/*
   * Dynamic byte queue limits.  See include/linux/dynamic_queue_limits.h
   *
   * Copyright (c) 2011, Tom Herbert <therbert@google.com>
   */
  #include <linux/module.h>
  #include <linux/types.h>
  #include <linux/ctype.h>
  #include <linux/kernel.h>
  #include <linux/jiffies.h>
  #include <linux/dynamic_queue_limits.h>
  
  #define POSDIFF(A, B) ((int)((A) - (B)) > 0 ? (A) - (B) : 0)
  #define AFTER_EQ(A, B) ((int)((A) - (B)) >= 0)
  
  /* Records completed count and recalculates the queue limit */
  void dql_completed(struct dql *dql, unsigned int count)
  {
  	unsigned int inprogress, prev_inprogress, limit;
  	unsigned int ovlimit, completed, num_queued;
  	bool all_prev_completed;
  
  	num_queued = ACCESS_ONCE(dql->num_queued);
  
  	/* Can't complete more than what's in queue */
  	BUG_ON(count > num_queued - dql->num_completed);
  
  	completed = dql->num_completed + count;
  	limit = dql->limit;
  	ovlimit = POSDIFF(num_queued - dql->num_completed, limit);
  	inprogress = num_queued - completed;
  	prev_inprogress = dql->prev_num_queued - dql->num_completed;
  	all_prev_completed = AFTER_EQ(completed, dql->prev_num_queued);
  
  	if ((ovlimit && !inprogress) ||
  	    (dql->prev_ovlimit && all_prev_completed)) {
  		/*
  		 * Queue considered starved if:
  		 *   - The queue was over-limit in the last interval,
  		 *     and there is no more data in the queue.
  		 *  OR
  		 *   - The queue was over-limit in the previous interval and
  		 *     when enqueuing it was possible that all queued data
  		 *     had been consumed.  This covers the case when queue
  		 *     may have becomes starved between completion processing
  		 *     running and next time enqueue was scheduled.
  		 *
  		 *     When queue is starved increase the limit by the amount
  		 *     of bytes both sent and completed in the last interval,
  		 *     plus any previous over-limit.
  		 */
  		limit += POSDIFF(completed, dql->prev_num_queued) +
  		     dql->prev_ovlimit;
  		dql->slack_start_time = jiffies;
  		dql->lowest_slack = UINT_MAX;
  	} else if (inprogress && prev_inprogress && !all_prev_completed) {
  		/*
  		 * Queue was not starved, check if the limit can be decreased.
  		 * A decrease is only considered if the queue has been busy in
  		 * the whole interval (the check above).
  		 *
  		 * If there is slack, the amount of execess data queued above
  		 * the the amount needed to prevent starvation, the queue limit
  		 * can be decreased.  To avoid hysteresis we consider the
  		 * minimum amount of slack found over several iterations of the
  		 * completion routine.
  		 */
  		unsigned int slack, slack_last_objs;
  
  		/*
  		 * Slack is the maximum of
  		 *   - The queue limit plus previous over-limit minus twice
  		 *     the number of objects completed.  Note that two times
  		 *     number of completed bytes is a basis for an upper bound
  		 *     of the limit.
  		 *   - Portion of objects in the last queuing operation that
  		 *     was not part of non-zero previous over-limit.  That is
  		 *     "round down" by non-overlimit portion of the last
  		 *     queueing operation.
  		 */
  		slack = POSDIFF(limit + dql->prev_ovlimit,
  		    2 * (completed - dql->num_completed));
  		slack_last_objs = dql->prev_ovlimit ?
  		    POSDIFF(dql->prev_last_obj_cnt, dql->prev_ovlimit) : 0;
  
  		slack = max(slack, slack_last_objs);
  
  		if (slack < dql->lowest_slack)
  			dql->lowest_slack = slack;
  
  		if (time_after(jiffies,
  			       dql->slack_start_time + dql->slack_hold_time)) {
  			limit = POSDIFF(limit, dql->lowest_slack);
  			dql->slack_start_time = jiffies;
  			dql->lowest_slack = UINT_MAX;
  		}
  	}
  
  	/* Enforce bounds on limit */
  	limit = clamp(limit, dql->min_limit, dql->max_limit);
  
  	if (limit != dql->limit) {
  		dql->limit = limit;
  		ovlimit = 0;
  	}
  
  	dql->adj_limit = limit + completed;
  	dql->prev_ovlimit = ovlimit;
  	dql->prev_last_obj_cnt = dql->last_obj_cnt;
  	dql->num_completed = completed;
  	dql->prev_num_queued = num_queued;
  }
  EXPORT_SYMBOL(dql_completed);
  
  void dql_reset(struct dql *dql)
  {
  	/* Reset all dynamic values */
  	dql->limit = 0;
  	dql->num_queued = 0;
  	dql->num_completed = 0;
  	dql->last_obj_cnt = 0;
  	dql->prev_num_queued = 0;
  	dql->prev_last_obj_cnt = 0;
  	dql->prev_ovlimit = 0;
  	dql->lowest_slack = UINT_MAX;
  	dql->slack_start_time = jiffies;
  }
  EXPORT_SYMBOL(dql_reset);
  
  int dql_init(struct dql *dql, unsigned hold_time)
  {
  	dql->max_limit = DQL_MAX_LIMIT;
  	dql->min_limit = 0;
  	dql->slack_hold_time = hold_time;
  	dql_reset(dql);
  	return 0;
  }
  EXPORT_SYMBOL(dql_init);