Interact with the driver

Software on Fuchsia interacts with driver components through their exposed service capabilities. Once a client connects to the driver's service, it receives an instance of the FIDL protocol representing that driver. For more details, see the Driver Communication guide.

In this section, you'll create a new eductl executable that discovers and interacts with the capabilities exposed by the qemu_edu driver.

Create a new tools component

Create a new project directory in your Bazel workspace for a new binary tool:

mkdir -p fuchsia-codelab/qemu_edu/tools

After you complete this section, the project should have the following directory structure:

//fuchsia-codelab/qemu_edu/tools
                  |- BUILD.bazel
                  |- eductl.cc

Create the qemu_edu/tools/BUILD.bazel file and add the following statement to include the necessary build rules from the Fuchsia SDK:

qemu_edu/tools/BUILD.bazel:

{% includecode gerrit_repo="fuchsia/sdk-samples/drivers" gerrit_path="src/qemu_edu/tools/BUILD.bazel" region_tag="imports" adjust_indentation="auto" %}

Create a new qemu_edu/tools/eductl.cc file with the following code to set up a basic command line executable:

qemu_edu/tools/eductl.cc:

{% includecode gerrit_repo="fuchsia/sdk-samples/drivers" gerrit_path="src/qemu_edu/tools/eductl.cc" region_tag="imports" adjust_indentation="auto" %}

{% includecode gerrit_repo="fuchsia/sdk-samples/drivers" gerrit_path="src/qemu_edu/tools/eductl.cc" region_tag="cli_helpers" adjust_indentation="auto" %}

int main(int argc, char* argv[]) {
  const char* cmd = basename(argv[0]);

  // ...

  return usage(cmd);
}

This executable supports two subcommands to execute the liveness check and factorial computation.

Add the following new rules to the bottom of the project's build configuration to build this new tool into a Fuchsia package:

qemu_edu/tools/BUILD.bazel:

{% includecode gerrit_repo="fuchsia/sdk-samples/drivers" gerrit_path="src/qemu_edu/tools/BUILD.bazel" region_tag="binary" adjust_indentation="auto" exclude_regexp="\/\/src\/qemu_edu" %}

{% includecode gerrit_repo="fuchsia/sdk-samples/drivers" gerrit_path="src/qemu_edu/tools/BUILD.bazel" region_tag="component" adjust_indentation="auto" %}

{% includecode gerrit_repo="fuchsia/sdk-samples/drivers" gerrit_path="src/qemu_edu/tools/BUILD.bazel" region_tag="package" adjust_indentation="auto" %}

Implement the client tool

Clients connect to the driver's exposed service to receive an instance of the FIDL protocol. Add the following code to eductl to connect to the edu device service:

qemu_edu/tools/eductl.cc:

{% includecode gerrit_repo="fuchsia/sdk-samples/drivers" gerrit_path="src/qemu_edu/tools/eductl.cc" region_tag="imports" adjust_indentation="auto" %}

{{ '<strong>' }}{% includecode gerrit_repo="fuchsia/sdk-samples/drivers" gerrit_path="src/qemu_edu/tools/eductl.cc" region_tag="fidl_imports" adjust_indentation="auto" %}{{ '</strong>' }}

{{ '<strong>' }}{% includecode gerrit_repo="fuchsia/sdk-samples/drivers" gerrit_path="src/qemu_edu/tools/eductl.cc" region_tag="service_client" adjust_indentation="auto" %}{{ '</strong>' }}

// ...

Add liveness_check() and compute_factorial() functions to call methods using the examples.qemuedu/Device FIDL protocol returned from ConnectToDevice(). Finally, update the tool's main() function to call the appropriate device function based on the argument passed on the command line:

qemu_edu/tools/eductl.cc:

// ...

{{ '<strong>' }}{% includecode gerrit_repo="fuchsia/sdk-samples/drivers" gerrit_path="src/qemu_edu/tools/eductl.cc" region_tag="liveness_check" adjust_indentation="auto" %}{{ '</strong>' }}

{{ '<strong>' }}{% includecode gerrit_repo="fuchsia/sdk-samples/drivers" gerrit_path="src/qemu_edu/tools/eductl.cc" region_tag="compute_factorial" adjust_indentation="auto" %}{{ '</strong>' }}

{% includecode gerrit_repo="fuchsia/sdk-samples/drivers" gerrit_path="src/qemu_edu/tools/eductl.cc" region_tag="main" adjust_indentation="auto" highlight="4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20" %}

Update the tool's build configuration to depend on the FIDL bindings for the examples.qemuedu library:

qemu_edu/tools/BUILD.bazel:

{% set build_bazel_snippet %} {% includecode gerrit_repo=“fuchsia/sdk-samples/drivers” gerrit_path=“src/qemu_edu/tools/BUILD.bazel” region_tag=“binary” adjust_indentation=“auto” highlight=“7” %} {% endset %}

{{ build_bazel_snippet|replace("//src/qemu_edu","//fuchsia-codelab/qemu_edu")|trim() }}

<<_common/_restart_femu.md>>

Reload the driver

Use the bazel run command to build and execute the driver component target:

bazel run //fuchsia-codelab/qemu_edu/drivers:pkg.component

Run the tool

Use the bazel run command to build and execute the tool, passing the arguments fact 12 to compute the factorial of 12:

bazel run //fuchsia-codelab/qemu_edu/tools:pkg.eductl_tool -- fact 12

The bazel run command performs the following steps:

Build the executable and package.
Publish the package to a local package repository.
Register the package repository with the target device.
Use ffx driver run-tool to run the binary inside the driver_playground component.

The command prints output similar to the following with the computation result the factorial:

Factorial(12) = 479001600

Congratulations! You‘ve successfully connected to your driver’s exposed services from a separate client.