ChibiOS/os/rt/include/chschd.h

/*
    ChibiOS - Copyright (C) 2006..2015 Giovanni Di Sirio.

    This file is part of ChibiOS.

    ChibiOS is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation; either version 3 of the License, or
    (at your option) any later version.

    ChibiOS is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with this program.  If not, see <http://www.gnu.org/licenses/>.
*/

/**
 * @file    chschd.h
 * @brief   Scheduler macros and structures.
 *
 * @addtogroup scheduler
 * @{
 */

#ifndef _CHSCHD_H_
#define _CHSCHD_H_

/*===========================================================================*/
/* Module constants.                                                         */
/*===========================================================================*/

/**
 * @name    Wakeup status codes
 * @{
 */
#define MSG_OK              0       /**< @brief Normal wakeup message.      */
#define MSG_TIMEOUT         -1      /**< @brief Wakeup caused by a timeout
                                         condition.                         */
#define MSG_RESET           -2      /**< @brief Wakeup caused by a reset
                                         condition.                         */
/** @} */

/**
 * @name    Priority constants
 * @{
 */
#define NOPRIO              0U      /**< @brief Ready list header priority. */
#define IDLEPRIO            1U      /**< @brief Idle thread priority.       */
#define LOWPRIO             2U      /**< @brief Lowest user priority.       */
#define NORMALPRIO          64U     /**< @brief Normal user priority.       */
#define HIGHPRIO            127U    /**< @brief Highest user priority.      */
#define ABSPRIO             255U    /**< @brief Greatest possible priority. */
/** @} */

/**
 * @name    Thread states
 * @{
 */
#define CH_STATE_READY      0U      /**< @brief Waiting on the ready list.  */
#define CH_STATE_CURRENT    1U      /**< @brief Currently running.          */
#define CH_STATE_WTSTART    2U      /**< @brief Created but not started.    */
#define CH_STATE_SUSPENDED  3U      /**< @brief Suspended state.            */
#define CH_STATE_QUEUED     4U      /**< @brief Waiting on an I/O queue.    */
#define CH_STATE_WTSEM      5U      /**< @brief Waiting on a semaphore.     */
#define CH_STATE_WTMTX      6U      /**< @brief Waiting on a mutex.         */
#define CH_STATE_WTCOND     7U      /**< @brief Waiting on a condition
                                         variable.                          */
#define CH_STATE_SLEEPING   8U      /**< @brief Waiting in @p chThdSleep()
                                         or @p chThdSleepUntil().           */
#define CH_STATE_WTEXIT     9U      /**< @brief Waiting in @p chThdWait().  */
#define CH_STATE_WTOREVT    10U     /**< @brief Waiting for an event.       */
#define CH_STATE_WTANDEVT   11U     /**< @brief Waiting for several events. */
#define CH_STATE_SNDMSGQ    12U     /**< @brief Sending a message, in queue.*/
#define CH_STATE_SNDMSG     13U     /**< @brief Sent a message, waiting
                                         answer.                            */
#define CH_STATE_WTMSG      14U     /**< @brief Waiting for a message.      */
#define CH_STATE_FINAL      15U     /**< @brief Thread terminated.          */

/**
 * @brief   Thread states as array of strings.
 * @details Each element in an array initialized with this macro can be
 *          indexed using the numeric thread state values.
 */
#define CH_STATE_NAMES                                                     \
  "READY", "CURRENT", "WTSTART", "SUSPENDED", "QUEUED", "WTSEM", "WTMTX",  \
  "WTCOND", "SLEEPING", "WTEXIT", "WTOREVT", "WTANDEVT", "SNDMSGQ",        \
  "SNDMSG", "WTMSG", "FINAL"
/** @} */

/**
 * @name    Thread flags and attributes
 * @{
 */
#define CH_FLAG_MODE_MASK       3U  /**< @brief Thread memory mode mask.    */
#define CH_FLAG_MODE_STATIC     0U  /**< @brief Static thread.              */
#define CH_FLAG_MODE_HEAP       1U  /**< @brief Thread allocated from a
                                         Memory Heap.                       */
#define CH_FLAG_MODE_MEMPOOL    2U  /**< @brief Thread allocated from a
                                         Memory Pool.                       */
#define CH_FLAG_TERMINATE       4U  /**< @brief Termination requested flag. */
/** @} */

/**
 * @name    Working Areas and Alignment
 */
/**
 * @brief   Enforces a correct alignment for a stack area size value.
 *
 * @param[in] n         the stack size to be aligned to the next stack
 *                      alignment boundary
 * @return              The aligned stack size.
 *
 * @api
 */
#define THD_ALIGN_STACK_SIZE(n)                                             \
  (((((size_t)(n)) - 1U) | (sizeof(stkalign_t) - 1U)) + 1U)

/**
 * @brief   Calculates the total Working Area size.
 *
 * @param[in] n         the stack size to be assigned to the thread
 * @return              The total used memory in bytes.
 *
 * @api
 */
#define THD_WORKING_AREA_SIZE(n)                                            \
  THD_ALIGN_STACK_SIZE(sizeof(thread_t) + PORT_WA_SIZE(n))

/**
 * @brief   Static working area allocation.
 * @details This macro is used to allocate a static thread working area
 *          aligned as both position and size.
 *
 * @param[in] s         the name to be assigned to the stack array
 * @param[in] n         the stack size to be assigned to the thread
 *
 * @api
 */
#define THD_WORKING_AREA(s, n)                                              \
  stkalign_t s[THD_WORKING_AREA_SIZE(n) / sizeof(stkalign_t)]
/** @} */

/**
 * @name    Threads abstraction macros
 */
/**
 * @brief   Thread declaration macro.
 * @note    Thread declarations should be performed using this macro because
 *          the port layer could define optimizations for thread functions.
 */
#define THD_FUNCTION(tname, arg) PORT_THD_FUNCTION(tname, arg)
/** @} */

/*===========================================================================*/
/* Module pre-compile time settings.                                         */
/*===========================================================================*/

/*===========================================================================*/
/* Derived constants and error checks.                                       */
/*===========================================================================*/

/*===========================================================================*/
/* Module data structures and types.                                         */
/*===========================================================================*/

/**
 * @brief   Generic threads single link list, it works like a stack.
 */
struct ch_threads_list {
  thread_t              *p_next;    /**< @brief Next in the list/queue.     */
};

/**
 * @brief   Generic threads bidirectional linked list header and element.
 */
struct ch_threads_queue{
  thread_t              *p_next;    /**< @brief Next in the list/queue.     */
  thread_t              *p_prev;    /**< @brief Previous in the queue.      */
};

/**
 * @brief   Structure representing a thread.
 * @note    Not all the listed fields are always needed, by switching off some
 *          not needed ChibiOS/RT subsystems it is possible to save RAM space
 *          by shrinking this structure.
 */
struct ch_thread {
  thread_t              *p_next;    /**< @brief Next in the list/queue.     */
  /* End of the fields shared with the threads_list_t structure.*/
  thread_t              *p_prev;    /**< @brief Previous in the queue.      */
  /* End of the fields shared with the threads_queue_t structure.*/
  tprio_t               p_prio;     /**< @brief Thread priority.            */
  struct context        p_ctx;      /**< @brief Processor context.          */
#if (CH_CFG_USE_REGISTRY == TRUE) || defined(__DOXYGEN__)
  thread_t              *p_newer;   /**< @brief Newer registry element.     */
  thread_t              *p_older;   /**< @brief Older registry element.     */
#endif
  /* End of the fields shared with the ReadyList structure. */
#if (CH_CFG_USE_REGISTRY == TRUE) || defined(__DOXYGEN__)
  /**
   * @brief Thread name or @p NULL.
   */
  const char            *p_name;
#endif
#if (CH_DBG_ENABLE_STACK_CHECK == TRUE) || defined(__DOXYGEN__)
  /**
   * @brief Thread stack boundary.
   */
  stkalign_t            *p_stklimit;
#endif
  /**
   * @brief Current thread state.
   */
  tstate_t              p_state;
  /**
   * @brief Various thread flags.
   */
  tmode_t               p_flags;
#if (CH_CFG_USE_DYNAMIC == TRUE) || defined(__DOXYGEN__)
  /**
   * @brief References to this thread.
   */
  trefs_t               p_refs;
#endif
  /**
   * @brief Number of ticks remaining to this thread.
   */
#if (CH_CFG_TIME_QUANTUM > 0) || defined(__DOXYGEN__)
  tslices_t             p_preempt;
#endif
#if (CH_DBG_THREADS_PROFILING == TRUE) || defined(__DOXYGEN__)
  /**
   * @brief Thread consumed time in ticks.
   * @note  This field can overflow.
   */
  volatile systime_t    p_time;
#endif
  /**
   * @brief State-specific fields.
   * @note  All the fields declared in this union are only valid in the
   *        specified state or condition and are thus volatile.
   */
  union {
    /**
     * @brief Thread wakeup code.
     * @note  This field contains the low level message sent to the thread
     *        by the waking thread or interrupt handler. The value is valid
     *        after exiting the @p chSchWakeupS() function.
     */
    msg_t               rdymsg;
    /**
     * @brief Thread exit code.
     * @note  The thread termination code is stored in this field in order
     *        to be retrieved by the thread performing a @p chThdWait() on
     *        this thread.
     */
    msg_t               exitcode;
    /**
     * @brief Pointer to a generic "wait" object.
     * @note  This field is used to get a generic pointer to a synchronization
     *        object and is valid when the thread is in one of the wait
     *        states.
     */
    void                *wtobjp;
    /**
     * @brief Pointer to a generic thread reference object.
     * @note  This field is used to get a pointer to a synchronization
     *        object and is valid when the thread is in @p CH_STATE_SUSPENDED
     *        state.
     */
    thread_reference_t  *wttrp;
#if (CH_CFG_USE_SEMAPHORES == TRUE) || defined(__DOXYGEN__)
    /**
     * @brief Pointer to a generic semaphore object.
     * @note  This field is used to get a pointer to a synchronization
     *        object and is valid when the thread is in @p CH_STATE_WTSEM
     *        state.
     */
    struct ch_semaphore *wtsemp;
#endif
#if (CH_CFG_USE_MUTEXES == TRUE) || defined(__DOXYGEN__)
    /**
     * @brief Pointer to a generic mutex object.
     * @note  This field is used to get a pointer to a synchronization
     *        object and is valid when the thread is in @p CH_STATE_WTMTX
     *        state.
     */
    struct ch_mutex     *wtmtxp;
#endif
#if (CH_CFG_USE_EVENTS == TRUE) || defined(__DOXYGEN__)
    /**
     * @brief Enabled events mask.
     * @note  This field is only valid while the thread is in the
     *        @p CH_STATE_WTOREVT or @p CH_STATE_WTANDEVT states.
     */
    eventmask_t         ewmask;
#endif
  }                     p_u;
#if (CH_CFG_USE_WAITEXIT == TRUE) || defined(__DOXYGEN__)
  /**
   * @brief Termination waiting list.
   */
  threads_list_t        p_waiting;
#endif
#if (CH_CFG_USE_MESSAGES == TRUE) || defined(__DOXYGEN__)
  /**
   * @brief Messages queue.
   */
  threads_queue_t       p_msgqueue;
  /**
   * @brief Thread message.
   */
  msg_t                 p_msg;
#endif
#if (CH_CFG_USE_EVENTS == TRUE) || defined(__DOXYGEN__)
  /**
   * @brief Pending events mask.
   */
  eventmask_t           p_epending;
#endif
#if (CH_CFG_USE_MUTEXES == TRUE) || defined(__DOXYGEN__)
  /**
   * @brief List of the mutexes owned by this thread.
   * @note  The list is terminated by a @p NULL in this field.
   */
  struct ch_mutex       *p_mtxlist;
  /**
   * @brief Thread's own, non-inherited, priority.
   */
  tprio_t               p_realprio;
#endif
#if ((CH_CFG_USE_DYNAMIC == TRUE) && (CH_CFG_USE_MEMPOOLS == TRUE)) ||      \
    defined(__DOXYGEN__)
  /**
   * @brief Memory Pool where the thread workspace is returned.
   */
  void                  *p_mpool;
#endif
#if (CH_DBG_STATISTICS == TRUE) || defined(__DOXYGEN__)
  /**
   * @brief Thread statistics.
   */
  time_measurement_t    p_stats;
#endif
#if defined(CH_CFG_THREAD_EXTRA_FIELDS)
  /* Extra fields defined in chconf.h.*/
  CH_CFG_THREAD_EXTRA_FIELDS
#endif
};

/**
 * @extends virtual_timers_list_t
 *
 * @brief   Virtual Timer descriptor structure.
 */
struct ch_virtual_timer {
  virtual_timer_t       *vt_next;   /**< @brief Next timer in the list.     */
  virtual_timer_t       *vt_prev;   /**< @brief Previous timer in the list. */
  systime_t             vt_delta;   /**< @brief Time delta before timeout.  */
  vtfunc_t              vt_func;    /**< @brief Timer callback function
                                                pointer.                    */
  void                  *vt_par;    /**< @brief Timer callback function
                                                parameter.                  */
};

/**
 * @brief   Virtual timers list header.
 * @note    The timers list is implemented as a double link bidirectional list
 *          in order to make the unlink time constant, the reset of a virtual
 *          timer is often used in the code.
 */
struct ch_virtual_timers_list {
  virtual_timer_t       *vt_next;   /**< @brief Next timer in the delta
                                                list.                       */
  virtual_timer_t       *vt_prev;   /**< @brief Last timer in the delta
                                                list.                       */
  systime_t             vt_delta;   /**< @brief Must be initialized to -1.  */
#if (CH_CFG_ST_TIMEDELTA == 0) || defined(__DOXYGEN__)
  volatile systime_t    vt_systime; /**< @brief System Time counter.        */
#endif
#if (CH_CFG_ST_TIMEDELTA > 0) || defined(__DOXYGEN__)
  /**
   * @brief   System time of the last tick event.
   */
  systime_t             vt_lasttime;/**< @brief System time of the last
                                                tick event.                 */
#endif
};

/**
 * @extends threads_queue_t
 */
struct ch_ready_list {
  threads_queue_t       r_queue;    /**< @brief Threads queue.              */
  tprio_t               r_prio;     /**< @brief This field must be
                                                initialized to zero.        */
  struct context        r_ctx;      /**< @brief Not used, present because
                                                offsets.                    */
#if (CH_CFG_USE_REGISTRY == TRUE) || defined(__DOXYGEN__)
  thread_t              *r_newer;   /**< @brief Newer registry element.     */
  thread_t              *r_older;   /**< @brief Older registry element.     */
#endif
  /* End of the fields shared with the thread_t structure.*/
  thread_t              *r_current; /**< @brief The currently running
                                                thread.                     */
};

/**
 * @brief   System debug data structure.
 */
struct ch_system_debug {
  /**
   * @brief   Pointer to the panic message.
   * @details This pointer is meant to be accessed through the debugger, it is
   *          written once and then the system is halted.
   * @note    Accesses to this pointer must never be optimized out so the
   *          field itself is declared volatile.
   */
  const char            * volatile panic_msg;
#if (CH_DBG_SYSTEM_STATE_CHECK == TRUE) || defined(__DOXYGEN__)
  /**
   * @brief   ISR nesting level.
   */
  cnt_t                 isr_cnt;
  /**
   * @brief   Lock nesting level.
   */
  cnt_t                 lock_cnt;
#endif
#if (CH_DBG_ENABLE_TRACE == TRUE) || defined(__DOXYGEN__)
  /**
   * @brief   Public trace buffer.
   */
  ch_trace_buffer_t     trace_buffer;
#endif
};

/**
 * @brief   System data structure.
 * @note    This structure contain all the data areas used by the OS except
 *          stacks.
 */
struct ch_system {
  /**
   * @brief   Ready list header.
   */
  ready_list_t          rlist;
  /**
   * @brief   Virtual timers delta list header.
   */
  virtual_timers_list_t vtlist;
  /**
   * @brief   System debug.
   */
  system_debug_t        dbg;
  /**
   * @brief   Main thread descriptor.
   */
  thread_t              mainthread;
#if (CH_CFG_USE_TM == TRUE) || defined(__DOXYGEN__)
  /**
   * @brief   Time measurement calibration data.
   */
  tm_calibration_t      tm;
#endif
#if (CH_DBG_STATISTICS == TRUE) || defined(__DOXYGEN__)
  /**
   * @brief   Global kernel statistics.
   */
  kernel_stats_t        kernel_stats;
#endif
#if CH_CFG_NO_IDLE_THREAD == FALSE
  /**
   * @brief   Idle thread working area.
   */
  THD_WORKING_AREA(idle_thread_wa, PORT_IDLE_THREAD_STACK_SIZE);
#endif
};

/*===========================================================================*/
/* Module macros.                                                            */
/*===========================================================================*/

/**
 * @brief   Returns the priority of the first thread on the given ready list.
 *
 * @notapi
 */
#define firstprio(rlp)  ((rlp)->p_next->p_prio)

/**
 * @brief   Current thread pointer access macro.
 * @note    This macro is not meant to be used in the application code but
 *          only from within the kernel, use the @p chThdSelf() API instead.
 * @note    It is forbidden to use this macro in order to change the pointer
 *          (currp = something), use @p setcurrp() instead.
 */
#define currp ch.rlist.r_current

/**
 * @brief   Current thread pointer change macro.
 * @note    This macro is not meant to be used in the application code but
 *          only from within the kernel.
 *
 * @notapi
 */
#define setcurrp(tp) (currp = (tp))

/*===========================================================================*/
/* External declarations.                                                    */
/*===========================================================================*/

#if !defined(__DOXYGEN__)
extern ch_system_t ch;
#endif

/*
 * Scheduler APIs.
 */
#ifdef __cplusplus
extern "C" {
#endif
  void _scheduler_init(void);
  thread_t *chSchReadyI(thread_t *tp);
  void chSchGoSleepS(tstate_t newstate);
  msg_t chSchGoSleepTimeoutS(tstate_t newstate, systime_t time);
  void chSchWakeupS(thread_t *ntp, msg_t msg);
  void chSchRescheduleS(void);
  bool chSchIsPreemptionRequired(void);
  void chSchDoRescheduleBehind(void);
  void chSchDoRescheduleAhead(void);
  void chSchDoReschedule(void);
#if CH_CFG_OPTIMIZE_SPEED == FALSE
  void queue_prio_insert(thread_t *tp, threads_queue_t *tqp);
  void queue_insert(thread_t *tp, threads_queue_t *tqp);
  thread_t *queue_fifo_remove(threads_queue_t *tqp);
  thread_t *queue_lifo_remove(threads_queue_t *tqp);
  thread_t *queue_dequeue(thread_t *tp);
  void list_insert(thread_t *tp, threads_list_t *tlp);
  thread_t *list_remove(threads_list_t *tlp);
#endif /* CH_CFG_OPTIMIZE_SPEED == FALSE */
#ifdef __cplusplus
}
#endif

/*===========================================================================*/
/* Module inline functions.                                                  */
/*===========================================================================*/

/**
 * @brief   Threads list initialization.
 *
 * @param[in] tlp       pointer to the threads list object
 *
 * @notapi
 */
static inline void list_init(threads_list_t *tlp) {

  /*lint -save -e9087 -e740 [11.3, 1.3] Cast required by list handling.*/
  tlp->p_next = (thread_t *)tlp;
  /*lint -restore*/
}

/**
 * @brief   Evaluates to @p true if the specified threads list is empty.
 *
 * @param[in] tlp       pointer to the threads list object
 * @return              The status of the list.
 *
 * @notapi
 */
static inline bool list_isempty(threads_list_t *tlp) {

  /*lint -save -e9087 -e740 [11.3, 1.3] Cast required by list handling.*/
  return (bool)(tlp->p_next == (thread_t *)tlp);
  /*lint -restore*/
}

/**
 * @brief   Evaluates to @p true if the specified threads list is not empty.
 *
 * @param[in] tlp       pointer to the threads list object
 * @return              The status of the list.
 *
 * @notapi
 */
static inline bool list_notempty(threads_list_t *tlp) {

  /*lint -save -e9087 -e740 [11.3, 1.3] Cast required by list handling.*/
  return (bool)(tlp->p_next != (thread_t *)tlp);
  /*lint -restore*/
}

/**
 * @brief   Threads queue initialization.
 *
 * @param[in] tqp       pointer to the threads queue object
 *
 * @notapi
 */
static inline void queue_init(threads_queue_t *tqp) {

  /*lint -save -e9087 -e740 [11.3, 1.3] Cast required by list handling.*/
  tqp->p_next = (thread_t *)tqp;
  tqp->p_prev = (thread_t *)tqp;
  /*lint -restore*/
}

/**
 * @brief   Evaluates to @p true if the specified threads queue is empty.
 *
 * @param[in] tqp       pointer to the threads queue object
 * @return              The status of the queue.
 *
 * @notapi
 */
static inline bool queue_isempty(threads_queue_t *tqp) {

  /*lint -save -e9087 -e740 [11.3, 1.3] Cast required by list handling.*/
  return (bool)(tqp->p_next == (thread_t *)tqp);
  /*lint -restore*/
}

/**
 * @brief   Evaluates to @p true if the specified threads queue is not empty.
 *
 * @param[in] tqp       pointer to the threads queue object
 * @return              The status of the queue.
 *
 * @notapi
 */
static inline bool queue_notempty(threads_queue_t *tqp) {

  /*lint -save -e9087 -e740 [11.3, 1.3] Cast required by list handling.*/
  return (bool)(tqp->p_next != (thread_t *)tqp);
  /*lint -restore*/
}

/* If the performance code path has been chosen then all the following
   functions are inlined into the various kernel modules.*/
#if CH_CFG_OPTIMIZE_SPEED == TRUE
static inline void list_insert(thread_t *tp, threads_list_t *tlp) {

  tp->p_next = tlp->p_next;
  tlp->p_next = tp;
}

static inline thread_t *list_remove(threads_list_t *tlp) {

  thread_t *tp = tlp->p_next;
  tlp->p_next = tp->p_next;

  return tp;
}

static inline void queue_prio_insert(thread_t *tp, threads_queue_t *tqp) {

  /*lint -save -e9087 -e740 [11.3, 1.3] Cast required by list handling.*/
  thread_t *cp = (thread_t *)tqp;
  do {
    cp = cp->p_next;
  } while ((cp != (thread_t *)tqp) && (cp->p_prio >= tp->p_prio));
  /*lint -restore*/
  tp->p_next = cp;
  tp->p_prev = cp->p_prev;
  tp->p_prev->p_next = tp;
  cp->p_prev = tp;
}

static inline void queue_insert(thread_t *tp, threads_queue_t *tqp) {

  /*lint -save -e9087 -e740 [11.3, 1.3] Cast required by list handling.*/
  tp->p_next = (thread_t *)tqp;
  /*lint -restore*/
  tp->p_prev = tqp->p_prev;
  tp->p_prev->p_next = tp;
  tqp->p_prev = tp;
}

static inline thread_t *queue_fifo_remove(threads_queue_t *tqp) {
  thread_t *tp = tqp->p_next;

  tqp->p_next = tp->p_next;
  /*lint -save -e9087 -e740 [11.3, 1.3] Cast required by list handling.*/
  tqp->p_next->p_prev = (thread_t *)tqp;
  /*lint -restore*/

  return tp;
}

static inline thread_t *queue_lifo_remove(threads_queue_t *tqp) {
  thread_t *tp = tqp->p_prev;

  tqp->p_prev = tp->p_prev;
  /*lint -save -e9087 -e740 [11.3, 1.3] Cast required by list handling.*/
  tqp->p_prev->p_next = (thread_t *)tqp;
  /*lint -restore*/

  return tp;
}

static inline thread_t *queue_dequeue(thread_t *tp) {

  tp->p_prev->p_next = tp->p_next;
  tp->p_next->p_prev = tp->p_prev;

  return tp;
}
#endif /* CH_CFG_OPTIMIZE_SPEED == TRUE */

/**
 * @brief   Determines if the current thread must reschedule.
 * @details This function returns @p true if there is a ready thread with
 *          higher priority.
 *
 * @return              The priorities situation.
 * @retval false        if rescheduling is not necessary.
 * @retval true         if there is a ready thread at higher priority.
 *
 * @iclass
 */
static inline bool chSchIsRescRequiredI(void) {

  chDbgCheckClassI();

  return firstprio(&ch.rlist.r_queue) > currp->p_prio;
}

/**
 * @brief   Determines if yielding is possible.
 * @details This function returns @p true if there is a ready thread with
 *          equal or higher priority.
 *
 * @return              The priorities situation.
 * @retval false        if yielding is not possible.
 * @retval true         if there is a ready thread at equal or higher priority.
 *
 * @sclass
 */
static inline bool chSchCanYieldS(void) {

  chDbgCheckClassS();

  return firstprio(&ch.rlist.r_queue) >= currp->p_prio;
}

/**
 * @brief   Yields the time slot.
 * @details Yields the CPU control to the next thread in the ready list with
 *          equal or higher priority, if any.
 *
 * @sclass
 */
static inline void chSchDoYieldS(void) {

  chDbgCheckClassS();

  if (chSchCanYieldS()) {
    chSchDoRescheduleBehind();
  }
}

/**
 * @brief   Inline-able preemption code.
 * @details This is the common preemption code, this function must be invoked
 *          exclusively from the port layer.
 *
 * @special
 */
static inline void chSchPreemption(void) {
  tprio_t p1 = firstprio(&ch.rlist.r_queue);
  tprio_t p2 = currp->p_prio;

#if CH_CFG_TIME_QUANTUM > 0
  if (currp->p_preempt > 0U) {
    if (p1 > p2) {
      chSchDoRescheduleAhead();
    }
  }
  else {
    if (p1 >= p2) {
      chSchDoRescheduleBehind();
    }
  }
#else /* CH_CFG_TIME_QUANTUM == 0 */
  if (p1 >= p2) {
    chSchDoRescheduleAhead();
  }
#endif /* CH_CFG_TIME_QUANTUM == 0 */
}

#endif /* _CHSCHD_H_ */

/** @} */