Device Driver Model

Introduction

The Zephyr kernel supports a variety of device drivers. Whether a driver is available depends on the board and the driver.

The Zephyr device model provides a consistent device model for configuring the drivers that are part of a system. The device model is responsible for initializing all the drivers configured into the system.

Each type of driver (UART, SPI, I2C) is supported by a generic type API.

In this model the driver fills in the pointer to the structure containing the function pointers to its API functions during driver initialization. These structures are placed into the RAM section in initialization level order.

Standard Drivers

Device drivers which are present on all supported board configurations are listed below.

  • Interrupt controller: This device driver is used by the kernel’s interrupt management subsystem.

  • Timer: This device driver is used by the kernel’s system clock and hardware clock subsystem.

  • Serial communication: This device driver is used by the kernel’s system console subsystem.

  • Random number generator: This device driver provides a source of random numbers.

    Important

    Certain implementations of this device driver do not generate sequences of values that are truly random.

Synchronous Calls

Zephyr provides a set of device drivers for multiple boards. Each driver should support an interrupt-based implementation, rather than polling, unless the specific hardware does not provide any interrupt.

High-level calls accessed through device-specific APIs, such as i2c.h or spi.h, are usually intended as synchronous. Thus, these calls should be blocking.

Driver APIs

The following APIs for device drivers are provided by device.h. The APIs are intended for use in device drivers only and should not be used in applications.

DEVICE_INIT()
create device object and set it up for boot time initialization.
DEVICE_AND_API_INIT()
Create device object and set it up for boot time initialization. This also takes a pointer to driver API struct for link time pointer assignment.
DEVICE_NAME_GET()
Expands to the full name of a global device object.
DEVICE_GET()
Obtain a pointer to a device object by name.
DEVICE_DECLARE()
Declare a device object.

Driver Data Structures

The device initialization macros populate some data structures at build time which are split into read-only and runtime-mutable parts. At a high level we have:

struct device {
      struct device_config *config;
      void *driver_api;
      void *driver_data;
};

struct device_config {
      char    *name;
      int (*init)(struct device *device);
      const void *config_info;
  [...]
};

The config member is for read-only configuration data set at build time. For example, base memory mapped IO addresses, IRQ line numbers, or other fixed physical characteristics of the device. This is the config_info structure passed to the DEVICE_*INIT() macros.

The driver_data struct is kept in RAM, and is used by the driver for per-instance runtime housekeeping. For example, it may contain reference counts, semaphores, scratch buffers, etc.

The driver_api struct maps generic subsystem APIs to the device-specific implementations in the driver. It is typically read-only and populated at build time. The next section describes this in more detail.

Subsystems and API Structures

Most drivers will be implementing a device-independent subsystem API. Applications can simply program to that generic API, and application code is not specific to any particular driver implementation.

A subsystem API definition typically looks like this:

typedef int (*subsystem_do_this_t)(struct device *device, int foo, int bar);
typedef void (*subsystem_do_that_t)(struct device *device, void *baz);

struct subsystem_api {
      subsystem_do_this_t do_this;
      subsystem_do_that_t do_that;
};

static inline int subsystem_do_this(struct device *device, int foo, int bar)
{
      struct subsystem_api *api;

      api = (struct subsystem_api *)device->driver_api;
      return api->do_this(device, foo, bar);
}

static inline void subsystem_do_that(struct device *device, void *baz)
{
      struct subsystem_api *api;

      api = (struct subsystem_api *)device->driver_api;
      api->do_that(device, foo, bar);
}

A driver implementing a particular subsystem will define the real implementation of these APIs, and populate an instance of subsystem_api structure:

static int my_driver_do_this(struct device *device, int foo, int bar)
{
      ...
}

static void my_driver_do_that(struct device *device, void *baz)
{
      ...
}

static struct subsystem_api my_driver_api_funcs = {
      .do_this = my_driver_do_this,
      .do_that = my_driver_do_that
};

The driver would then pass my_driver_api_funcs as the api argument to DEVICE_AND_API_INIT(), or manually assign it to device->driver_api in the driver init function.

Note

Since pointers to the API functions are referenced in the driver_api struct, they will always be included in the binary even if unused; gc-sections linker option will always see at least one reference to them. Providing for link-time size optimizations with driver APIs in most cases requires that the optional feature be controlled by a Kconfig option.

Single Driver, Multiple Instances

Some drivers may be instantiated multiple times in a given system. For example there can be multiple GPIO banks, or multiple UARTS. Each instance of the driver will have a different config_info struct and driver_data struct.

Configuring interrupts for multiple drivers instances is a special case. If each instance needs to configure a different interrupt line, this can be accomplished through the use of per-instance configuration functions, since the parameters to IRQ_CONNECT() need to be resolvable at build time.

For example, let’s say we need to configure two instances of my_driver, each with a different interrupt line. In drivers/subsystem/subsystem_my_driver.h:

typedef void (*my_driver_config_irq_t)(struct device *device);

struct my_driver_config {
      u32_t base_addr;
      my_driver_config_irq_t config_func;
};

In the implementation of the common init function:

void my_driver_isr(struct device *device)
{
      /* Handle interrupt */
      ...
}

int my_driver_init(struct device *device)
{
      const struct my_driver_config *config = device->config->config_info;

      /* Do other initialization stuff */
      ...

      config->config_func(device);

      return 0;
}

Then when the particular instance is declared:

#if CONFIG_MY_DRIVER_0

DEVICE_DECLARE(my_driver_0);

static void my_driver_config_irq_0(void)
{
      IRQ_CONNECT(MY_DRIVER_0_IRQ, MY_DRIVER_0_PRI, my_driver_isr,
                  DEVICE_GET(my_driver_0), MY_DRIVER_0_FLAGS);
}

const static struct my_driver_config my_driver_config_0 = {
      .base_addr = MY_DRIVER_0_BASE_ADDR,
      .config_func = my_driver_config_irq_0
}

static struct my_driver_data_0;

DEVICE_AND_API_INIT(my_driver_0, MY_DRIVER_0_NAME, my_driver_init,
                    &my_driver_data_0, &my_driver_config_0, POST_KERNEL,
                    MY_DRIVER_0_PRIORITY, &my_driver_api_funcs);

#endif /* CONFIG_MY_DRIVER_0 */

Note the use of DEVICE_DECLARE() to avoid a circular dependency on providing the IRQ handler argument and the definition of the device itself.

Initialization Levels

Drivers may depend on other drivers being initialized first, or require the use of kernel services. The DEVICE_INIT() APIs allow the user to specify at what time during the boot sequence the init function will be executed. Any driver will specify one of five initialization levels:

PRE_KERNEL_1
Used for devices that have no dependencies, such as those that rely solely on hardware present in the processor/SOC. These devices cannot use any kernel services during configuration, since the kernel services are not yet available. The interrupt subsystem will be configured however so it’s OK to set up interrupts. Init functions at this level run on the interrupt stack.
PRE_KERNEL_2
Used for devices that rely on the initialization of devices initialized as part of the PRE_KERNEL_1 level. These devices cannot use any kernel services during configuration, since the kernel services are not yet available. Init functions at this level run on the interrupt stack.
POST_KERNEL
Used for devices that require kernel services during configuration. Init functions at this level run in context of the kernel main task.
APPLICATION
Used for application components (i.e. non-kernel components) that need automatic configuration. These devices can use all services provided by the kernel during configuration. Init functions at this level run on the kernel main task.

Within each initialization level you may specify a priority level, relative to other devices in the same initialization level. The priority level is specified as an integer value in the range 0 to 99; lower values indicate earlier initialization. The priority level must be a decimal integer literal without leading zeroes or sign (e.g. 32), or an equivalent symbolic name (e.g. \#define MY_INIT_PRIO 32); symbolic expressions are not permitted (e.g. CONFIG_KERNEL_INIT_PRIORITY_DEFAULT + 5).

System Drivers

In some cases you may just need to run a function at boot. Special SYS_* macros exist that map to DEVICE_*INIT() calls. For SYS_INIT() there are no config or runtime data structures and there isn’t a way to later get a device pointer by name. The same policies for initialization level and priority apply.

For SYS_DEVICE_DEFINE() you can obtain pointers by name, see power management section.

SYS_INIT()

SYS_DEVICE_DEFINE()

Error handling

In general, it’s best to use __ASSERT() macros instead of propagating return values unless the failure is expected to occur during the normal course of operation (such as a storage device full). Bad parameters, programming errors, consistency checks, pathological/unrecoverable failures, etc., should be handled by assertions.

When it is appropriate to return error conditions for the caller to check, 0 should be returned on success and a POSIX errno.h code returned on failure. See https://github.com/zephyrproject-rtos/zephyr/wiki/Naming-Conventions#return-codes for details about this.

API Reference

group device_model

Device Model APIs.

Defines

Z_DEVICE_MAX_NAME_LEN
DEVICE_INIT(dev_name, drv_name, init_fn, data, cfg_info, level, prio)

Create device object and set it up for boot time initialization.

This macro defines a device object that is automatically configured by the kernel during system initialization. Note that devices set up with this macro will not be accessible from user mode since the API is not specified; whenever possible, use DEVICE_AND_API_INIT instead.

Parameters
  • dev_name: Device name. This must be less than Z_DEVICE_MAX_NAME_LEN characters in order to be looked up from user mode with device_get_binding().
  • drv_name: The name this instance of the driver exposes to the system.
  • init_fn: Address to the init function of the driver.
  • data: Pointer to the device’s configuration data.
  • cfg_info: The address to the structure containing the configuration information for this instance of the driver.
  • level: The initialization level at which configuration occurs. Must be one of the following symbols, which are listed in the order they are performed by the kernel:
    • PRE_KERNEL_1: Used for devices that have no dependencies, such as those that rely solely on hardware present in the processor/SOC. These devices cannot use any kernel services during configuration, since they are not yet available.
    • PRE_KERNEL_2: Used for devices that rely on the initialization of devices initialized as part of the PRE_KERNEL_1 level. These devices cannot use any kernel services during configuration, since they are not yet available.
    • POST_KERNEL: Used for devices that require kernel services during configuration.
    • APPLICATION: Used for application components (i.e. non-kernel components) that need automatic configuration. These devices can use all services provided by the kernel during configuration.
  • prio: The initialization priority of the device, relative to other devices of the same initialization level. Specified as an integer value in the range 0 to 99; lower values indicate earlier initialization. Must be a decimal integer literal without leading zeroes or sign (e.g. 32), or an equivalent symbolic name (e.g. #define MY_INIT_PRIO 32); symbolic expressions are not permitted (e.g. CONFIG_KERNEL_INIT_PRIORITY_DEFAULT + 5).

DEVICE_AND_API_INIT(dev_name, drv_name, init_fn, data, cfg_info, level, prio, api)

Create device object and set it up for boot time initialization, with the option to set driver_api.

This macro defines a device object that is automatically configured by the kernel during system initialization. Note that devices set up with this macro will not be accessible from user mode since the API is not specified; whenever possible, use DEVICE_AND_API_INIT instead.

The driver api is also set here, eliminating the need to do that during initialization.

Parameters
  • dev_name: Device name. This must be less than Z_DEVICE_MAX_NAME_LEN characters in order to be looked up from user mode with device_get_binding().
  • drv_name: The name this instance of the driver exposes to the system.
  • init_fn: Address to the init function of the driver.
  • data: Pointer to the device’s configuration data.
  • cfg_info: The address to the structure containing the configuration information for this instance of the driver.
  • level: The initialization level at which configuration occurs. Must be one of the following symbols, which are listed in the order they are performed by the kernel:
    • PRE_KERNEL_1: Used for devices that have no dependencies, such as those that rely solely on hardware present in the processor/SOC. These devices cannot use any kernel services during configuration, since they are not yet available.
    • PRE_KERNEL_2: Used for devices that rely on the initialization of devices initialized as part of the PRE_KERNEL_1 level. These devices cannot use any kernel services during configuration, since they are not yet available.
    • POST_KERNEL: Used for devices that require kernel services during configuration.
    • APPLICATION: Used for application components (i.e. non-kernel components) that need automatic configuration. These devices can use all services provided by the kernel during configuration.
  • prio: The initialization priority of the device, relative to other devices of the same initialization level. Specified as an integer value in the range 0 to 99; lower values indicate earlier initialization. Must be a decimal integer literal without leading zeroes or sign (e.g. 32), or an equivalent symbolic name (e.g. #define MY_INIT_PRIO 32); symbolic expressions are not permitted (e.g. CONFIG_KERNEL_INIT_PRIORITY_DEFAULT + 5).
  • api: Provides an initial pointer to the API function struct used by the driver. Can be NULL.

DEVICE_DEFINE(dev_name, drv_name, init_fn, pm_control_fn, data, cfg_info, level, prio, api)

Create device object and set it up for boot time initialization, with the option to device_pm_control. In case of Device Idle Power Management is enabled, make sure the device is in suspended state after initialization.

This macro defines a device object that is automatically configured by the kernel during system initialization. Note that devices set up with this macro will not be accessible from user mode since the API is not specified; whenever possible, use DEVICE_AND_API_INIT instead.

The driver api is also set here, eliminating the need to do that during initialization.

Parameters
  • dev_name: Device name. This must be less than Z_DEVICE_MAX_NAME_LEN characters in order to be looked up from user mode with device_get_binding().
  • drv_name: The name this instance of the driver exposes to the system.
  • init_fn: Address to the init function of the driver.
  • data: Pointer to the device’s configuration data.
  • cfg_info: The address to the structure containing the configuration information for this instance of the driver.
  • level: The initialization level at which configuration occurs. Must be one of the following symbols, which are listed in the order they are performed by the kernel:
    • PRE_KERNEL_1: Used for devices that have no dependencies, such as those that rely solely on hardware present in the processor/SOC. These devices cannot use any kernel services during configuration, since they are not yet available.
    • PRE_KERNEL_2: Used for devices that rely on the initialization of devices initialized as part of the PRE_KERNEL_1 level. These devices cannot use any kernel services during configuration, since they are not yet available.
    • POST_KERNEL: Used for devices that require kernel services during configuration.
    • APPLICATION: Used for application components (i.e. non-kernel components) that need automatic configuration. These devices can use all services provided by the kernel during configuration.
  • prio: The initialization priority of the device, relative to other devices of the same initialization level. Specified as an integer value in the range 0 to 99; lower values indicate earlier initialization. Must be a decimal integer literal without leading zeroes or sign (e.g. 32), or an equivalent symbolic name (e.g. #define MY_INIT_PRIO 32); symbolic expressions are not permitted (e.g. CONFIG_KERNEL_INIT_PRIORITY_DEFAULT + 5).
  • api: Provides an initial pointer to the API function struct used by the driver. Can be NULL.
Parameters
  • pm_control_fn: Pointer to device_pm_control function. Can be empty function (device_pm_control_nop) if not implemented.

DEVICE_NAME_GET(name)

Expands to the full name of a global device object.

Return the full name of a device object symbol created by DEVICE_INIT(), using the dev_name provided to DEVICE_INIT().

It is meant to be used for declaring extern symbols pointing on device objects before using the DEVICE_GET macro to get the device object.

Return
The expanded name of the device object created by DEVICE_INIT()
Parameters
  • name: The same as dev_name provided to DEVICE_INIT()

DEVICE_GET(name)

Obtain a pointer to a device object by name.

Return the address of a device object created by DEVICE_INIT(), using the dev_name provided to DEVICE_INIT().

Return
A pointer to the device object created by DEVICE_INIT()
Parameters
  • name: The same as dev_name provided to DEVICE_INIT()

DEVICE_DECLARE(name)

Declare a static device object.

This macro can be used at the top-level to declare a device, such that DEVICE_GET() may be used before the full declaration in DEVICE_INIT().

This is often useful when configuring interrupts statically in a device’s init or per-instance config function, as the init function itself is required by DEVICE_INIT() and use of DEVICE_GET() inside it creates a circular dependency.

Parameters
  • name: Device name

Typedefs

typedef void (*device_pm_cb)(struct device *dev, int status, void *context, void *arg)

Functions

void z_sys_device_do_config_level(s32_t level)
struct device *device_get_binding(const char *name)

Retrieve the device structure for a driver by name.

Device objects are created via the DEVICE_INIT() macro and placed in memory by the linker. If a driver needs to bind to another driver it can use this function to retrieve the device structure of the lower level driver by the name the driver exposes to the system.

Return
pointer to device structure; NULL if not found or cannot be used.
Parameters
  • name: device name to search for.

struct device_pm
#include <device.h>

Device PM info.

Parameters
  • dev: pointer to device structure
  • lock: lock to synchronize the get/put operations
  • enable: device pm enable flag
  • usage: device usage count
  • fsm_state: device idle internal power state
  • event: event object to listen to the sync request events
  • signal: signal to notify the Async API callers

struct device_config
#include <device.h>

Static device information (In ROM) Per driver instance.

Parameters
  • name: name of the device
  • init: init function for the driver
  • config_info: address of driver instance config information

struct device
#include <device.h>

Runtime device structure (In memory) Per driver instance.

Parameters
  • device_config: Build time config information
  • driver_api: pointer to structure containing the API functions for the device type. This pointer is filled in by the driver at init time.
  • driver_data: driver instance data. For driver use only