Driver binding

For writing bind rules, refer to the Bind Rules Tutorial.

In Fuchsia, the driver framework maintains a tree of drivers and devices in the system. In this tree, a device represents access to some hardware available to the OS. A driver both publishes and binds to devices. For example, a USB driver might bind to a PCI device (its parent) and publish an ethernet device (its child). In order to determine which devices a driver can bind to, each driver has a bind rule and each device has a set of properties. The bind rule defines a condition that matches the properties of devices that it wants to bind to.

Bind rules and the conditions they refer to are defined by a domain specific language. The bind compiler consumes this language and produces bytecode for bind rules. The language has two kinds of source files: rules, and libraries. Libraries are used to share property definitions between drivers and bind rules. The compiler also produces FIDL files from bind libraries so that drivers may refer to device properties in code.

One thing to note about this stage of the migration is that there is no support for defining device property keys in bind libraries (see below). Instead, the keys from the old driver binding system (lib/ddk/binding.h) are available to be extended. These keys are hardcoded into the bind compiler and are available under the fuchsia namespace. For example, the PCI vendor ID key is fuchsia.BIND_PCI_VID. Eventually the hardcoded keys will be removed from this namespace and all node property keys will be defined in bind libraries.

The compiler

The compiler takes a list of library sources, and one rule source. For example:

fx bindc compile \
  --include src/devices/bind/fuchsia.usb/fuchsia.usb.bind \
  --output tools/bindc/examples/gizmo.h \

Currently, it produces a C header file that may be included by a driver. The header file defines a macro:

ZIRCON_DRIVER(Driver, Ops, VendorName, Version);
  • Driver is the name of the driver.
  • Ops is a zx_driver_ops, which are the driver operation hooks
  • VendorName is a string representing the name of the driver vendor.
  • Version is a string representing the version of the driver.

For more details, see the driver development documentation.

Bind rules

A bind rule defines the conditions to call a driver's bind() hook. Each statement in the bind rule is a condition over the properties of the device that must hold true in order for the driver to bind. If the bind rules finish executing and all conditions are true, then the device coordinator will call the driver's bind() hook.

A bind rule should be thought of as a declarative expression of the conditions under which a driver should bind. As such, the order of execution of condition expressions is not relevant to its final evaluation. It may help to consider the bind rule to be a Boolean formula.

There are four kinds of statements:

  • Condition statements are equality (or inequality) expressions of the form <key> == <value> (or <key> != <value>).
  • Accept statements are lists of permissible values for a given key.
  • If statements provide simple branching.
  • True and false statements can be used to explicitly evaluate a bind rule.


This example bind rule can be found at //tools/bindc/examples/gizmo.bind.

using fuchsia.usb;

// The device must be a USB device.

if fuchsia.BIND_USB_VID == fuchsia.usb.BIND_USB_VID.INTEL {
  // If the device's vendor is Intel, the device class must be audio.
  fuchsia.BIND_USB_CLASS == fuchsia.usb.BIND_USB_CLASS.AUDIO;
} else if fuchsia.BIND_USB_VID == fuchsia.usb.BIND_USB_VID.REALTEK {
  // If the device's vendor is Realtek, the device class must be one of the following values:
  accept fuchsia.BIND_USB_CLASS {
} else {
  // If the vendor is neither Intel or Realtek, do not bind.

Language restrictions

There are some restrictions on the language that are imposed to improve readability and ensure that bind rules are simple representations of the conditions under which a driver should bind.

  • Empty blocks are not allowed. It's ambiguous whether an empty block should mean that the driver will bind or abort. The author should use an explicit true or false statement.

  • If statements must have else blocks and are terminal. This restriction increases readability by making explicit the branches of execution. Since no statement may follow an if statement, it is easy to trace a path through the bind rules.

  • True and false statements must be the only statement in their scope. Bind rules are not imperative programs and the order of evaluation is not important. Mixing boolean statements (particularly true) with other conditions may lead to situations where this is not clear.


rule = using-list , ( statement )+ ;

using-list = ( using , ";" )* ;

using = "using" , compound-identifier , ( "as" , IDENTIFIER ) ;

statement = condition , ";" | accept | if-statement | true | false ;

condition = compound-identifier , condition-op , value ;

condition-op = "==" | "!=" ;

accept = "accept" , compound-identifier , "{" ( value , "," )+ "}" ;

if-statement = "if" , condition , "{" , ( statement )+ , "}" ,
                ( "else if" , "{" , ( statement )+ , "}" )* ,
                "else" , "{" , ( statement )+ , "}" ;

true = "true" , ";" ;

false = "false" , ";" ;

compound-identifier = IDENTIFIER ( "." , IDENTIFIER )* ;

value = compound-identifier | STRING-LITERAL | NUMERIC-LITERAL | "true" | "false" ;

An identifier matches the regex [a-zA-Z]([a-zA-Z0-9_]*[a-zA-Z0-9])? and must not match any keyword. The list of keywords is:


A string literal matches the regex ”[^”]*”, and a numeric literal matches the regex [0-9]+ or 0x[0-9A-F]+.

The bind compiler will ignore (treat as whitespace) any line prefixed by //, and any multiple lines delimited by /* and */.

Composite Bind

In addition to binding drivers to devices, drivers in Fuchsia can use bind rules to create a composite device from nodes. The bind rules follow the same language specification as non-composite bind, but separated into nodes that contain a name and set of statements.

There must be exactly one primary node in the bind rules. The composite driver will be started in the same driver host as the primary node.

An example composite bind rule file can be found at //tools/bindc/examples/composite-gizmo.bind.

composite gizmo_sysmem;

using fuchsia.platform;
using fuchsia.sysmem;
using fuchsia.tee;

primary node "sysmem" {
  fuchsia.BIND_PROTOCOL == fuchsia.sysmem.BIND_PROTOCOL.DEVICE;

node "tee" {
  if fuchsia.BIND_PROTOCOL == fuchsia.tee.BIND_PROTOCOL.DEVICE {
  } else {
    fuchsia.BIND_PLATFORM_DEV_VID == fuchsia.platform.BIND_PLATFORM_DEV_VID.QEMU;

The grammar for composite bind is:

composite-bind-rules = [composite-device], using-list , ( node )+ ;

composite-device = “composite” , IDENTIFIER;

node = [ "primary" ], "node" , STRING-LITERAL , "{" , ( statement )+ , "}"

Build targets

To declare bind rules within the Fuchsia build system, use the following build target:

driver_bind_rules("bind") {
  rules = <bind rules filename>
  bind_output = <generated bind binary filename>
  deps = [ <list of bind library targets> ]

For more details, refer to //build/bind/bind.gni.


The bind compiler supports a data-driven unit test framework for bind rules that allows you to test your bind rules in isolation from the driver. A test case for a bind rule consists of a device specification and an expected result, i.e. bind or abort. Test cases are passed to the bind compiler in the form of JSON specification files and the compiler executes each test case by running the debugger.

The JSON specification must be a list of test case objects, where each object contains:

  • name A string for the name of the test case.
  • expected The expected result. Must be “match” or “abort”.
  • device A list of string key value pairs describing the properties of a device. This is similar to the debugger's device specifications (see this example).

If the test is for a composite device, then each node in the device can have a list of test case objects. The JSON specification for the unit tests will be a list of node objects instead. Each node object contains:

  • node A string for the node name. It must match a node in the bind rules tests.
  • tests A list of test case objects.


This is an example test case, the full set of tests is at //tools/bindc/examples/test.json. This case checks that the bind rules match a device with the listed properties, i.e. an Intel USB audio device.

    "name": "Intel",
    "expected": "match",
    "device": {
      "fuchsia.BIND_PROTOCOL": "fuchsia.usb.BIND_PROTOCOL.INTERFACE",
      "fuchsia.BIND_USB_VID": "fuchsia.usb.BIND_USB_VID.INTEL",
      "fuchsia.BIND_USB_CLASS": "fuchsia.usb.BIND_USB_CLASS.AUDIO"

Here is an example of a composite bind node with test cases. The full set of tests is located in `//tools/bindc/examples/composite-tests.json. Each test case checks if the node’s bind rules match a device with the listed properties.

        "node": "sysmem",
        "tests": [
                "name": "Match",
                "expected": "match",
                "device": {
                    "fuchsia.BIND_PROTOCOL": "fuchsia.sysmem.BIND_PROTOCOL.DEVICE"
                "name": "Abort sysmem",
                "expected": "abort",
                "device": {
                    "fuchsia.BIND_PROTOCOL": "fuchsia.tee.BIND_PROTOCOL.DEVICE"


Define a test build target like so

bind_test("example_bind_test") {
  rules = <bind rules filename>
  tests = <test specification filename>
  deps = [ <list of bind library targets> ]

Alternatively, you can simply add a tests argument to your existing bind_rules to generate a test target. It’s name will be the original target’s name plus _test. For example, the following would generate example_bind_test.

driver_bind_rules("example_bind") {
  rules = "gizmo.bind"
  bind_output = “gizmo.bindbc”
  tests = "tests.json"
  deps = [ "//src/devices/bind/fuchsia.usb" ]


If you have defined a build target for your test then you can run the tests as usual with fx test.

fx test example_bind_test

Otherwise you can run the bind tool directly. For example:

fx bindc test \
  tools/bindc/examples/gizmo.bind \
  --test-spec tools/bindc/examples/tests.json \
  --include src/devices/bind/fuchsia.usb/fuchsia.usb.bind

Bind libraries

A bind library defines a set of properties that drivers may assign to their children. Also, bind rules may refer to bind libraries.


A bind library begins by defining its namespace:

library <vendor>.<library>;

Every namespace must begin with a vendor and each vendor should ensure that there are no clashes within their own namespace. However, the language allows for one vendor to extend the library of another. Google will use fuchsia for public libraries.

Any values introduced by a library are namespaced. For example, the following library defines a new PCI device ID GIZMO_VER_1.

library gizmotronics.gizmo;

using fuchsia.pci as pci;

extend uint pci.device_id {
  GIZMO_VER_1 = 0x4242,

To refer to this value the driver author should use the fully qualified name, as follows.

using fuchsia.pci as pci;
using gizmotronics.gizmo;

pci.device_id == gizmotronics.gizmo.device_id.GIZMO_VER_1

Keys and values

Device property definitions look similar to variable declarations in other languages.

<type> <name>;
<type> <name> {

A bind library may also extend properties from other libraries.

extend <type> <name> {

Each key has a type, and all values that correspond to that key must be of that type. The language supports primitive types: one of uint, string, or bool; and enumerations (enum). When defining keys you should prefer enumerations except when values will be provided by an external source, such as hardware.

When defining a primitive value use the form <identifier> = <literal>, and for enumerations only an identifier is necessary. It is valid to define multiple primitive values with the same literal.


library = library-header , using-list , declaration-list ;

library-header = "library" , compound-identifier , ";" ;

using-list = ( using , ";" )* ;

using = "using" , compound-identifier , ( "as" , IDENTIFIER ) ;

compound-identifier = IDENTIFIER ( "." , IDENTIFIER )* ;

declaration-list = ( declaration , ";" )* ;

declaration = primitive-declaration | enum-declaration ;

primitive-declaration = ( "extend" ) , type , compound-identifier ,
                        ( "{" primitive-value-list "}" ) ;

type = "uint" | "string" | "bool";

primitive-value-list = ( IDENTIFIER , "=" , literal , "," )* ;

enum-declaration = ( "extend" ) , "enum" , compound-identifier ,
                   ( "{" , enum-value-list , "}" ) ;

enum-value-list = ( IDENTIFIER , "," )* ;

literal = STRING-LITERAL | NUMERIC-LITERAL | "true" | "false" ;

An identifier matches the regex [a-zA-Z]([a-zA-Z0-9_]*[a-zA-Z0-9])? and must not match any keyword. The list of keywords is:


A string literal matches the regex ”[^”]*”, and a numeric literal matches the regex [0-9]+ or 0x[0-9A-F]+.

The bind compiler will ignore (treat as whitespace) any line prefixed by //, and any multiple lines delimited by /* and */.

Build targets

To declare a bind library within the Fuchsia build system, use the following build target:

bind_library(<library name>) {
  source = <bind library filename>
  public_deps = [ <list of bind library targets> ]

For more details, refer to //build/bind/bind.gni.