/*

   * Functions to sequence FLUSH and FUA writes.

   *
   * Copyright (C) 2011		Max Planck Institute for Gravitational Physics
   * Copyright (C) 2011		Tejun Heo <tj@kernel.org>
   *
   * This file is released under the GPLv2.
   *
   * REQ_{FLUSH|FUA} requests are decomposed to sequences consisted of three
   * optional steps - PREFLUSH, DATA and POSTFLUSH - according to the request
   * properties and hardware capability.
   *
   * If a request doesn't have data, only REQ_FLUSH makes sense, which
   * indicates a simple flush request.  If there is data, REQ_FLUSH indicates
   * that the device cache should be flushed before the data is executed, and
   * REQ_FUA means that the data must be on non-volatile media on request
   * completion.
   *
   * If the device doesn't have writeback cache, FLUSH and FUA don't make any
   * difference.  The requests are either completed immediately if there's no
   * data or executed as normal requests otherwise.
   *
   * If the device has writeback cache and supports FUA, REQ_FLUSH is
   * translated to PREFLUSH but REQ_FUA is passed down directly with DATA.
   *
   * If the device has writeback cache and doesn't support FUA, REQ_FLUSH is
   * translated to PREFLUSH and REQ_FUA to POSTFLUSH.
   *
   * The actual execution of flush is double buffered.  Whenever a request
   * needs to execute PRE or POSTFLUSH, it queues at
   * q->flush_queue[q->flush_pending_idx].  Once certain criteria are met, a
   * flush is issued and the pending_idx is toggled.  When the flush
   * completes, all the requests which were pending are proceeded to the next
   * step.  This allows arbitrary merging of different types of FLUSH/FUA
   * requests.
   *
   * Currently, the following conditions are used to determine when to issue
   * flush.
   *
   * C1. At any given time, only one flush shall be in progress.  This makes
   *     double buffering sufficient.
   *
   * C2. Flush is deferred if any request is executing DATA of its sequence.
   *     This avoids issuing separate POSTFLUSHes for requests which shared
   *     PREFLUSH.
   *
   * C3. The second condition is ignored if there is a request which has
   *     waited longer than FLUSH_PENDING_TIMEOUT.  This is to avoid
   *     starvation in the unlikely case where there are continuous stream of
   *     FUA (without FLUSH) requests.
   *
   * For devices which support FUA, it isn't clear whether C2 (and thus C3)
   * is beneficial.
   *
   * Note that a sequenced FLUSH/FUA request with DATA is completed twice.
   * Once while executing DATA and again after the whole sequence is
   * complete.  The first completion updates the contained bio but doesn't
   * finish it so that the bio submitter is notified only after the whole
   * sequence is complete.  This is implemented by testing REQ_FLUSH_SEQ in
   * req_bio_endio().
   *
   * The above peculiarity requires that each FLUSH/FUA request has only one
   * bio attached to it, which is guaranteed as they aren't allowed to be
   * merged in the usual way.

   */

  #include <linux/kernel.h>
  #include <linux/module.h>
  #include <linux/bio.h>
  #include <linux/blkdev.h>

  #include <linux/gfp.h>

  
  #include "blk.h"

  /* FLUSH/FUA sequences */
  enum {

  	REQ_FSEQ_PREFLUSH	= (1 << 0), /* pre-flushing in progress */
  	REQ_FSEQ_DATA		= (1 << 1), /* data write in progress */
  	REQ_FSEQ_POSTFLUSH	= (1 << 2), /* post-flushing in progress */
  	REQ_FSEQ_DONE		= (1 << 3),
  
  	REQ_FSEQ_ACTIONS	= REQ_FSEQ_PREFLUSH | REQ_FSEQ_DATA |
  				  REQ_FSEQ_POSTFLUSH,
  
  	/*
  	 * If flush has been pending longer than the following timeout,
  	 * it's issued even if flush_data requests are still in flight.
  	 */
  	FLUSH_PENDING_TIMEOUT	= 5 * HZ,

  };

  static bool blk_kick_flush(struct request_queue *q);

  static unsigned int blk_flush_policy(unsigned int fflags, struct request *rq)

  {

  	unsigned int policy = 0;

  	if (blk_rq_sectors(rq))
  		policy |= REQ_FSEQ_DATA;

  	if (fflags & REQ_FLUSH) {
  		if (rq->cmd_flags & REQ_FLUSH)
  			policy |= REQ_FSEQ_PREFLUSH;

  		if (!(fflags & REQ_FUA) && (rq->cmd_flags & REQ_FUA))
  			policy |= REQ_FSEQ_POSTFLUSH;

  	}

  	return policy;

  }

  static unsigned int blk_flush_cur_seq(struct request *rq)

  {

  	return 1 << ffz(rq->flush.seq);
  }

  static void blk_flush_restore_request(struct request *rq)
  {

  	/*

  	 * After flush data completion, @rq->bio is %NULL but we need to
  	 * complete the bio again.  @rq->biotail is guaranteed to equal the
  	 * original @rq->bio.  Restore it.

  	 */

  	rq->bio = rq->biotail;
  
  	/* make @rq a normal request */
  	rq->cmd_flags &= ~REQ_FLUSH_SEQ;

  	rq->end_io = rq->flush.saved_end_io;

  }

  /**
   * blk_flush_complete_seq - complete flush sequence
   * @rq: FLUSH/FUA request being sequenced
   * @seq: sequences to complete (mask of %REQ_FSEQ_*, can be zero)
   * @error: whether an error occurred
   *
   * @rq just completed @seq part of its flush sequence, record the
   * completion and trigger the next step.
   *
   * CONTEXT:
   * spin_lock_irq(q->queue_lock)
   *
   * RETURNS:
   * %true if requests were added to the dispatch queue, %false otherwise.
   */
  static bool blk_flush_complete_seq(struct request *rq, unsigned int seq,
  				   int error)

  {

  	struct request_queue *q = rq->q;
  	struct list_head *pending = &q->flush_queue[q->flush_pending_idx];
  	bool queued = false;
  
  	BUG_ON(rq->flush.seq & seq);
  	rq->flush.seq |= seq;
  
  	if (likely(!error))
  		seq = blk_flush_cur_seq(rq);
  	else
  		seq = REQ_FSEQ_DONE;
  
  	switch (seq) {
  	case REQ_FSEQ_PREFLUSH:
  	case REQ_FSEQ_POSTFLUSH:
  		/* queue for flush */
  		if (list_empty(pending))
  			q->flush_pending_since = jiffies;
  		list_move_tail(&rq->flush.list, pending);
  		break;
  
  	case REQ_FSEQ_DATA:
  		list_move_tail(&rq->flush.list, &q->flush_data_in_flight);
  		list_add(&rq->queuelist, &q->queue_head);
  		queued = true;
  		break;
  
  	case REQ_FSEQ_DONE:
  		/*
  		 * @rq was previously adjusted by blk_flush_issue() for
  		 * flush sequencing and may already have gone through the
  		 * flush data request completion path.  Restore @rq for
  		 * normal completion and end it.
  		 */
  		BUG_ON(!list_empty(&rq->queuelist));
  		list_del_init(&rq->flush.list);
  		blk_flush_restore_request(rq);
  		__blk_end_request_all(rq, error);
  		break;
  
  	default:
  		BUG();
  	}
  
  	return blk_kick_flush(q) | queued;

  }

  static void flush_end_io(struct request *flush_rq, int error)

  {

  	struct request_queue *q = flush_rq->q;
  	struct list_head *running = &q->flush_queue[q->flush_running_idx];

  	bool queued = false;
  	struct request *rq, *n;
  
  	BUG_ON(q->flush_pending_idx == q->flush_running_idx);
  
  	/* account completion of the flush request */
  	q->flush_running_idx ^= 1;
  	elv_completed_request(q, flush_rq);
  
  	/* and push the waiting requests to the next stage */
  	list_for_each_entry_safe(rq, n, running, flush.list) {
  		unsigned int seq = blk_flush_cur_seq(rq);
  
  		BUG_ON(seq != REQ_FSEQ_PREFLUSH && seq != REQ_FSEQ_POSTFLUSH);
  		queued |= blk_flush_complete_seq(rq, seq, error);
  	}

  	/*

  	 * Kick the queue to avoid stall for two cases:
  	 * 1. Moving a request silently to empty queue_head may stall the
  	 * queue.
  	 * 2. When flush request is running in non-queueable queue, the
  	 * queue is hold. Restart the queue after flush request is finished
  	 * to avoid stall.
  	 * This function is called from request completion path and calling
  	 * directly into request_fn may confuse the driver.  Always use
  	 * kblockd.

  	 */

  	if (queued || q->flush_queue_delayed)

  		blk_run_queue_async(q);

  	q->flush_queue_delayed = 0;

  }

  /**
   * blk_kick_flush - consider issuing flush request
   * @q: request_queue being kicked
   *
   * Flush related states of @q have changed, consider issuing flush request.
   * Please read the comment at the top of this file for more info.
   *
   * CONTEXT:
   * spin_lock_irq(q->queue_lock)
   *
   * RETURNS:
   * %true if flush was issued, %false otherwise.
   */
  static bool blk_kick_flush(struct request_queue *q)

  {

  	struct list_head *pending = &q->flush_queue[q->flush_pending_idx];
  	struct request *first_rq =
  		list_first_entry(pending, struct request, flush.list);
  
  	/* C1 described at the top of this file */
  	if (q->flush_pending_idx != q->flush_running_idx || list_empty(pending))
  		return false;
  
  	/* C2 and C3 */
  	if (!list_empty(&q->flush_data_in_flight) &&
  	    time_before(jiffies,
  			q->flush_pending_since + FLUSH_PENDING_TIMEOUT))
  		return false;
  
  	/*
  	 * Issue flush and toggle pending_idx.  This makes pending_idx
  	 * different from running_idx, which means flush is in flight.
  	 */
  	blk_rq_init(q, &q->flush_rq);
  	q->flush_rq.cmd_type = REQ_TYPE_FS;
  	q->flush_rq.cmd_flags = WRITE_FLUSH | REQ_FLUSH_SEQ;
  	q->flush_rq.rq_disk = first_rq->rq_disk;
  	q->flush_rq.end_io = flush_end_io;
  
  	q->flush_pending_idx ^= 1;

  	list_add_tail(&q->flush_rq.queuelist, &q->queue_head);

  	return true;

  }

  static void flush_data_end_io(struct request *rq, int error)

  {

  	struct request_queue *q = rq->q;

  	/*
  	 * After populating an empty queue, kick it to avoid stall.  Read
  	 * the comment in flush_end_io().
  	 */

  	if (blk_flush_complete_seq(rq, REQ_FSEQ_DATA, error))

  		blk_run_queue_async(q);

  }

  /**
   * blk_insert_flush - insert a new FLUSH/FUA request
   * @rq: request to insert
   *

   * To be called from __elv_add_request() for %ELEVATOR_INSERT_FLUSH insertions.

   * @rq is being submitted.  Analyze what needs to be done and put it on the
   * right queue.
   *
   * CONTEXT:
   * spin_lock_irq(q->queue_lock)
   */
  void blk_insert_flush(struct request *rq)

  {

  	struct request_queue *q = rq->q;
  	unsigned int fflags = q->flush_flags;	/* may change, cache */
  	unsigned int policy = blk_flush_policy(fflags, rq);

  	/*
  	 * @policy now records what operations need to be done.  Adjust
  	 * REQ_FLUSH and FUA for the driver.
  	 */
  	rq->cmd_flags &= ~REQ_FLUSH;
  	if (!(fflags & REQ_FUA))
  		rq->cmd_flags &= ~REQ_FUA;
  
  	/*

  	 * An empty flush handed down from a stacking driver may
  	 * translate into nothing if the underlying device does not
  	 * advertise a write-back cache.  In this case, simply
  	 * complete the request.
  	 */
  	if (!policy) {
  		__blk_end_bidi_request(rq, 0, 0, 0);
  		return;
  	}

  	BUG_ON(rq->bio != rq->biotail); /*assumes zero or single bio rq */

  
  	/*

  	 * If there's data but flush is not necessary, the request can be
  	 * processed directly without going through flush machinery.  Queue
  	 * for normal execution.
  	 */
  	if ((policy & REQ_FSEQ_DATA) &&
  	    !(policy & (REQ_FSEQ_PREFLUSH | REQ_FSEQ_POSTFLUSH))) {

  		list_add_tail(&rq->queuelist, &q->queue_head);

  		return;

  	}

  	/*
  	 * @rq should go through flush machinery.  Mark it part of flush
  	 * sequence and submit for further processing.
  	 */
  	memset(&rq->flush, 0, sizeof(rq->flush));
  	INIT_LIST_HEAD(&rq->flush.list);

  	rq->cmd_flags |= REQ_FLUSH_SEQ;

  	rq->flush.saved_end_io = rq->end_io; /* Usually NULL */

  	rq->end_io = flush_data_end_io;
  
  	blk_flush_complete_seq(rq, REQ_FSEQ_ACTIONS & ~policy, 0);

  }

  /**
   * blk_abort_flushes - @q is being aborted, abort flush requests
   * @q: request_queue being aborted
   *
   * To be called from elv_abort_queue().  @q is being aborted.  Prepare all
   * FLUSH/FUA requests for abortion.
   *
   * CONTEXT:
   * spin_lock_irq(q->queue_lock)
   */
  void blk_abort_flushes(struct request_queue *q)

  {

  	struct request *rq, *n;
  	int i;

  	/*

  	 * Requests in flight for data are already owned by the dispatch
  	 * queue or the device driver.  Just restore for normal completion.

  	 */

  	list_for_each_entry_safe(rq, n, &q->flush_data_in_flight, flush.list) {
  		list_del_init(&rq->flush.list);
  		blk_flush_restore_request(rq);

  	}

  	/*

  	 * We need to give away requests on flush queues.  Restore for
  	 * normal completion and put them on the dispatch queue.

  	 */

  	for (i = 0; i < ARRAY_SIZE(q->flush_queue); i++) {
  		list_for_each_entry_safe(rq, n, &q->flush_queue[i],
  					 flush.list) {
  			list_del_init(&rq->flush.list);
  			blk_flush_restore_request(rq);
  			list_add_tail(&rq->queuelist, &q->queue_head);
  		}

  	}

  }

  static void bio_end_flush(struct bio *bio, int err)

  {

  	if (err)

  		clear_bit(BIO_UPTODATE, &bio->bi_flags);

  	if (bio->bi_private)
  		complete(bio->bi_private);
  	bio_put(bio);

  }
  
  /**
   * blkdev_issue_flush - queue a flush
   * @bdev:	blockdev to issue flush for

   * @gfp_mask:	memory allocation flags (for bio_alloc)

   * @error_sector:	error sector
   *
   * Description:
   *    Issue a flush for the block device in question. Caller can supply
   *    room for storing the error offset in case of a flush error, if they

   *    wish to. If WAIT flag is not passed then caller may check only what
   *    request was pushed in some internal queue for later handling.

   */

  int blkdev_issue_flush(struct block_device *bdev, gfp_t gfp_mask,

  		sector_t *error_sector)

  {
  	DECLARE_COMPLETION_ONSTACK(wait);
  	struct request_queue *q;
  	struct bio *bio;

  	int ret = 0;

  
  	if (bdev->bd_disk == NULL)
  		return -ENXIO;
  
  	q = bdev_get_queue(bdev);
  	if (!q)
  		return -ENXIO;

  	/*
  	 * some block devices may not have their queue correctly set up here
  	 * (e.g. loop device without a backing file) and so issuing a flush
  	 * here will panic. Ensure there is a request function before issuing

  	 * the flush.

  	 */
  	if (!q->make_request_fn)
  		return -ENXIO;

  	bio = bio_alloc(gfp_mask, 0);

  	bio->bi_end_io = bio_end_flush;

  	bio->bi_bdev = bdev;

  	bio->bi_private = &wait;

  	bio_get(bio);

  	submit_bio(WRITE_FLUSH, bio);

  	wait_for_completion(&wait);
  
  	/*
  	 * The driver must store the error location in ->bi_sector, if
  	 * it supports it. For non-stacked drivers, this should be
  	 * copied from blk_rq_pos(rq).
  	 */
  	if (error_sector)
                 *error_sector = bio->bi_sector;

  	if (!bio_flagged(bio, BIO_UPTODATE))

  		ret = -EIO;
  
  	bio_put(bio);
  	return ret;
  }

  EXPORT_SYMBOL(blkdev_issue_flush);