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:
For more DFv2-related features, see Additional tasks.
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
)
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.
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
)
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.
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.
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:
manifest
field to the location of the driver's .cml
file.info
field to the location of the driver component information JSON file.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
Note: The instructions below are based on the Simple driver.
This section provides additional features you can add to your minimal DFv2 driver:
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()));
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()); }
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"); }
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.