/* SPDX-License-Identifier: GPL-2.0 */

  /*
   * Dynamic queue limits (dql) - Definitions
   *
   * Copyright (c) 2011, Tom Herbert <therbert@google.com>
   *
   * This header file contains the definitions for dynamic queue limits (dql).
   * dql would be used in conjunction with a producer/consumer type queue
   * (possibly a HW queue).  Such a queue would have these general properties:
   *
   *   1) Objects are queued up to some limit specified as number of objects.
   *   2) Periodically a completion process executes which retires consumed
   *      objects.
   *   3) Starvation occurs when limit has been reached, all queued data has
   *      actually been consumed, but completion processing has not yet run
   *      so queuing new data is blocked.
   *   4) Minimizing the amount of queued data is desirable.
   *
   * The goal of dql is to calculate the limit as the minimum number of objects
   * needed to prevent starvation.
   *
   * The primary functions of dql are:
   *    dql_queued - called when objects are enqueued to record number of objects
   *    dql_avail - returns how many objects are available to be queued based
   *      on the object limit and how many objects are already enqueued
   *    dql_completed - called at completion time to indicate how many objects
   *      were retired from the queue
   *
   * The dql implementation does not implement any locking for the dql data
   * structures, the higher layer should provide this.  dql_queued should
   * be serialized to prevent concurrent execution of the function; this
   * is also true for  dql_completed.  However, dql_queued and dlq_completed  can
   * be executed concurrently (i.e. they can be protected by different locks).
   */
  
  #ifndef _LINUX_DQL_H
  #define _LINUX_DQL_H
  
  #ifdef __KERNEL__
  
  struct dql {
  	/* Fields accessed in enqueue path (dql_queued) */
  	unsigned int	num_queued;		/* Total ever queued */
  	unsigned int	adj_limit;		/* limit + num_completed */
  	unsigned int	last_obj_cnt;		/* Count at last queuing */
  
  	/* Fields accessed only by completion path (dql_completed) */
  
  	unsigned int	limit ____cacheline_aligned_in_smp; /* Current limit */
  	unsigned int	num_completed;		/* Total ever completed */
  
  	unsigned int	prev_ovlimit;		/* Previous over limit */
  	unsigned int	prev_num_queued;	/* Previous queue total */
  	unsigned int	prev_last_obj_cnt;	/* Previous queuing cnt */
  
  	unsigned int	lowest_slack;		/* Lowest slack found */
  	unsigned long	slack_start_time;	/* Time slacks seen */
  
  	/* Configuration */
  	unsigned int	max_limit;		/* Max limit */
  	unsigned int	min_limit;		/* Minimum limit */
  	unsigned int	slack_hold_time;	/* Time to measure slack */
  };
  
  /* Set some static maximums */
  #define DQL_MAX_OBJECT (UINT_MAX / 16)
  #define DQL_MAX_LIMIT ((UINT_MAX / 2) - DQL_MAX_OBJECT)
  
  /*
   * Record number of objects queued. Assumes that caller has already checked
   * availability in the queue with dql_avail.
   */
  static inline void dql_queued(struct dql *dql, unsigned int count)
  {
  	BUG_ON(count > DQL_MAX_OBJECT);

  	dql->last_obj_cnt = count;

  
  	/* We want to force a write first, so that cpu do not attempt
  	 * to get cache line containing last_obj_cnt, num_queued, adj_limit
  	 * in Shared state, but directly does a Request For Ownership
  	 * It is only a hint, we use barrier() only.
  	 */
  	barrier();
  
  	dql->num_queued += count;

  }
  
  /* Returns how many objects can be queued, < 0 indicates over limit. */
  static inline int dql_avail(const struct dql *dql)
  {

  	return READ_ONCE(dql->adj_limit) - READ_ONCE(dql->num_queued);

  }
  
  /* Record number of completed objects and recalculate the limit. */
  void dql_completed(struct dql *dql, unsigned int count);
  
  /* Reset dql state */
  void dql_reset(struct dql *dql);
  
  /* Initialize dql state */

  void dql_init(struct dql *dql, unsigned int hold_time);

  
  #endif /* _KERNEL_ */
  
  #endif /* _LINUX_DQL_H */