Zircon Driver Development

Zircon drivers are shared libraries that are dynamically loaded in Device Host processes in user space. The process of loading a driver is controlled by the Device Coordinator. See Device Model for more information on Device Hosts, Device Coordinator and the driver and device lifecycles.

Directory structure

Zircon drivers are found under system/dev. They are grouped based on the protocols they implement. The driver protocols are defined in ddk/include/ddk/protodefs.h. For example, a USB ethernet driver goes in system/dev/ethernet rather than system/dev/usb because it implements an ethernet protocol. However, drivers that implement the USB stack are in system/dev/usb because they implement USB protocols.

In the driver's rules.mk, the MODULE_TYPE should be driver. This will install the driver shared lib in /boot/driver/.

If your driver is built outside Zircon, install them in /system/driver/ . The Device Coordinator looks in those directories for loadable drivers.

Declaring a driver

At a minimum, a driver should contain the driver declaration and implement the bind() driver op.

Drivers are loaded and bound to a device when the Device Coordinator successfully finds a matching driver for a device. A driver declares the devices it is compatible with via bindings. The following bind program declares the AHCI driver:

ZIRCON_DRIVER_BEGIN(ahci, ahci_driver_ops, "zircon", "0.1", 4)

The AHCI driver has 4 directives in the bind program. "zircon" is the vendor id and "0.1" is the driver version. It binds with ZX_PROTOCOL_PCI devices with PCI class 1, subclass 6, interface 1.

The PCI driver publishes the matching device with the following properties:

zx_device_prop_t device_props[] = {
    {BIND_PCI_VID, 0, info.vendor_id},
    {BIND_PCI_DID, 0, info.device_id},
    {BIND_PCI_CLASS, 0, info.base_class},
    {BIND_PCI_SUBCLASS, 0, info.sub_class},
    {BIND_PCI_INTERFACE, 0, info.program_interface},
    {BIND_PCI_REVISION, 0, info.revision_id},
    {BIND_PCI_BDF_ADDR, 0, BIND_PCI_BDF_PACK(info.bus_id, info.dev_id,

Binding variables and macros are defined in zircon/driver/binding.h. If you are introducing a new device class, you may need to introduce new binding variables in that file. Binding variables are 32-bit values. If your variable value requires greater than a 32-bit value, split them into multiple 32-bit variables. An example is ACPI HID values, which are 8 characters (64-bits) long. It is split into BIND_ACPI_HID_0_3 and BIND_ACPI_HID_4_7.

Binding directives are evaluated sequentially. The branching directives BI_GOTO() and BI_GOTO_IF() allow you to jump forward to the matching label, defined by BI_LABEL().

BI_ABORT_IF_AUTOBIND may be used (usually as the first instruction) to prevent the default automatic binding behaviour. In that case, a driver can be bound to a device using fuchsia.device.Controller/Bind FIDL call

Driver binding

A driver’s bind() function is called when it is matched to a device. Generally a driver will initialize any data structures needed for the device and initialize hardware in this function. It should not perform any time-consuming tasks or block in this function, because it is invoked from the devhost's RPC thread and it will not be able to service other requests in the meantime. Instead, it should spawn a new thread to perform lengthy tasks.

The driver should make no assumptions about the state of the hardware in bind(), resetting the hardware or otherwise ensuring it is in a known state. Because the system recovers from a driver crash by re-spawning the devhost, the hardware may be in an unknown state when bind() is invoked.

A driver is required to publish a zx_device_t in bind() by calling device_add(). This is necessary for the Device Coordinator to keep track of the device lifecycle. If the driver is not able to publish a functional device in bind(), for example if it is initializing the full device in a thread, it should publish an invisible device, and make this device visible when initialization is completed. See DEVICE_ADD_INVISIBLE and device_make_visible() in zircon/ddk/driver.h.

There are generally four outcomes from bind():

  1. The driver determines the device is supported and does not need to do any heavy lifting, so publishes a new device via device_add() and returns ZX_OK.

  2. The driver determines that even though the bind program matched, the device cannot be supported (maybe due to checking hw version bits or whatnot) and returns an error.

  3. The driver needs to do further initialization before the device is ready or it’s sure it can support it, so it publishes an invisible device and kicks off a thread to keep working, while returning ZX_OK. That thread will eventually make the device visible or, if it cannot successfully initialize it, remove it.

  4. The driver represents a bus or controller with 0..n children which may dynamically appear or disappear. In this case it should publish a device immediately representing the bus or controller, and then dynamically publish children (that downstream drivers will bind to) representing hardware on that bus. Examples: AHCI/SATA, USB, etc.

After a device is added and made visible by the system, it is made available to client processes and for binding by compatible drivers.

Device protocols

A driver provides a set of device ops and optional protocol ops to a device. Device ops implement the device lifecycle methods and the external interface to the device that are called by other user space applications and services. Protocol ops implement the ddk-internal protocols of the device that are called by other drivers.

You can pass one set of protocol ops for the device in device_add_args_t. If a device supports multiple protocols, implement the get_protocol() device op. A device can only have one protocol id. The protocol id corresponds to the class the device is published under in devfs.

Device protocol headers are found in ddk/protocol/. Ops and any data structures passed between drivers should be defined in this header.

Driver operation

A driver generally operates by servicing client requests from children drivers or other processes. It fulfills those requests either by communicating directly with hardware (for example, via MMIO) or by communicating with its parent device (for example, queuing a USB transaction).

External client requests from processes outside the devhost are fulfilled by the device ops message(), read(), write(), and ioctl(). Requests from children drivers, generally in the same process, are fulfilled by device protocols corresponding to the device class. Driver-to-driver requests should use device protocols instead of device ops.

A device can get a protocol supported by its parent by calling device_get_protocol() on its parent device.

Device interrupts

Device interrupts are implemented by interrupt objects, which are a type of kernel objects. A driver requests a handle to the device interrupt from its parent device in a device protocol method. The handle returned will be bound to the appropriate interrupt for the device, as defined by a parent driver. For example, the PCI protocol implements map_interrupt() for PCI children. A driver should spawn a thread to wait on the interrupt handle.

The kernel will automatically handle masking and unmasking the interrupt as appropriate, depending on whether the interrupt is edge-triggered or level-triggered. For level-triggered hardware interrupts, zx_interrupt_wait() will mask the interrupt before returning and unmask the interrupt when it is called again the next time. For edge-triggered interrupts, the interrupt remains unmasked.

The interrupt thread should not perform any long-running tasks. For drivers that perform lengthy tasks, use a worker thread.

You can signal an interrupt handle with zx_interrupt_signal() on slot ZX_INTERRUPT_SLOT_USER to return from zx_interrupt_wait(). This is necessary to shut down the interrupt thread during driver clean up.

FIDL Messages

Messages for each device class are defined in the FIDL language. Each device implements zero or more FIDL protocols, multiplexed over a single channel per client. The driver is given the opportunity to interpret FIDL messages via the message() hook.


Ioctls are deprecated in favor of messages. No new ioctls should be implemented.

Ioctls for each device class are defined in zircon/device/. Ioctls may accept or return handles. The IOCTL_KIND_* defines in zircon/device/ioctl.h, used in the ioctl declaration, defines whether the ioctl accepts or returns handles and how many. The driver owns the handles passed in and should close the handles when they’re no longer needed, unless it returns ZX_ERR_NOT_SUPPORTED in which case the devhost RPC layer will close the handles.

Protocol ops vs. FIDL messages

Protocol ops define the DDK-internal API for a device. FIDL messages define the external API. Define a protocol op if the function is meant to be called by other drivers. A driver should call a protocol op on its parent to make use of those functions.

Isolate devices

Devices that are added with DEVICE_ADD_MUST_ISOLATE spawn a new proxy devhost. The device exists in both the parent devhost and as the root of the new devhost. Devmgr attempts to load .proxy.so into this proxy devhost. For example, PCI is supplied by libpci.so so devmgr would look to load libpci.proxy.so. The driver is provided a channel in create() when it creates the proxy device (the “bottom half” that runs in the new devhost). The proxy device should cache this channel for when it needs to communicate with the top half (e.g. if it needs to call API on the parent device).

rxrpc() is invoked on the top half when this channel is written to by the bottom half. There is no common wire protocol for this channel. For an example, refer to the PCI driver.

NOTE: This is a mechanism used by various bus devices and not something general drivers should have to worry about. (please ping swetland if you think you need to use this)


ddk/debug.h defines the zxlogf(<log_level>,...) macro. The log messages are printed to the system debuglog over the network and on the serial port if available for the device. By default, ERROR and INFO are always printed. You can control the log level for a driver by passing the boot cmdline driver.<driver_name>.log=+<level>,-<level>. For example, driver.sdhci.log=-info,+trace,+spew enables the TRACE and SPEW logs and disable the INFO logs for the sdhci driver.

The log levels prefixed by “L” (LERROR, LINFO, etc.) do not get sent over the network and is useful for network logging.

Driver testing

ZX_PROTOCOL_TEST provides a mechanism to test drivers by running the driver under test in an emulated environment. Write a driver that binds to a ZX_PROTOCOL_TEST device. This driver should publish a device that the driver under test can bind to, and it should implement the protocol functions the driver under test invokes in normal operation. This helper driver should be declared with BI_ABORT_IF_AUTOBIND in the bindings.

The test harness calls fuchsia.device.test.RootDevice.CreateDevice() on /dev/test/test, which will create a ZX_PROTOCOL_TEST device and return its path. Then it calls ioctl_device_bind() with the helper driver on the newly created device. This approach generally works better for mid-layer protocol drivers. It's possible to emulate real hardware with the same approach but it may not be as useful.

The functions defined in ddk/protocol/test.h are for testing libraries that run as part of a driver. For an example, refer to system/ulib/ddk/test. The test harness for these tests is system/utest/driver-tests/main.c

Driver rights

Although drivers run in user space processes, they have a more restricted set of rights than normal processes. Drivers are not allowed to access the filesystem, including devfs. That means a driver cannot interact with arbitrary devices. If your driver needs to do this, consider writing a service instead. For example, the virtual console is implemented by the virtcon service.

Privileged operations such as zx_vmo_create_contiguous() and zx_interrupt_create require a root resource handle. This handle is not available to drivers other than the system driver (ACPI on x86 systems and platform on ARM systems). A device should request its parent to perform such operations for it. Contact the author of the parent driver if its protocol does not address this use case.

Similarly, a driver is not allowed to request arbitrary MMIO ranges, interrupts or GPIOs. Bus drivers such as PCI and platform only return the resources associated to the child device.