The Device Protocol

Please refer to the header comments for descriptions of the methods.

Hook ordering guarantees

Hook ordering guarantees

The hooks that a driver implements will be invoked by other drivers and by the runtime. These invocations in some occasions may occur in parallel with invocations of other or even the same hook. This section will describe the ordering properties that you may rely on.


This section uses the terms unsequenced, indeterminately sequenced, and sequenced before as they are used in the C++ execution model.

Driver Initialization

The zx_driver_ops_t init hook will execute completely before any other hooks for that driver.

Driver Teardown

The zx_driver_ops_t release hook will begin execution only after all devices created by this driver have been released.

Driver Bind

If tests are enabled, the zx_driver_ops_t bind hook will begin execution only after the run_unit_tests hook.

Device Lifecycle

The device lifecycle begins when some driver successfully invokes device_add(). This may occur on any thread. No zx_device_ops_t hooks will run before the device's lifecycle has begun or after it has ended.

The device lifecycle ends when the device's release hook has begun executing.

The zx_device_ops_t hooks are unsequenced with respect to each other unless otherwise specified.

Note: This means that any code that occurs after a call to device_add(), even in bind hooks, is unsequenced with respect to the end of the created device's lifecycle.

Device Connection Lifecycle

A device connection lifecycle begins when the zx_device_ops_t open hook begins executing. None of the zx_device_ops_t read/write/message/close hooks will be invoked if the number of alive device connections is 0.

A device connection lifecycle ends when the zx_device_ops_t close hook begins executing. Any execution of read/write/message hooks is sequenced before this.

Since the read/write/message hooks only execute on the devhost's main thread, they will never be executed concurrently but the processing of outstanding requests from different connections will be indeterminately sequenced.

Device Power Management

The zx_device_ops_t suspend hook is sequenced before itself (e.g. if a request to suspend to D1 happens, and while that is being executed a request to suspend to D2 happens, the first will finish before the latter begins). It is also sequenced before the resume hook.

The set_performance_state hook is sequenced before itself. It has no particular ordering with suspend/resume hooks. After the driver returns from the set_performance_state hook with success, it is assumed by power manager that the device is operating at the requested performance state whenever the device is in working state. Since the hook only executes on the devhost's main thread, multiple requests are not executed concurrently. On success, the out_state and the requested_state is same. If the device is in a working state, the performance state will be changed to requested_state immediately. If the device is in non-working state, the performance state will be the requested_state whenever the device transitions to working state. On failure, the out_state will have the state that the device can go into.

The configure_autosuspend hook is sequenced before itself and is used to configure whether devices can suspend or resume themselves depending on their idleness. The hook is called with the deepest sleep state the device is expected to be in which is when the device is suspended. If the entire system is being suspended to a sleep state, the driver should expect suspend hook to be called, even if the auto suspend is configured. It is not supported to selectively suspend a device when auto suspend is configured.

Misc Device APIs

The zx_device_ops_t get_size and get_protocol hooks are unsequenced with respect to all hooks (including concurrent invocations of themselves). The one exception to this is that they are sequenced before the release hook.