9. Clock Driver¶

9.1. Introduction¶

The purpose of the clock driver is to provide two services for the operating system.

A steady time basis to the kernel, so that the RTEMS primitives that need a clock tick work properly. See the Clock Manager chapter of the RTEMS Application C User’s Guide for more details.
An optional time counter to generate timestamps of the uptime and wall clock time.

The clock driver is usually located in the clock directory of the BSP. Clock drivers should use the Clock Driver Shell available via the clockdrv_shell.h include file.

9.2. Clock Driver Shell¶

The Clock Driver Shell include file defines the clock driver functions declared in #include <rtems/clockdrv.h> which are used by RTEMS configuration file #include <rtems/confdefs.h>. In case the application configuration defines #define CONFIGURE_APPLICATION_NEEDS_CLOCK_DRIVER, then the clock driver is registered and should provide its services to the operating system. A hardware specific clock driver must provide some functions, defines and macros for the Clock Driver Shell which are explained here step by step. A clock driver file looks in general like this.

/*
 * A section with functions, defines and macros to provide hardware specific
 * functions for the Clock Driver Shell.
 */
#include "../../../shared/clockdrv_shell.h"

9.2.1. Initialization¶

Depending on the hardware capabilities one out of three clock driver variants must be selected.

The most basic clock driver provides only a periodic interrupt service routine which calls rtems_clock_tick(). The interval is determined by the application configuration via #define CONFIGURE_MICROSECONDS_PER_TICK and can be obtained via rtems_configuration_get_microseconds_per_tick(). The timestamp resolution is limited to the clock tick interval.
In case the hardware lacks support for a free running counter, then the module used for the clock tick may provide support for timestamps with a resolution below the clock tick interval. For this so called simple timecounters can be used.
The desired variant uses a free running counter to provide accurate timestamps. This variant is mandatory on SMP configurations.

9.2.1.1. Clock Tick Only Variant¶

static void some_support_initialize_hardware( void )
{
  /* Initialize hardware */
}

#define Clock_driver_support_initialize_hardware() \
          some_support_initialize_hardware()

/* Indicate that this clock driver lacks a proper timecounter in hardware */

#define CLOCK_DRIVER_USE_DUMMY_TIMECOUNTER

#include "../../../shared/clockdrv_shell.h"

9.2.1.2. Simple Timecounter Variant¶

#include <rtems/timecounter.h>

static rtems_timecounter_simple some_tc;

static uint32_t some_tc_get( rtems_timecounter_simple *tc )
{
  return some.counter;
}

static bool some_tc_is_pending( rtems_timecounter_simple *tc )
{
  return some.is_pending;
}

static uint32_t some_tc_get_timecount( struct timecounter *tc )
{
  return rtems_timecounter_simple_downcounter_get(
    tc,
    some_tc_get,
    some_tc_is_pending
  );
}

static void some_tc_tick( void )
{
  rtems_timecounter_simple_downcounter_tick( &some_tc, some_tc_get );
}

static void some_support_initialize_hardware( void )
{
  uint32_t frequency = 123456;
  uint64_t us_per_tick = rtems_configuration_get_microseconds_per_tick();
  uint32_t timecounter_ticks_per_clock_tick =
                         ( frequency * us_per_tick ) / 1000000;

  /* Initialize hardware */
  rtems_timecounter_simple_install(
    &some_tc,
    frequency,
    timecounter_ticks_per_clock_tick,
    some_tc_get_timecount
  );
}

#define Clock_driver_support_initialize_hardware() \
          some_support_initialize_hardware()
#define Clock_driver_timecounter_tick() \
          some_tc_tick()

#include "../../../shared/clockdrv_shell.h"

9.2.1.3. Timecounter Variant¶

This variant is preferred since it is the most efficient and yields the most accurate timestamps. It is also mandatory on SMP configurations to obtain valid timestamps. The hardware must provide a periodic interrupt to service the clock tick and a free running counter for the timecounter. The free running counter must have a power of two period. The tc_counter_mask must be initialized to the free running counter period minus one, e.g. for a 32-bit counter this is 0xffffffff. The tc_get_timecount function must return the current counter value (the counter values must increase, so if the counter counts down, a conversion is necessary). Use RTEMS_TIMECOUNTER_QUALITY_CLOCK_DRIVER for the tc_quality. Set tc_frequency to the frequency of the free running counter in Hz. All other fields of the struct timecounter must be zero initialized. Install the initialized timecounter via rtems_timecounter_install().

#include <rtems/timecounter.h>

static struct timecounter some_tc;

static uint32_t some_tc_get_timecount( struct timecounter *tc )
{
  some.free_running_counter;
}

static void some_support_initialize_hardware( void )
{
  uint64_t us_per_tick = rtems_configuration_get_microseconds_per_tick();
  uint32_t frequency = 123456;

  /*
   * The multiplication must be done in 64-bit arithmetic to avoid an integer
   * overflow on targets with a high enough counter frequency.
   */
  uint32_t interval = (uint32_t) ( ( frequency * us_per_tick ) / 1000000 );

  /*
   * Initialize hardware and set up a periodic interrupt for the configuration
   * based interval.
   */
  some_tc.tc_get_timecount = some_tc_get_timecount;
  some_tc.tc_counter_mask = 0xffffffff;
  some_tc.tc_frequency = frequency;
  some_tc.tc_quality = RTEMS_TIMECOUNTER_QUALITY_CLOCK_DRIVER;
  rtems_timecounter_install( &some_tc );
}

#define Clock_driver_support_initialize_hardware() \
          some_support_initialize_hardware()

#include "../../../shared/clockdrv_shell.h"

9.2.2. Install Clock Tick Interrupt Service Routine¶

The clock driver must provide a function to install the clock tick interrupt service routine via Clock_driver_support_install_isr().

#include <bsp/irq.h>
#include <bsp/fatal.h>

static void some_support_install_isr( rtems_interrupt_handler isr )
{
  rtems_status_code sc;
  sc = rtems_interrupt_handler_install(
    SOME_IRQ,
    "Clock",
    RTEMS_INTERRUPT_UNIQUE,
    isr,
    NULL
  );
  if ( sc != RTEMS_SUCCESSFUL ) {
    bsp_fatal( SOME_FATAL_IRQ_INSTALL );
  }
}

#define Clock_driver_support_install_isr( isr, old ) \
          some_support_install_isr( isr )

#include "../../../shared/clockdrv_shell.h"

9.2.3. Support At Tick¶

The hardware specific support at tick is specified by Clock_driver_support_at_tick().

static void some_support_at_tick( void )
{
  /* Clear interrupt */
}

#define Clock_driver_support_at_tick() \
          some_support_at_tick()

#include "../../../shared/clockdrv_shell.h"

9.2.4. System Shutdown Support¶

The Clock Driver Shell provides the routine Clock_exit() that is scheduled to be run during system shutdown via the atexit() routine. The hardware specific shutdown support is specified by Clock_driver_support_shutdown_hardware() which is used by Clock_exit(). It should disable the clock tick source if it was enabled. This can be used to prevent clock ticks after the system is shutdown.

static void some_support_shutdown_hardware( void )
{
  /* Shutdown hardware */
}

#define Clock_driver_support_shutdown_hardware() \
          some_support_shutdown_hardware()

#include "../../../shared/clockdrv_shell.h"

9.2.5. Multiple Clock Driver Ticks Per Clock Tick¶

In case the hardware needs more than one clock driver tick per clock tick (e.g. due to a limited range of the hardware timer), then this can be specified with the optional #define CLOCK_DRIVER_ISRS_PER_TICK and #define CLOCK_DRIVER_ISRS_PER_TICK_VALUE defines. This is currently used only for x86 and it hopefully remains that way.

/* Enable multiple clock driver ticks per clock tick */
#define CLOCK_DRIVER_ISRS_PER_TICK 1

/* Specifiy the clock driver ticks per clock tick value */
#define CLOCK_DRIVER_ISRS_PER_TICK_VALUE 123

#include "../../../shared/clockdrv_shell.h"

9.2.6. Clock Driver Ticks Counter¶

The Clock Driver Shell provide a global variable that is simply a count of the number of clock driver interrupt service routines that have occurred. This information is valuable when debugging a system. This variable is declared as follows:

volatile uint32_t Clock_driver_ticks;