Idle Task Configuration

26.17. Idle Task Configuration#

This section describes configuration options related to the idle tasks.

26.17.1. CONFIGURE_IDLE_TASK_BODY#

CONSTANT:

CONFIGURE_IDLE_TASK_BODY

OPTION TYPE:

This configuration option is an initializer define.

DEFAULT VALUE:

If the CONFIGURE_DISABLE_BSP_SETTINGS configuration option is not defined and BSP_IDLE_TASK_BODY is provided by the BSP, then the default value is defined by BSP_IDLE_TASK_BODY, otherwise the default value is _CPU_Thread_Idle_body.

DESCRIPTION:

The value of this configuration option initializes the IDLE thread body.

NOTES:

IDLE threads shall not block. A blocking IDLE thread results in undefined system behaviour because the scheduler assume that at least one ready thread exists.

IDLE threads can be used to initialize the application, see configuration option CONFIGURE_IDLE_TASK_INITIALIZES_APPLICATION.

The BSP may have knowledge of the specific CPU model, system controller logic, and peripheral buses, so a BSP-specific IDLE task may be capable of turning components off to save power during extended periods of no task activity.

CONSTRAINTS:

The value of the configuration option shall be defined to a valid function pointer of the type void *( *idle_body )( uintptr_t ).

26.17.2. CONFIGURE_IDLE_TASK_INITIALIZES_APPLICATION#

CONSTANT:

CONFIGURE_IDLE_TASK_INITIALIZES_APPLICATION

OPTION TYPE:

This configuration option is a boolean feature define.

DEFAULT CONFIGURATION:

If this configuration option is undefined, then the user is assumed to provide one or more initialization tasks.

DESCRIPTION:

This configuration option is defined to indicate that the user has configured no user initialization tasks or threads and that the user provided IDLE task will perform application initialization and then transform itself into an IDLE task.

NOTES:

If you use this option be careful, the user IDLE task cannot block at all during the initialization sequence. Further, once application initialization is complete, it shall make itself preemptible and enter an idle body loop.

The IDLE task shall run at the lowest priority of all tasks in the system.

If this configuration option is defined, then it is mandatory to configure a user IDLE task with the CONFIGURE_IDLE_TASK_BODY configuration option, otherwise a compile time error in the configuration file will occur.

The application shall define at least one of the following configuration options

otherwise a compile time error in the configuration file will occur.

If no Classic API initialization task and no POSIX API initialization thread is configured, then no Global Construction is performed.

26.17.3. CONFIGURE_IDLE_TASK_STACK_SIZE#

CONSTANT:

CONFIGURE_IDLE_TASK_STACK_SIZE

OPTION TYPE:

This configuration option is an integer define.

DEFAULT VALUE:

If the CONFIGURE_DISABLE_BSP_SETTINGS configuration option is not defined and BSP_IDLE_TASK_STACK_SIZE is provided by the BSP, then the default value is defined by BSP_IDLE_TASK_STACK_SIZE, otherwise the default value is defined by the CONFIGURE_MINIMUM_TASK_STACK_SIZE configuration option.

DESCRIPTION:

The value of this configuration option defines the task stack size for an IDLE task.

NOTES:

In SMP configurations, there is one IDLE task per configured processor, see CONFIGURE_MAXIMUM_PROCESSORS.

CONSTRAINTS:

The following constraints apply to this configuration option:

  • The value of the configuration option shall be greater than or equal to a BSP-specific and application-specific minimum value.

  • The value of the configuration option shall be small enough so that the IDLE task stack area calculation carried out by <rtems/confdefs.h> does not overflow an integer of type size_t.

26.17.4. CONFIGURE_IDLE_TASK_STORAGE_SIZE#

CONSTANT:

CONFIGURE_IDLE_TASK_STORAGE_SIZE

OPTION TYPE:

This configuration option is an integer define.

DEFAULT VALUE:

This configuration option has no default value. If it is not specified, then the task storage area for each IDLE task will allocated from the RTEMS Workspace or through a custom IDLE task stack allocator.

DESCRIPTION:

If this configuration option is specified, then the task storage areas for the IDLE tasks are statically allocated by <rtems/confdefs.h>. The value of this configuration option defines the size in bytes of the task storage area of each IDLE task in the system.

NOTES:

By default, the IDLE task storage areas are allocated from the RTEMS Workspace. Applications which do not want to use a heap allocator can use this configuration option to use statically allocated memory for the IDLE task storage areas. The task storage area contains the task stack, the thread-local storage, and the floating-point context on architectures with a separate floating-point context. The size of the thread-local storage area is defined at link time or by the CONFIGURE_MAXIMUM_THREAD_LOCAL_STORAGE_SIZE configuration option. You have to estimate the actual thread-local storage size if you want to use this configuration option. If the IDLE task stack size would be less than the value defined by the CONFIGURE_IDLE_TASK_STACK_SIZE configuration option, for example because the thread-local storage size is larger than expected, then the system terminates with the INTERNAL_ERROR_CORE fatal source and the INTERNAL_ERROR_IDLE_THREAD_STACK_TOO_SMALL fatal code during system initialization.

The value of this configuration option is passed to RTEMS_TASK_STORAGE_SIZE() by <rtems/confdefs.h> to determine the actual size of the statically allocated area to take architecture-specific overheads into account.

The

configuration options are mutually exclusive.

CONSTRAINTS:

The value of the configuration option shall be greater than or equal to CONFIGURE_IDLE_TASK_STACK_SIZE.