20.2. Background

20.2.1. Device Driver Table

Each application utilizing the RTEMS I/O manager must specify the address of a Device Driver Table in its Configuration Table. This table contains each device driver’s entry points that is to be initialised by RTEMS during initialization. Each device driver may contain the following entry points:

  • Initialization

  • Open

  • Close

  • Read

  • Write

  • Control

If the device driver does not support a particular entry point, then that entry in the Configuration Table should be NULL. RTEMS will return RTEMS_SUCCESSFUL as the executive’s and zero (0) as the device driver’s return code for these device driver entry points.

Applications can register and unregister drivers with the RTEMS I/O manager avoiding the need to have all drivers statically defined and linked into this table.

The confdefs.h entry CONFIGURE_MAXIMUM_DRIVERS configures the number of driver slots available to the application.

20.2.2. Major and Minor Device Numbers

Each call to the I/O manager must provide a device’s major and minor numbers as arguments. The major number is the index of the requested driver’s entry points in the Device Driver Table, and is used to select a specific device driver. The exact usage of the minor number is driver specific, but is commonly used to distinguish between a number of devices controlled by the same driver.

The data types rtems_device_major_number and rtems_device_minor_number are used to manipulate device major and minor numbers, respectively.

20.2.3. Device Names

The I/O Manager provides facilities to associate a name with a particular device. Directives are provided to register the name of a device and to look up the major/minor number pair associated with a device name.

20.2.4. Device Driver Environment

Application developers, as well as device driver developers, must be aware of the following regarding the RTEMS I/O Manager:

  • A device driver routine executes in the context of the invoking task. Thus if the driver blocks, the invoking task blocks.

  • The device driver is free to change the modes of the invoking task, although the driver should restore them to their original values.

  • Device drivers may be invoked from ISRs.

  • Only local device drivers are accessible through the I/O manager.

  • A device driver routine may invoke all other RTEMS directives, including I/O directives, on both local and global objects.

Although the RTEMS I/O manager provides a framework for device drivers, it makes no assumptions regarding the construction or operation of a device driver.

20.2.5. Runtime Driver Registration

Board support package and application developers can select wether a device driver is statically entered into the default device table or registered at runtime.

Dynamic registration helps applications where:

  • The BSP and kernel libraries are common to a range of applications for a specific target platform. An application may be built upon a common library with all drivers. The application selects and registers the drivers. Uniform driver name lookup protects the application.

  • The type and range of drivers may vary as the application probes a bus during initialization.

  • Support for hot swap bus system such as Compact PCI.

  • Support for runtime loadable driver modules.

20.2.6. Device Driver Interface

When an application invokes an I/O manager directive, RTEMS determines which device driver entry point must be invoked. The information passed by the application to RTEMS is then passed to the correct device driver entry point. RTEMS will invoke each device driver entry point assuming it is compatible with the following prototype:

rtems_device_driver io_entry(
    rtems_device_major_number  major,
    rtems_device_minor_number  minor,
    void                      *argument_block

The format and contents of the parameter block are device driver and entry point dependent.

It is recommended that a device driver avoid generating error codes which conflict with those used by application components. A common technique used to generate driver specific error codes is to make the most significant part of the status indicate a driver specific code.

20.2.7. Device Driver Initialization

RTEMS automatically initializes all device drivers when multitasking is initiated via the rtems_initialize_executive directive. RTEMS initializes the device drivers by invoking each device driver initialization entry point with the following parameters:


the major device number for this device driver.




will point to the Configuration Table.

The returned status will be ignored by RTEMS. If the driver cannot successfully initialize the device, then it should invoke the fatal_error_occurred directive.