Device drivers are loaded into driver host processes when it is determined they are needed. What determines if they are loaded or not is the binding program, which is a description of what device a driver can bind to. The binding program is defined using a small domain specific language, which is compiled to bytecode that is distributed with the driver.

An example binding program from the Intel Ethernet driver:

fuchsia.device.protocol == fuchsia.pci.protocol.PCI_DEVICE;
fuchsia.pci.vendor == fuchsia.pci.vendor.INTEL;
accept fuchsia.pci.device {
    0x100E, // Qemu
    0x15A3, // Broadwell
    0x1570, // Skylake
    0x1533, // I210 standalone
    0x15b7, // Skull Canyon NUC
    0x15b8, // I219
    0x15d8, // Kaby Lake NUC
}

The bind compiler takes a binding program and outputs a C header file that defines a macro, ZIRCON_DRIVER. The ZIRCON_DRIVER macro includes the necessary compiler directives to put the binding program into an ELF NOTE section, allowing it to be inspected by the Device Coordinator without needing to fully load the driver into its process.

The second parameter to ZIRCON_DRIVER is a zx_driver_ops_t structure pointer (defined by lib/ddk/driver.h, which defines the init, bind, create, and release methods.

init() is invoked when a driver is loaded into a Driver Host process and allows for any global initialization. Typically none is required. If the init() method is implemented and fails, the driver load will fail.

bind() is invoked to offer the driver a device to bind to. The device is one that has matched the bind rules the driver has published. If the bind() method succeeds, the driver must create a new device and add it as a child of the device passed in to the bind() method. See Device Lifecycle for more information.

create() is invoked for platform/system bus drivers or proxy drivers. For the vast majority of drivers, this method is not required.

release() is invoked before the driver is unloaded, after all devices it may have created in bind() and elsewhere have been destroyed. Currently this method is never invoked. Drivers, once loaded, remain loaded for the life of a Driver Host process.

Device Lifecycle

Within a Driver Host process, devices exist as a tree of zx_device_t structures, which are opaque to the driver. These are created with device_add(), which the driver provides a zx_protocol_device_t structure to. The methods defined by the function pointers in this structure are the "device ops". The various structures and functions are defined in device.h

The device_add() function creates a new device, adding it as a child to the provided parent device. That parent device must be either the device passed in to the bind() method of a device driver, or another device which has been created by the same device driver.

A side-effect of device_add() is that the newly created device will be added to the global Device Filesystem maintained by the Device Coordinator. If the device has not implemented an init() hook, the device will be immediately accessible through opening its node in devfs.

The init() hook is invoked following device_add(). This is useful for drivers that have to do extended initialization or probing and do not want to visibly publish their device(s) until that succeeds (and quietly remove them if that fails). The driver should call device_init_reply() once they have completed initialization. This reply does not necessarily need to be called from the init() hook. The device will remain invisible and is guaranteed not to be removed until this point.

Devices are reference counted. A reference is acquired when a driver creates the device with device_add() and when the device is opened by a remote process through the Device Filesystem.

From the moment that device_init_reply() is called, or device_add() is called without an implemented init() hook, other device ops may be called by the Driver Host.

When device_async_remove() is called on a device, this schedules the removal of the device and its descendants.

The removal of a device consists of four parts: running the device's unbind() hook, removal of the device from the Device Filesystem, dropping the reference acquired by device_add() and running the device's release() hook.

When the unbind() method is invoked, this signals to the driver it should start shutting the device down, and call device_unbind_reply() once it has finished unbinding. Unbind also acts as a hard barrier for FIDL transactions. The FDF will not permit any new FIDL transactions or connections to be created when Unbind is called. Drivers are responsible for closing or replying to any outstanding transactions in their unbind hook if they handle FIDL messages. This is an optional hook. If it is not implemented, it is treated as device_unbind_reply() was called immediately. When device_unbind_reply is called, all FIDL connections will be terminated.

Since a child device may have work in progress when its unbind() method is called, it's possible that the parent device (which already completed unbinding) could continue to receive device method calls or protocol method calls on behalf of that child. It is advisable that before completing unbinding, the parent device should arrange for these methods to return errors, so that calls from a child before the child removal is completed do not start more work or cause unexpected interactions.

The release() method is only called after the creating driver has completed unbinding, all open instances of that device have been closed, and all children of that device have been unbound and released. This is the last opportunity for the driver to destroy or free any resources associated with the device. It is not valid to refer to the zx_device_t for that device after release() returns. Calling any device methods or protocol methods for protocols obtained from the parent device past this point is illegal and will likely result in a crash.

An Example of the Tear-Down Sequence

To explain how the unbind() and release() work during the tear-down process, below is an example of how a USB WLAN driver would usually handle it. In short, the unbind() call sequence is top-down while the release() sequence is bottom-up.

Note that this is just an example. This might not match what exactly the real WLAN driver is doing.

Assume a WLAN device is plugged in as a USB device, and a PHY interface has been created under the USB device. In addition to the PHY interface, 2 MAC interfaces have been created under the PHY interface.

            +------------+
            | USB Device | .unbind()
            +------------+ .release()
                  |
            +------------+
            |  WLAN PHY  | .unbind()
            +------------+ .release()
              |        |
    +------------+  +------------+
    | WLAN MAC 0 |  | WLAN MAC 1 | .unbind()
    +------------+  +------------+ .release()

Now, we unplug this USB WLAN device.

• The USB XHCI detects the removal and calls device_async_remove(usb_device).

• This will lead to the USB device's unbind() being called. Once it completes unbinding, it would call device_unbind_reply().

    usb_device_unbind(void* ctx) {
        // Stop interrupt or anything to prevent incoming requests.
        ...

        device_unbind_reply(usb_dev);
    }

• When the USB device completes unbinding, the WLAN PHY's unbind() is called. Once it completes unbinding, it would call device_unbind_reply().

    wlan_phy_unbind(void* ctx) {
        // Stop interrupt or anything to prevent incoming requests.
        ...

        device_unbind_reply(wlan_phy);
    }

• When wlan_phy completes unbinding, unbind() will be called on all of its children (wlan_mac_0, wlan_mac_1).

    wlan_mac_unbind(void* ctx) {
        // Stop accepting new requests, and notify clients that this device is offline (often just
        // by returning a ZX_ERR_IO_NOT_PRESENT to any requests that happen after unbind).
        ...

        device_unbind_reply(iface_mac_X);
    }

• Once all the clients of a device have been removed, and that device has no children, its refcount will reach zero and its release() method will be called.

• WLAN MAC 0 and 1's release() are called.

    wlan_mac_release(void* ctx) {
        // Release sources allocated at creation.
        ...

        // Delete the object here.
        ...
    }

• The wlan_phy has no open connections, but still has child devices (wlan_mac_0 and wlan_mac_1). Once they have both been released, its refcount finally reaches zero and its release() method is invoked.

    wlan_phy_release(void* ctx) {
        // Release sources allocated at creation.
        ...

        // Delete the object here.
        ...
    }

• Once the USB device now has no child devices or open connections, its release() would be called.