Using Inspect for drivers


In case you are not familiarized with Inspect, it is recommend that you read the following pages:


Inspect can be used by drivers. However, there are some special considerations that must be made given that drivers are not components.

Consider the following system topology diagram:

Figure 1: example topology

Diagram legend:

Gray: Components. Blue: Component namespace that shows the out/diagnostics directory contents. Purple: Contents of the out/diagnostics/* inspect files.

Components can expose inspect data in the following ways:

  • VMO file: Typically called root.inspect
  • Tree service: A service file for the fuchsia.inspect.Tree protocol

A component can expose one or more of these files. In the diagram above, components foo and echo expose a single file. However, driver_manager exposes multiple files, one VMO file for each device.

The reason for this is that each device is not a component, therefore driver_manager aggregates their inspect VMOs and publishes them itself. This is a major difference between exposing inspect data from a component and from a driver that has implications in how the data can be queried through selectors.

If you wanted to query data for the foo and echo components, you can express that in a specific selector. For example, core/foo:root/child1:prop1, core/echo:root/child1:prop1. You have a unique way of querying each even if the properties and nodes are called the same way given that you can uniquely identify them using their moniker.

However, when it comes to drivers, it’s recommended that all devices expose an inspect hierarchy where the root has a single child with the name of the device for a better experience using selectors to query their data. All other properties and nodes are children of this child.

In the diagram above, you can query: bootstrap/driver_manager:root/device_a:failures and bootstrap/driver_manager:root/device_b:failures uniquely. If neither device_a nor device_b exposed their inspect VMO with a root/{device name} node where all properties are located, you wouldn’t have a way of differencing between both and the selector bootstrap/driver_manager:root:failures would match both of them.

Including Inspect in drivers

These steps walk you through how to include Inspect in your drivers. For a full example of a driver with Inspect included, see the following example test driver.

  1. Add the zircon inspect library to your driver dependencies in

    deps = [
  2. Include this header in your driver:

    #include <lib/inspect/cpp/inspect.h>
  3. Create an Inspector instance in your device class:

    class TestDevice {
         inspect::Inspector inspect_;

    Use this to create properties and children to build the inspect tree:

    TestDevice::TestDevice() {
        state_ = inspect_.GetRoot().CreateString("state","invalid");
        // inspect is a tree; You can add children and structure your data.
        performance_ = inspect_.GetRoot().CreateChild("performance");
        call_count_ = performance_.CreateUint("call_count",0);
        total_time_ = performance_.CreateUint("total_time(ms)",0);
    TestDevice::SetState(State s) {
        case kActive:

    Inspect currently supports a variety of property types like integers, strings, arrays, bool, double, histograms.

    Inspect is RAII so remember to hold references to properties that you will update, otherwise they’ll be removed from the inspect VMO.

    class TestDevice {
         inspect::Inspector inspect_;
         inspect::StringProperty state_;
         inspect::Node performance_;
         inspect::UintProperty call_count_;
         inspect::UintProperty total_time_;

    You can add properties and nodes to an inspect::ValueList instead of holding references to them. This will tie the lifetime of the properties and nodes with the lifetime of the inspect::ValueList. Note that inspect::Inspector happens to be a ValueList for convenience.

    inspect_.GetRoot().CreateString("name","test device",&inspect_);
  4. Export inspect VMO to driver manager.

    zx_status Bind() {

    You can publish one inspect VMO per device; Instance devices do not count. To add inspect for instance devices, you can create child nodes from the devices’ inspect and add to it.

  5. Done. Now you can view Inspect data for the driver.

    • The device inspect file is hosted in /dev/diagnostics/class//xxx.inspect
    • Check the inspect data using iquery
    fx iquery show-file /dev/diagnostics/class/ethernet/000.inspect
    // To view all of driver_manager and driver host
    fx iquery show 'bootstrap/driver_manager'
  6. Run fx snapshot and check if your inspect data is present in inspect.json. Note that the feedback component is not part of bringup, so taking snapshots is not very useful when working only with a bringup build. For these situations, prefer using iquery which is available in bootfs.

Isolated devmgr

You can view devfs with /hub, which enables tools like iquery on tests using isolated devmgr.

  1. If you are using src/lib/isolated_devmgr/isolated_devmgr.gni, then it’ll automatically be hosted in the test’s out directory.


    iquery show-file  /hub/r/test_env_**/**/c/fs-management-devmgr.cmx/**/out/dev/diagnostics/driver_manager/dm.inspect
  2. If you are using zircon/system/ulib/devmgr-integration-test/isolated_devmgr.gni, you will need to call AddDevfsToOutgoingDir on the devmgr instance (example) to host devfs in the test’s out directory. Once this is done, viewing the inspect file is similar to the previous point.