Device Drivers and Device 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.