same address. This allows us to test both the controller and the target end of the communication on the same image.

Note

The #forward-cells attribute should always be 1. Each entry in the forwards attribute consists of the phandle followed by the address. In the example above, <&i2c1 0x20> will forward all read/write operations made to i2c0 at port 0x20 to i2c1 on the same port. Since no additional cells are used by the emulated controller, the number of cells should remain 1.

Samples

Here are some examples present in Zephyr:

  1. Bosch BMI160 sensor driver connected via both I2C and SPI to an emulator:

    # From the root of the zephyr repository
west build -b native_sim tests/drivers/sensor/bmi160

  2. The same test can be built with a second EEPROM which is an Atmel AT24 EEPROM driver connected via I2C an emulator:

    # From the root of the zephyr repository
west build -b native_sim tests/drivers/eeprom/api -- -DDTC_OVERLAY_FILE=at2x_emul.overlay -DOVERLAY_CONFIG=at2x_emul.conf

API Reference

Emulator interface