Write a minimal DFv2 driver

This guide walks through the steps involved in creating a minimal DFv2 driver.

The instructions in this guide are based on the minimal skeleton driver, which provides the minimum implementation necessary to build, load, and register a new DFv2 driver in a Fuchsia system.

The steps are:

Create a driver header file.
Create a driver source file.
Add the driver export macro.
Create a build file.
Write bind rules.
Create a driver component.

For more DFv2-related features, see Additional tasks.

Create a driver header file

To create a header file for your DFv2 driver, do the following:

Create a new header file (.h) for the driver (for example, skeleton_driver.h).

Include the following interface to the header file:

#include <lib/driver/component/cpp/driver_base.h>

Add an interface for the DriverBase class, for example:

#include <lib/driver/component/cpp/driver_base.h>

namespace skeleton {

class SkeletonDriver : public fdf::DriverBase {
 public:
  SkeletonDriver(fdf::DriverStartArgs start_args, fdf::UnownedSynchronizedDispatcher driver_dispatcher);

  // Called by the driver framework to initialize the driver instance.
  zx::result<> SkeletonDriver::Start() override;
};

}  // namespace skeleton

(Source: skeleton_driver.h)

Create a driver source file

To implement the basic methods for the DriverBase class, do the following:

Create a new source file (.cc) for the driver (for example, skeleton_driver.cc).
Include the header file created for the driver, for example:
```
#include "skeleton_driver.h"
```

Implement the basic methods for the class, for example:

#include "skeleton_driver.h"

namespace skeleton {

SkeletonDriver::SkeletonDriver(fdf::DriverStartArgs start_args,
                           fdf::UnownedSynchronizedDispatcher driver_dispatcher)
    : DriverBase("skeleton_driver", std::move(start_args),
          std::move(driver_dispatcher)) {
}

zx::result<> SkeletonDriver::Start() {
  return zx::ok();
}

}  // namespace skeleton

(Source: skeleton_driver.cc)

This driver constructor needs to pass the driver name (for example, "skeleton_driver"), start_args, and driver_dispatcher to the DriverBase class.

Add the driver export macro

To add the driver export macro, do the following:

In the driver source file, include the following header file:
```
#include <lib/driver/component/cpp/driver_export.h>
```

Add the following macro (which exports the driver class) at the bottom of the driver source file:

FUCHSIA_DRIVER_EXPORT(skeleton::SkeletonDriver);

For example:

#include <lib/driver/component/cpp/driver_base.h>
#include <lib/driver/component/cpp/driver_export.h>

#include "skeleton_driver.h"

namespace skeleton {

SkeletonDriver::SkeletonDriver(fdf::DriverStartArgs start_args,
                           fdf::UnownedSynchronizedDispatcher driver_dispatcher)
    : DriverBase("skeleton_driver", std::move(start_args),
          std::move(driver_dispatcher)) {
}

zx::result<> SkeletonDriver::Start() {
  return zx::ok();
}

}  // namespace skeleton

FUCHSIA_DRIVER_EXPORT(skeleton::SkeletonDriver);

(Source: skeleton_driver.cc)

Create a build file

To create a build file for the driver, do the following:

Create a new BUILD.gn file.
Include the following line to import the driver build rules:
```
import("//build/drivers.gni")
```

Add a target for the driver, for example:

fuchsia_driver("driver") {
  output_name = "skeleton_driver"
  sources = [ "skeleton_driver.cc" ]
  deps = [
    "//sdk/lib/driver/component/cpp",
    "//src/devices/lib/driver:driver_runtime",
  ]
}

(Source: BUILD.gn)

The output_name field must be unique among all drivers.

Write bind rules

To write bind rules for your driver, do the following:

Create a new bind rule file (.bind) for the driver (for example, skeleton_driver.bind).

Add basic bind rules, for example:

using gizmo.example;
gizmo.example.TEST_NODE_ID == "skeleton_driver";

(Source: skeleton_driver.bind)

In the BUILD.gn file, include the following line to import the bind build rules:
```
import("//build/bind/bind.gni")
```
In the BUILD.gn file, add a target for the driver's bind rules, for example:
```
driver_bind_rules("bind") {
  rules = "skeleton.bind"
  bind_output = "skeleton_driver.bindbc"
  deps = [ "//examples/drivers/bind_library:gizmo.example" ]
}
```
(Source: BUILD.gn)

The bind_output field must be unique among all drivers.

Note: To learn more about finding node properties and writing bind rules, see Bind Rules Tutorial.

Create a driver component

To create a Fuchsia component for the driver, do the following:

Create a new component manifest file (.cml) in the meta directory (for example, skeleton_driver.cml).

Include the following component shards:

{
    include: [
        "inspect/client.shard.cml",
        "syslog/client.shard.cml",
    ],
}

Add the driver's program information using the following format:

{
    program: {
        runner: "driver",
        binary: "driver/<OUTPUT_NAME>.so",
        bind: "meta/bind/<BIND_OUTPUT>",
    },
}

The binary field must match the output_name field in the fuchsia_driver target of the BUILD.gn file, and the bind field must match bind_output in the driver_bind_rules target, for example:

{
    include: [
        "inspect/client.shard.cml",
        "syslog/client.shard.cml",
    ],
    program: {
        runner: "driver",
        binary: "driver/skeleton_driver.so",
        bind: "meta/bind/skeleton.bindbc",
    },
}

(Source: skeleton_driver.cml)

Create a new JSON file to provide the component's information (for example, component-info.json).

Add the driver component's information in JSON format, for example:

{
    "short_description": "Driver Framework example for a skeleton DFv2 driver",
    "manufacturer": "",
    "families": [],
    "models": [],
    "areas": [
        "DriverFramework"
    ]
}

(Source: component-info.json)

In the BUILD.gn file, include the following line to import the component build rules:
```
import("//build/components.gni")
```
In the BUILD.gn file, add a target for the driver component, for example:
```
fuchsia_driver_component("component") {
  component_name = "skeleton"
  manifest = "meta/skeleton.cml"
  deps = [
     ":bind",
     ":driver"
  ]
  info = "component-info.json"
}
```
(Source: BUILD.gn)

See the rules for these fields below:
- Set the manifest field to the location of the driver's .cml file.
- Set the info field to the location of the driver component information JSON file.
- Set the deps array to include the fuchsia_driver and driver_bind_rules targets from the BUILD.gn file.

You can now build, load, and register this DFv2 driver in a Fuchsia system

Additional tasks

This section provides additional features you can add to your minimal DFv2 driver:

Add logs
Add a child node
Clean up the driver
Add a compat device server

Add logs

By default, to print logs from a DFv2 driver, use the FDF_LOG macro, for example:

FDF_LOG(INFO, "Starting SimpleDriver")

In addition to using the FDF_LOG macro, you can also print logs using Fuchsia's structured logger library (structured_logger.h), which uses the FDF_SLOG macro.

To use structured logs from your DFv2 driver, do the following:

Include the following header:

#include <lib/driver/logging/cpp/structured_logger.h>

Use the FDF_SLOG macro to print logs, for example:

FDF_SLOG(ERROR, "Failed to add child", KV("status", result.status_string()));

Add a child node

A DFv2 driver can add child nodes using the following Node protocol in the fuchsia.driver.framework FIDL library:

open protocol Node {
    flexible AddChild(resource struct {
        args NodeAddArgs;
        controller server_end:NodeController;
        node server_end:<Node, optional>;
    }) -> () error NodeError;
};

The driver can connect to the Node protocol using the node() value in the DriverStartArgs object or from the DriverBase class's node() function.

In addition to using the Node protocol, you need to create NodeController endpoints. The AddChild() method requires the server end of the fuchsia.driver.framework NodeController protocol. On the other hand, the driver is responsible for storing and keeping the client end of the protocol. If this client end is deallocated, the driver framework removes the child node.

To add a child node using the Node protocol, do the following:

In your DFv2 driver, create an NodeAddArgs object, which takes the following arguments:
```
type NodeAddArgs = resource table {
    /// Name of the node.
    1: name NodeName;
    2: offers vector<fuchsia.component.decl.Offer>:MAX_OFFER_COUNT;
    3: symbols vector<NodeSymbol>:MAX_SYMBOL_COUNT;
    4: properties NodePropertyVector;
    5: devfs_args DevfsAddArgs;
};
```
(Source: topology.fidl)
- name is the name of the child node.
- offers is the capabilities the parent is offering to the child node. You can create them with the MakeOffer() function in the node_adds_args library.
- symbols is the functions to be provided to the driver. It can be ignored for DFv2 drivers.
- properties is the child node properties, which determines which driver becomes bound to the child node (for more information, see Bind rules tutorial). You can create node properties with the MakeProperty() function in the node_adds_args library.
- devfs_args is required if the child node needs access to devfs.
The example below creates an NodeAddArgs object:
```
fidl::Arena arena;
auto properties = std::vector{fdf::MakeProperty(arena, bind_fuchsia_test::TEST_CHILD, "simple")};
  auto args = fuchsia_driver_framework::wire::NodeAddArgs::Builder(arena)
                  .name(arena, "simple_child")
                  .properties(arena, std::move(properties))
                  .Build();
```
Update the driver's DriverBase class to add a FIDL client object.

The example below uses a fidl::WireSyncClient object:
```
class SimpleDriver : public fdf::DriverBase {
<...>
private:
<...>
  fidl::WireSyncClient<fuchsia_driver_framework::NodeController> child_controller_;
};
```
This setup allows you to store the client end in the driver.

Create the server and client endpoints with the fidl::CreateEndpoints function, for example:

zx::result controller_endpoints =
    fidl::CreateEndpoints<fuchsia_driver_framework::NodeController>();
ZX_ASSERT_MSG(controller_endpoints.is_ok(), "Failed to create endpoints: %s",
              controller_endpoints.status_string());

Bind the client end to the FIDL client object, for example:
```
child_controller_.Bind(std::move(endpoints->client));
```
You can now connect to the Node server using the Node object,

Connect to the Node server and call the AddChild() method, for example:

 fidl::WireResult result =
      fidl::WireCall(node())->AddChild(args, std::move(controller_endpoints->server), {});

Putting all the steps together looks like the following example:

void SimpleDriver::Start(fdf::StartCompleter completer) {
  fidl::Arena arena;
  auto properties = std::vector{fdf::MakeProperty(arena, bind_fuchsia_test::TEST_CHILD, "skeleton")};
  auto args = fuchsia_driver_framework::wire::NodeAddArgs::Builder(arena)
                  .name(arena, "skeleton_child")
                  .properties(arena, std::move(properties))
                  .Build();

  auto controller_endpoints = fidl::Endpoints<fuchsia_driver_framework::NodeController>::Create();
  controller_.Bind(std::move(controller_endpoints.client));

  fidl::WireResult result =
      fidl::WireCall(node())->AddChild(args, std::move(controller_endpoints.server), {});
  if (!result.ok()) {
    FDF_LOG(ERROR, "Failed to add child %s", result.status_string());
    return completer(result.status());
  }

  completer(zx::ok());
}

Clean up the driver

If a DFv2 driver needs to perform teardowns before it is stopped (for example, stopping threads), then you need to override and implement additional DriverBase methods: PrepareStop() and Stop()

The PrepareStop() function is called before the driver's fdf dispatchers are shut down and the driver is deallocated. Therefore, the driver needs to implement PrepareStop()if it needs to perform certain operations before the driver's dispatchers shut down, for example:

void SimpleDriver::PrepareStop(fdf::PrepareStopCompleter completer) {
 // Teardown threads
  FDF_LOG(INFO, "Preparing to stop SimpleDriver");
  completer(zx::ok());
}

The Stop() function is called after all dispatchers belonging to this driver are shut down, for example:

void SimpleDriver::Stop() {
  FDF_LOG(INFO, "Stopping SimpleDriver");
}

Add a compat device server

If your DFv2 driver has descendant DFv1 drivers that haven't yet migrated to DFv2, you need to use the compatibility shim to enable your DFv2 driver to talk to other DFv1 drivers in the system. For more details, see the Set up the compat device server in a DFv2 driver guide.