scsi_lib.c 57.3 KB
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/*
 *  scsi_lib.c Copyright (C) 1999 Eric Youngdale
 *
 *  SCSI queueing library.
 *      Initial versions: Eric Youngdale (eric@andante.org).
 *                        Based upon conversations with large numbers
 *                        of people at Linux Expo.
 */

#include <linux/bio.h>
#include <linux/blkdev.h>
#include <linux/completion.h>
#include <linux/kernel.h>
#include <linux/mempool.h>
#include <linux/slab.h>
#include <linux/init.h>
#include <linux/pci.h>
#include <linux/delay.h>

#include <scsi/scsi.h>
#include <scsi/scsi_dbg.h>
#include <scsi/scsi_device.h>
#include <scsi/scsi_driver.h>
#include <scsi/scsi_eh.h>
#include <scsi/scsi_host.h>
#include <scsi/scsi_request.h>

#include "scsi_priv.h"
#include "scsi_logging.h"


#define SG_MEMPOOL_NR		(sizeof(scsi_sg_pools)/sizeof(struct scsi_host_sg_pool))
#define SG_MEMPOOL_SIZE		32

struct scsi_host_sg_pool {
	size_t		size;
	char		*name; 
	kmem_cache_t	*slab;
	mempool_t	*pool;
};

#if (SCSI_MAX_PHYS_SEGMENTS < 32)
#error SCSI_MAX_PHYS_SEGMENTS is too small
#endif

#define SP(x) { x, "sgpool-" #x } 
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static struct scsi_host_sg_pool scsi_sg_pools[] = {
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	SP(8),
	SP(16),
	SP(32),
#if (SCSI_MAX_PHYS_SEGMENTS > 32)
	SP(64),
#if (SCSI_MAX_PHYS_SEGMENTS > 64)
	SP(128),
#if (SCSI_MAX_PHYS_SEGMENTS > 128)
	SP(256),
#if (SCSI_MAX_PHYS_SEGMENTS > 256)
#error SCSI_MAX_PHYS_SEGMENTS is too large
#endif
#endif
#endif
#endif
}; 	
#undef SP

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static void scsi_run_queue(struct request_queue *q);
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/*
 * Function:	scsi_unprep_request()
 *
 * Purpose:	Remove all preparation done for a request, including its
 *		associated scsi_cmnd, so that it can be requeued.
 *
 * Arguments:	req	- request to unprepare
 *
 * Lock status:	Assumed that no locks are held upon entry.
 *
 * Returns:	Nothing.
 */
static void scsi_unprep_request(struct request *req)
{
	struct scsi_cmnd *cmd = req->special;

	req->flags &= ~REQ_DONTPREP;
	req->special = (req->flags & REQ_SPECIAL) ? cmd->sc_request : NULL;

	scsi_put_command(cmd);
}
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/*
 * Function:    scsi_queue_insert()
 *
 * Purpose:     Insert a command in the midlevel queue.
 *
 * Arguments:   cmd    - command that we are adding to queue.
 *              reason - why we are inserting command to queue.
 *
 * Lock status: Assumed that lock is not held upon entry.
 *
 * Returns:     Nothing.
 *
 * Notes:       We do this for one of two cases.  Either the host is busy
 *              and it cannot accept any more commands for the time being,
 *              or the device returned QUEUE_FULL and can accept no more
 *              commands.
 * Notes:       This could be called either from an interrupt context or a
 *              normal process context.
 */
int scsi_queue_insert(struct scsi_cmnd *cmd, int reason)
{
	struct Scsi_Host *host = cmd->device->host;
	struct scsi_device *device = cmd->device;
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	struct request_queue *q = device->request_queue;
	unsigned long flags;
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	SCSI_LOG_MLQUEUE(1,
		 printk("Inserting command %p into mlqueue\n", cmd));

	/*
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	 * Set the appropriate busy bit for the device/host.
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	 *
	 * If the host/device isn't busy, assume that something actually
	 * completed, and that we should be able to queue a command now.
	 *
	 * Note that the prior mid-layer assumption that any host could
	 * always queue at least one command is now broken.  The mid-layer
	 * will implement a user specifiable stall (see
	 * scsi_host.max_host_blocked and scsi_device.max_device_blocked)
	 * if a command is requeued with no other commands outstanding
	 * either for the device or for the host.
	 */
	if (reason == SCSI_MLQUEUE_HOST_BUSY)
		host->host_blocked = host->max_host_blocked;
	else if (reason == SCSI_MLQUEUE_DEVICE_BUSY)
		device->device_blocked = device->max_device_blocked;

	/*
	 * Decrement the counters, since these commands are no longer
	 * active on the host/device.
	 */
	scsi_device_unbusy(device);

	/*
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	 * Requeue this command.  It will go before all other commands
	 * that are already in the queue.
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	 *
	 * NOTE: there is magic here about the way the queue is plugged if
	 * we have no outstanding commands.
	 * 
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	 * Although we *don't* plug the queue, we call the request
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	 * function.  The SCSI request function detects the blocked condition
	 * and plugs the queue appropriately.
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         */
	spin_lock_irqsave(q->queue_lock, flags);
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	blk_requeue_request(q, cmd->request);
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	spin_unlock_irqrestore(q->queue_lock, flags);

	scsi_run_queue(q);

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	return 0;
}

/*
 * Function:    scsi_do_req
 *
 * Purpose:     Queue a SCSI request
 *
 * Arguments:   sreq	  - command descriptor.
 *              cmnd      - actual SCSI command to be performed.
 *              buffer    - data buffer.
 *              bufflen   - size of data buffer.
 *              done      - completion function to be run.
 *              timeout   - how long to let it run before timeout.
 *              retries   - number of retries we allow.
 *
 * Lock status: No locks held upon entry.
 *
 * Returns:     Nothing.
 *
 * Notes:	This function is only used for queueing requests for things
 *		like ioctls and character device requests - this is because
 *		we essentially just inject a request into the queue for the
 *		device.
 *
 *		In order to support the scsi_device_quiesce function, we
 *		now inject requests on the *head* of the device queue
 *		rather than the tail.
 */
void scsi_do_req(struct scsi_request *sreq, const void *cmnd,
		 void *buffer, unsigned bufflen,
		 void (*done)(struct scsi_cmnd *),
		 int timeout, int retries)
{
	/*
	 * If the upper level driver is reusing these things, then
	 * we should release the low-level block now.  Another one will
	 * be allocated later when this request is getting queued.
	 */
	__scsi_release_request(sreq);

	/*
	 * Our own function scsi_done (which marks the host as not busy,
	 * disables the timeout counter, etc) will be called by us or by the
	 * scsi_hosts[host].queuecommand() function needs to also call
	 * the completion function for the high level driver.
	 */
	memcpy(sreq->sr_cmnd, cmnd, sizeof(sreq->sr_cmnd));
	sreq->sr_bufflen = bufflen;
	sreq->sr_buffer = buffer;
	sreq->sr_allowed = retries;
	sreq->sr_done = done;
	sreq->sr_timeout_per_command = timeout;

	if (sreq->sr_cmd_len == 0)
		sreq->sr_cmd_len = COMMAND_SIZE(sreq->sr_cmnd[0]);

	/*
	 * head injection *required* here otherwise quiesce won't work
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	 *
	 * Because users of this function are apt to reuse requests with no
	 * modification, we have to sanitise the request flags here
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	 */
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	sreq->sr_request->flags &= ~REQ_DONTPREP;
	blk_insert_request(sreq->sr_device->request_queue, sreq->sr_request,
		       	   1, sreq);
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}
EXPORT_SYMBOL(scsi_do_req);

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/**
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 * scsi_execute - insert request and wait for the result
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 * @sdev:	scsi device
 * @cmd:	scsi command
 * @data_direction: data direction
 * @buffer:	data buffer
 * @bufflen:	len of buffer
 * @sense:	optional sense buffer
 * @timeout:	request timeout in seconds
 * @retries:	number of times to retry request
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 * @flags:	or into request flags;
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 *
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 * returns the req->errors value which is the the scsi_cmnd result
 * field.
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 **/
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int scsi_execute(struct scsi_device *sdev, const unsigned char *cmd,
		 int data_direction, void *buffer, unsigned bufflen,
		 unsigned char *sense, int timeout, int retries, int flags)
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{
	struct request *req;
	int write = (data_direction == DMA_TO_DEVICE);
	int ret = DRIVER_ERROR << 24;

	req = blk_get_request(sdev->request_queue, write, __GFP_WAIT);

	if (bufflen &&	blk_rq_map_kern(sdev->request_queue, req,
					buffer, bufflen, __GFP_WAIT))
		goto out;

	req->cmd_len = COMMAND_SIZE(cmd[0]);
	memcpy(req->cmd, cmd, req->cmd_len);
	req->sense = sense;
	req->sense_len = 0;
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	req->retries = retries;
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	req->timeout = timeout;
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	req->flags |= flags | REQ_BLOCK_PC | REQ_SPECIAL | REQ_QUIET;
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	/*
	 * head injection *required* here otherwise quiesce won't work
	 */
	blk_execute_rq(req->q, NULL, req, 1);

	ret = req->errors;
 out:
	blk_put_request(req);

	return ret;
}
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EXPORT_SYMBOL(scsi_execute);
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int scsi_execute_req(struct scsi_device *sdev, const unsigned char *cmd,
		     int data_direction, void *buffer, unsigned bufflen,
		     struct scsi_sense_hdr *sshdr, int timeout, int retries)
{
	char *sense = NULL;
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	int result;
	
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	if (sshdr) {
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		sense = kmalloc(SCSI_SENSE_BUFFERSIZE, GFP_NOIO);
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		if (!sense)
			return DRIVER_ERROR << 24;
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		memset(sense, 0, SCSI_SENSE_BUFFERSIZE);
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	}
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	result = scsi_execute(sdev, cmd, data_direction, buffer, bufflen,
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				  sense, timeout, retries, 0);
	if (sshdr)
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		scsi_normalize_sense(sense, SCSI_SENSE_BUFFERSIZE, sshdr);
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	kfree(sense);
	return result;
}
EXPORT_SYMBOL(scsi_execute_req);

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struct scsi_io_context {
	void *data;
	void (*done)(void *data, char *sense, int result, int resid);
	char sense[SCSI_SENSE_BUFFERSIZE];
};

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static kmem_cache_t *scsi_io_context_cache;

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static void scsi_end_async(struct request *req)
{
	struct scsi_io_context *sioc = req->end_io_data;

	if (sioc->done)
		sioc->done(sioc->data, sioc->sense, req->errors, req->data_len);

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	kmem_cache_free(scsi_io_context_cache, sioc);
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	__blk_put_request(req->q, req);
}

static int scsi_merge_bio(struct request *rq, struct bio *bio)
{
	struct request_queue *q = rq->q;

	bio->bi_flags &= ~(1 << BIO_SEG_VALID);
	if (rq_data_dir(rq) == WRITE)
		bio->bi_rw |= (1 << BIO_RW);
	blk_queue_bounce(q, &bio);

	if (!rq->bio)
		blk_rq_bio_prep(q, rq, bio);
	else if (!q->back_merge_fn(q, rq, bio))
		return -EINVAL;
	else {
		rq->biotail->bi_next = bio;
		rq->biotail = bio;
		rq->hard_nr_sectors += bio_sectors(bio);
		rq->nr_sectors = rq->hard_nr_sectors;
	}

	return 0;
}

static int scsi_bi_endio(struct bio *bio, unsigned int bytes_done, int error)
{
	if (bio->bi_size)
		return 1;

	bio_put(bio);
	return 0;
}

/**
 * scsi_req_map_sg - map a scatterlist into a request
 * @rq:		request to fill
 * @sg:		scatterlist
 * @nsegs:	number of elements
 * @bufflen:	len of buffer
 * @gfp:	memory allocation flags
 *
 * scsi_req_map_sg maps a scatterlist into a request so that the
 * request can be sent to the block layer. We do not trust the scatterlist
 * sent to use, as some ULDs use that struct to only organize the pages.
 */
static int scsi_req_map_sg(struct request *rq, struct scatterlist *sgl,
			   int nsegs, unsigned bufflen, gfp_t gfp)
{
	struct request_queue *q = rq->q;
	int nr_pages = (bufflen + PAGE_SIZE - 1) >> PAGE_SHIFT;
	unsigned int data_len = 0, len, bytes, off;
	struct page *page;
	struct bio *bio = NULL;
	int i, err, nr_vecs = 0;

	for (i = 0; i < nsegs; i++) {
		page = sgl[i].page;
		off = sgl[i].offset;
		len = sgl[i].length;
		data_len += len;

		while (len > 0) {
			bytes = min_t(unsigned int, len, PAGE_SIZE - off);

			if (!bio) {
				nr_vecs = min_t(int, BIO_MAX_PAGES, nr_pages);
				nr_pages -= nr_vecs;

				bio = bio_alloc(gfp, nr_vecs);
				if (!bio) {
					err = -ENOMEM;
					goto free_bios;
				}
				bio->bi_end_io = scsi_bi_endio;
			}

			if (bio_add_pc_page(q, bio, page, bytes, off) !=
			    bytes) {
				bio_put(bio);
				err = -EINVAL;
				goto free_bios;
			}

			if (bio->bi_vcnt >= nr_vecs) {
				err = scsi_merge_bio(rq, bio);
				if (err) {
					bio_endio(bio, bio->bi_size, 0);
					goto free_bios;
				}
				bio = NULL;
			}

			page++;
			len -= bytes;
			off = 0;
		}
	}

	rq->buffer = rq->data = NULL;
	rq->data_len = data_len;
	return 0;

free_bios:
	while ((bio = rq->bio) != NULL) {
		rq->bio = bio->bi_next;
		/*
		 * call endio instead of bio_put incase it was bounced
		 */
		bio_endio(bio, bio->bi_size, 0);
	}

	return err;
}

/**
 * scsi_execute_async - insert request
 * @sdev:	scsi device
 * @cmd:	scsi command
 * @data_direction: data direction
 * @buffer:	data buffer (this can be a kernel buffer or scatterlist)
 * @bufflen:	len of buffer
 * @use_sg:	if buffer is a scatterlist this is the number of elements
 * @timeout:	request timeout in seconds
 * @retries:	number of times to retry request
 * @flags:	or into request flags
 **/
int scsi_execute_async(struct scsi_device *sdev, const unsigned char *cmd,
		       int data_direction, void *buffer, unsigned bufflen,
		       int use_sg, int timeout, int retries, void *privdata,
		       void (*done)(void *, char *, int, int), gfp_t gfp)
{
	struct request *req;
	struct scsi_io_context *sioc;
	int err = 0;
	int write = (data_direction == DMA_TO_DEVICE);

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	sioc = kmem_cache_alloc(scsi_io_context_cache, gfp);
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	if (!sioc)
		return DRIVER_ERROR << 24;
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	memset(sioc, 0, sizeof(*sioc));
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	req = blk_get_request(sdev->request_queue, write, gfp);
	if (!req)
		goto free_sense;
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	req->flags |= REQ_BLOCK_PC | REQ_QUIET;
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	if (use_sg)
		err = scsi_req_map_sg(req, buffer, use_sg, bufflen, gfp);
	else if (bufflen)
		err = blk_rq_map_kern(req->q, req, buffer, bufflen, gfp);

	if (err)
		goto free_req;

	req->cmd_len = COMMAND_SIZE(cmd[0]);
	memcpy(req->cmd, cmd, req->cmd_len);
	req->sense = sioc->sense;
	req->sense_len = 0;
	req->timeout = timeout;
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	req->retries = retries;
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	req->end_io_data = sioc;

	sioc->data = privdata;
	sioc->done = done;

	blk_execute_rq_nowait(req->q, NULL, req, 1, scsi_end_async);
	return 0;

free_req:
	blk_put_request(req);
free_sense:
	kfree(sioc);
	return DRIVER_ERROR << 24;
}
EXPORT_SYMBOL_GPL(scsi_execute_async);

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/*
 * Function:    scsi_init_cmd_errh()
 *
 * Purpose:     Initialize cmd fields related to error handling.
 *
 * Arguments:   cmd	- command that is ready to be queued.
 *
 * Returns:     Nothing
 *
 * Notes:       This function has the job of initializing a number of
 *              fields related to error handling.   Typically this will
 *              be called once for each command, as required.
 */
static int scsi_init_cmd_errh(struct scsi_cmnd *cmd)
{
	cmd->serial_number = 0;

	memset(cmd->sense_buffer, 0, sizeof cmd->sense_buffer);

	if (cmd->cmd_len == 0)
		cmd->cmd_len = COMMAND_SIZE(cmd->cmnd[0]);

	/*
	 * We need saved copies of a number of fields - this is because
	 * error handling may need to overwrite these with different values
	 * to run different commands, and once error handling is complete,
	 * we will need to restore these values prior to running the actual
	 * command.
	 */
	cmd->old_use_sg = cmd->use_sg;
	cmd->old_cmd_len = cmd->cmd_len;
	cmd->sc_old_data_direction = cmd->sc_data_direction;
	cmd->old_underflow = cmd->underflow;
	memcpy(cmd->data_cmnd, cmd->cmnd, sizeof(cmd->cmnd));
	cmd->buffer = cmd->request_buffer;
	cmd->bufflen = cmd->request_bufflen;

	return 1;
}

/*
 * Function:   scsi_setup_cmd_retry()
 *
 * Purpose:    Restore the command state for a retry
 *
 * Arguments:  cmd	- command to be restored
 *
 * Returns:    Nothing
 *
 * Notes:      Immediately prior to retrying a command, we need
 *             to restore certain fields that we saved above.
 */
void scsi_setup_cmd_retry(struct scsi_cmnd *cmd)
{
	memcpy(cmd->cmnd, cmd->data_cmnd, sizeof(cmd->data_cmnd));
	cmd->request_buffer = cmd->buffer;
	cmd->request_bufflen = cmd->bufflen;
	cmd->use_sg = cmd->old_use_sg;
	cmd->cmd_len = cmd->old_cmd_len;
	cmd->sc_data_direction = cmd->sc_old_data_direction;
	cmd->underflow = cmd->old_underflow;
}

void scsi_device_unbusy(struct scsi_device *sdev)
{
	struct Scsi_Host *shost = sdev->host;
	unsigned long flags;

	spin_lock_irqsave(shost->host_lock, flags);
	shost->host_busy--;
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	if (unlikely(scsi_host_in_recovery(shost) &&
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		     shost->host_failed))
		scsi_eh_wakeup(shost);
	spin_unlock(shost->host_lock);
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	spin_lock(sdev->request_queue->queue_lock);
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	sdev->device_busy--;
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	spin_unlock_irqrestore(sdev->request_queue->queue_lock, flags);
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}

/*
 * Called for single_lun devices on IO completion. Clear starget_sdev_user,
 * and call blk_run_queue for all the scsi_devices on the target -
 * including current_sdev first.
 *
 * Called with *no* scsi locks held.
 */
static void scsi_single_lun_run(struct scsi_device *current_sdev)
{
	struct Scsi_Host *shost = current_sdev->host;
	struct scsi_device *sdev, *tmp;
	struct scsi_target *starget = scsi_target(current_sdev);
	unsigned long flags;

	spin_lock_irqsave(shost->host_lock, flags);
	starget->starget_sdev_user = NULL;
	spin_unlock_irqrestore(shost->host_lock, flags);

	/*
	 * Call blk_run_queue for all LUNs on the target, starting with
	 * current_sdev. We race with others (to set starget_sdev_user),
	 * but in most cases, we will be first. Ideally, each LU on the
	 * target would get some limited time or requests on the target.
	 */
	blk_run_queue(current_sdev->request_queue);

	spin_lock_irqsave(shost->host_lock, flags);
	if (starget->starget_sdev_user)
		goto out;
	list_for_each_entry_safe(sdev, tmp, &starget->devices,
			same_target_siblings) {
		if (sdev == current_sdev)
			continue;
		if (scsi_device_get(sdev))
			continue;

		spin_unlock_irqrestore(shost->host_lock, flags);
		blk_run_queue(sdev->request_queue);
		spin_lock_irqsave(shost->host_lock, flags);
	
		scsi_device_put(sdev);
	}
 out:
	spin_unlock_irqrestore(shost->host_lock, flags);
}

/*
 * Function:	scsi_run_queue()
 *
 * Purpose:	Select a proper request queue to serve next
 *
 * Arguments:	q	- last request's queue
 *
 * Returns:     Nothing
 *
 * Notes:	The previous command was completely finished, start
 *		a new one if possible.
 */
static void scsi_run_queue(struct request_queue *q)
{
	struct scsi_device *sdev = q->queuedata;
	struct Scsi_Host *shost = sdev->host;
	unsigned long flags;

	if (sdev->single_lun)
		scsi_single_lun_run(sdev);

	spin_lock_irqsave(shost->host_lock, flags);
	while (!list_empty(&shost->starved_list) &&
	       !shost->host_blocked && !shost->host_self_blocked &&
		!((shost->can_queue > 0) &&
		  (shost->host_busy >= shost->can_queue))) {
		/*
		 * As long as shost is accepting commands and we have
		 * starved queues, call blk_run_queue. scsi_request_fn
		 * drops the queue_lock and can add us back to the
		 * starved_list.
		 *
		 * host_lock protects the starved_list and starved_entry.
		 * scsi_request_fn must get the host_lock before checking
		 * or modifying starved_list or starved_entry.
		 */
		sdev = list_entry(shost->starved_list.next,
					  struct scsi_device, starved_entry);
		list_del_init(&sdev->starved_entry);
		spin_unlock_irqrestore(shost->host_lock, flags);

		blk_run_queue(sdev->request_queue);

		spin_lock_irqsave(shost->host_lock, flags);
		if (unlikely(!list_empty(&sdev->starved_entry)))
			/*
			 * sdev lost a race, and was put back on the
			 * starved list. This is unlikely but without this
			 * in theory we could loop forever.
			 */
			break;
	}
	spin_unlock_irqrestore(shost->host_lock, flags);

	blk_run_queue(q);
}

/*
 * Function:	scsi_requeue_command()
 *
 * Purpose:	Handle post-processing of completed commands.
 *
 * Arguments:	q	- queue to operate on
 *		cmd	- command that may need to be requeued.
 *
 * Returns:	Nothing
 *
 * Notes:	After command completion, there may be blocks left
 *		over which weren't finished by the previous command
 *		this can be for a number of reasons - the main one is
 *		I/O errors in the middle of the request, in which case
 *		we need to request the blocks that come after the bad
 *		sector.
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 * Notes:	Upon return, cmd is a stale pointer.
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 */
static void scsi_requeue_command(struct request_queue *q, struct scsi_cmnd *cmd)
{
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	struct request *req = cmd->request;
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	unsigned long flags;

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	scsi_unprep_request(req);
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	spin_lock_irqsave(q->queue_lock, flags);
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	blk_requeue_request(q, req);
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	spin_unlock_irqrestore(q->queue_lock, flags);
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	scsi_run_queue(q);
}

void scsi_next_command(struct scsi_cmnd *cmd)
{
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	struct scsi_device *sdev = cmd->device;
	struct request_queue *q = sdev->request_queue;

	/* need to hold a reference on the device before we let go of the cmd */
	get_device(&sdev->sdev_gendev);
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	scsi_put_command(cmd);
	scsi_run_queue(q);
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	/* ok to remove device now */
	put_device(&sdev->sdev_gendev);
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}

void scsi_run_host_queues(struct Scsi_Host *shost)
{
	struct scsi_device *sdev;

	shost_for_each_device(sdev, shost)
		scsi_run_queue(sdev->request_queue);
}

/*
 * Function:    scsi_end_request()
 *
 * Purpose:     Post-processing of completed commands (usually invoked at end
 *		of upper level post-processing and scsi_io_completion).
 *
 * Arguments:   cmd	 - command that is complete.
 *              uptodate - 1 if I/O indicates success, <= 0 for I/O error.
 *              bytes    - number of bytes of completed I/O
 *		requeue  - indicates whether we should requeue leftovers.
 *
 * Lock status: Assumed that lock is not held upon entry.
 *
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 * Returns:     cmd if requeue required, NULL otherwise.
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 *
 * Notes:       This is called for block device requests in order to
 *              mark some number of sectors as complete.
 * 
 *		We are guaranteeing that the request queue will be goosed
 *		at some point during this call.
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 * Notes:	If cmd was requeued, upon return it will be a stale pointer.
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 */
static struct scsi_cmnd *scsi_end_request(struct scsi_cmnd *cmd, int uptodate,
					  int bytes, int requeue)
{
	request_queue_t *q = cmd->device->request_queue;
	struct request *req = cmd->request;
	unsigned long flags;

	/*
	 * If there are blocks left over at the end, set up the command
	 * to queue the remainder of them.
	 */
	if (end_that_request_chunk(req, uptodate, bytes)) {
		int leftover = (req->hard_nr_sectors << 9);

		if (blk_pc_request(req))
			leftover = req->data_len;

		/* kill remainder if no retrys */
		if (!uptodate && blk_noretry_request(req))
			end_that_request_chunk(req, 0, leftover);
		else {
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			if (requeue) {
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				/*
				 * Bleah.  Leftovers again.  Stick the
				 * leftovers in the front of the
				 * queue, and goose the queue again.
				 */
				scsi_requeue_command(q, cmd);
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				cmd = NULL;
			}
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			return cmd;
		}
	}

	add_disk_randomness(req->rq_disk);

	spin_lock_irqsave(q->queue_lock, flags);
	if (blk_rq_tagged(req))
		blk_queue_end_tag(q, req);
	end_that_request_last(req);
	spin_unlock_irqrestore(q->queue_lock, flags);

	/*
	 * This will goose the queue request function at the end, so we don't
	 * need to worry about launching another command.
	 */
	scsi_next_command(cmd);
	return NULL;
}

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static struct scatterlist *scsi_alloc_sgtable(struct scsi_cmnd *cmd, gfp_t gfp_mask)
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{
	struct scsi_host_sg_pool *sgp;
	struct scatterlist *sgl;

	BUG_ON(!cmd->use_sg);

	switch (cmd->use_sg) {
	case 1 ... 8:
		cmd->sglist_len = 0;
		break;
	case 9 ... 16:
		cmd->sglist_len = 1;
		break;
	case 17 ... 32:
		cmd->sglist_len = 2;
		break;
#if (SCSI_MAX_PHYS_SEGMENTS > 32)
	case 33 ... 64:
		cmd->sglist_len = 3;
		break;
#if (SCSI_MAX_PHYS_SEGMENTS > 64)
	case 65 ... 128:
		cmd->sglist_len = 4;
		break;
#if (SCSI_MAX_PHYS_SEGMENTS  > 128)
	case 129 ... 256:
		cmd->sglist_len = 5;
		break;
#endif
#endif
#endif
	default:
		return NULL;
	}

	sgp = scsi_sg_pools + cmd->sglist_len;
	sgl = mempool_alloc(sgp->pool, gfp_mask);
	return sgl;
}

static void scsi_free_sgtable(struct scatterlist *sgl, int index)
{
	struct scsi_host_sg_pool *sgp;

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	BUG_ON(index >= SG_MEMPOOL_NR);
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	sgp = scsi_sg_pools + index;
	mempool_free(sgl, sgp->pool);
}

/*
 * Function:    scsi_release_buffers()
 *
 * Purpose:     Completion processing for block device I/O requests.
 *
 * Arguments:   cmd	- command that we are bailing.
 *
 * Lock status: Assumed that no lock is held upon entry.
 *
 * Returns:     Nothing
 *
 * Notes:       In the event that an upper level driver rejects a
 *		command, we must release resources allocated during
 *		the __init_io() function.  Primarily this would involve
 *		the scatter-gather table, and potentially any bounce
 *		buffers.
 */
static void scsi_release_buffers(struct scsi_cmnd *cmd)
{
	struct request *req = cmd->request;

	/*
	 * Free up any indirection buffers we allocated for DMA purposes. 
	 */
	if (cmd->use_sg)
		scsi_free_sgtable(cmd->request_buffer, cmd->sglist_len);
	else if (cmd->request_buffer != req->buffer)
		kfree(cmd->request_buffer);

	/*
	 * Zero these out.  They now point to freed memory, and it is
	 * dangerous to hang onto the pointers.
	 */
	cmd->buffer  = NULL;
	cmd->bufflen = 0;
	cmd->request_buffer = NULL;
	cmd->request_bufflen = 0;
}

/*
 * Function:    scsi_io_completion()
 *
 * Purpose:     Completion processing for block device I/O requests.
 *
 * Arguments:   cmd   - command that is finished.
 *
 * Lock status: Assumed that no lock is held upon entry.
 *
 * Returns:     Nothing
 *
 * Notes:       This function is matched in terms of capabilities to
 *              the function that created the scatter-gather list.
 *              In other words, if there are no bounce buffers
 *              (the normal case for most drivers), we don't need
 *              the logic to deal with cleaning up afterwards.
 *
 *		We must do one of several things here:
 *
 *		a) Call scsi_end_request.  This will finish off the
 *		   specified number of sectors.  If we are done, the
 *		   command block will be released, and the queue
 *		   function will be goosed.  If we are not done, then
 *		   scsi_end_request will directly goose the queue.
 *
 *		b) We can just use scsi_requeue_command() here.  This would
 *		   be used if we just wanted to retry, for example.
 */
void scsi_io_completion(struct scsi_cmnd *cmd, unsigned int good_bytes,
			unsigned int block_bytes)
{
	int result = cmd->result;
	int this_count = cmd->bufflen;
	request_queue_t *q = cmd->device->request_queue;
	struct request *req = cmd->request;
	int clear_errors = 1;
	struct scsi_sense_hdr sshdr;
	int sense_valid = 0;
	int sense_deferred = 0;

	if (blk_complete_barrier_rq(q, req, good_bytes >> 9))
		return;

	/*
	 * Free up any indirection buffers we allocated for DMA purposes. 
	 * For the case of a READ, we need to copy the data out of the
	 * bounce buffer and into the real buffer.
	 */
	if (cmd->use_sg)
		scsi_free_sgtable(cmd->buffer, cmd->sglist_len);
	else if (cmd->buffer != req->buffer) {
		if (rq_data_dir(req) == READ) {
			unsigned long flags;
			char *to = bio_kmap_irq(req->bio, &flags);
			memcpy(to, cmd->buffer, cmd->bufflen);
			bio_kunmap_irq(to, &flags);
		}
		kfree(cmd->buffer);
	}

	if (result) {
		sense_valid = scsi_command_normalize_sense(cmd, &sshdr);
		if (sense_valid)
			sense_deferred = scsi_sense_is_deferred(&sshdr);
	}
	if (blk_pc_request(req)) { /* SG_IO ioctl from block level */
		req->errors = result;
		if (result) {
			clear_errors = 0;
			if (sense_valid && req->sense) {
				/*
				 * SG_IO wants current and deferred errors
				 */
				int len = 8 + cmd->sense_buffer[7];

				if (len > SCSI_SENSE_BUFFERSIZE)
					len = SCSI_SENSE_BUFFERSIZE;
				memcpy(req->sense, cmd->sense_buffer,  len);
				req->sense_len = len;
			}
		} else
			req->data_len = cmd->resid;
	}

	/*
	 * Zero these out.  They now point to freed memory, and it is
	 * dangerous to hang onto the pointers.
	 */
	cmd->buffer  = NULL;
	cmd->bufflen = 0;
	cmd->request_buffer = NULL;
	cmd->request_bufflen = 0;

	/*
	 * Next deal with any sectors which we were able to correctly
	 * handle.
	 */
	if (good_bytes >= 0) {
		SCSI_LOG_HLCOMPLETE(1, printk("%ld sectors total, %d bytes done.\n",
					      req->nr_sectors, good_bytes));
		SCSI_LOG_HLCOMPLETE(1, printk("use_sg is %d\n", cmd->use_sg));

		if (clear_errors)
			req->errors = 0;
		/*
		 * If multiple sectors are requested in one buffer, then
		 * they will have been finished off by the first command.
		 * If not, then we have a multi-buffer command.
		 *
		 * If block_bytes != 0, it means we had a medium error
		 * of some sort, and that we want to mark some number of
		 * sectors as not uptodate.  Thus we want to inhibit
		 * requeueing right here - we will requeue down below
		 * when we handle the bad sectors.
		 */

		/*
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		 * If the command completed without error, then either
		 * finish off the rest of the command, or start a new one.
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		 */
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		if (scsi_end_request(cmd, 1, good_bytes, result == 0) == NULL)
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			return;
	}
	/*
	 * Now, if we were good little boys and girls, Santa left us a request
	 * sense buffer.  We can extract information from this, so we
	 * can choose a block to remap, etc.
	 */
	if (sense_valid && !sense_deferred) {
		switch (sshdr.sense_key) {
		case UNIT_ATTENTION:
			if (cmd->device->removable) {
				/* detected disc change.  set a bit 
				 * and quietly refuse further access.
				 */
				cmd->device->changed = 1;
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				scsi_end_request(cmd, 0,
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						this_count, 1);
				return;
			} else {
				/*
				* Must have been a power glitch, or a
				* bus reset.  Could not have been a
				* media change, so we just retry the
				* request and see what happens.  
				*/
				scsi_requeue_command(q, cmd);
				return;
			}
			break;
		case ILLEGAL_REQUEST:
			/*
		 	* If we had an ILLEGAL REQUEST returned, then we may
		 	* have performed an unsupported command.  The only
		 	* thing this should be would be a ten byte read where
			* only a six byte read was supported.  Also, on a
			* system where READ CAPACITY failed, we may have read
			* past the end of the disk.
		 	*/
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			if ((cmd->device->use_10_for_rw &&
			    sshdr.asc == 0x20 && sshdr.ascq == 0x00) &&
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			    (cmd->cmnd[0] == READ_10 ||
			     cmd->cmnd[0] == WRITE_10)) {
				cmd->device->use_10_for_rw = 0;
				/*
				 * This will cause a retry with a 6-byte
				 * command.
				 */
				scsi_requeue_command(q, cmd);
				result = 0;
			} else {
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				scsi_end_request(cmd, 0, this_count, 1);
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				return;
			}
			break;
		case NOT_READY:
			/*
			 * If the device is in the process of becoming ready,
			 * retry.
			 */
			if (sshdr.asc == 0x04 && sshdr.ascq == 0x01) {
				scsi_requeue_command(q, cmd);
				return;
			}
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			if (!(req->flags & REQ_QUIET))
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				scmd_printk(KERN_INFO, cmd,
					   "Device not ready.\n");
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			scsi_end_request(cmd, 0, this_count, 1);
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			return;
		case VOLUME_OVERFLOW:
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			if (!(req->flags & REQ_QUIET)) {
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				scmd_printk(KERN_INFO, cmd,
					   "Volume overflow, CDB: ");
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				__scsi_print_command(cmd->data_cmnd);
				scsi_print_sense("", cmd);
			}
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			scsi_end_request(cmd, 0, block_bytes, 1);
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			return;
		default:
			break;
		}
	}			/* driver byte != 0 */
	if (host_byte(result) == DID_RESET) {
		/*
		 * Third party bus reset or reset for error
		 * recovery reasons.  Just retry the request
		 * and see what happens.  
		 */
		scsi_requeue_command(q, cmd);
		return;
	}
	if (result) {
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		if (!(req->flags & REQ_QUIET)) {
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			scmd_printk(KERN_INFO, cmd,
				   "SCSI error: return code = 0x%x\n", result);
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			if (driver_byte(result) & DRIVER_SENSE)
				scsi_print_sense("", cmd);
		}
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		/*
		 * Mark a single buffer as not uptodate.  Queue the remainder.
		 * We sometimes get this cruft in the event that a medium error
		 * isn't properly reported.
		 */
		block_bytes = req->hard_cur_sectors << 9;
		if (!block_bytes)
			block_bytes = req->data_len;
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		scsi_end_request(cmd, 0, block_bytes, 1);
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	}
}
EXPORT_SYMBOL(scsi_io_completion);

/*
 * Function:    scsi_init_io()
 *
 * Purpose:     SCSI I/O initialize function.
 *
 * Arguments:   cmd   - Command descriptor we wish to initialize
 *
 * Returns:     0 on success
 *		BLKPREP_DEFER if the failure is retryable
 *		BLKPREP_KILL if the failure is fatal
 */
static int scsi_init_io(struct scsi_cmnd *cmd)
{
	struct request     *req = cmd->request;
	struct scatterlist *sgpnt;
	int		   count;

	/*
	 * if this is a rq->data based REQ_BLOCK_PC, setup for a non-sg xfer
	 */
	if ((req->flags & REQ_BLOCK_PC) && !req->bio) {
		cmd->request_bufflen = req->data_len;
		cmd->request_buffer = req->data;
		req->buffer = req->data;
		cmd->use_sg = 0;
		return 0;
	}

	/*
	 * we used to not use scatter-gather for single segment request,
	 * but now we do (it makes highmem I/O easier to support without
	 * kmapping pages)
	 */
	cmd->use_sg = req->nr_phys_segments;

	/*
	 * if sg table allocation fails, requeue request later.
	 */
	sgpnt = scsi_alloc_sgtable(cmd, GFP_ATOMIC);
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	if (unlikely(!sgpnt)) {
		scsi_unprep_request(req);
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		return BLKPREP_DEFER;
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	}
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	cmd->request_buffer = (char *) sgpnt;
	cmd->request_bufflen = req->nr_sectors << 9;
	if (blk_pc_request(req))
		cmd->request_bufflen = req->data_len;
	req->buffer = NULL;

	/* 
	 * Next, walk the list, and fill in the addresses and sizes of
	 * each segment.
	 */
	count = blk_rq_map_sg(req->q, req, cmd->request_buffer);

	/*
	 * mapped well, send it off
	 */
	if (likely(count <= cmd->use_sg)) {
		cmd->use_sg = count;
		return 0;
	}

	printk(KERN_ERR "Incorrect number of segments after building list\n");
	printk(KERN_ERR "counted %d, received %d\n", count, cmd->use_sg);
	printk(KERN_ERR "req nr_sec %lu, cur_nr_sec %u\n", req->nr_sectors,
			req->current_nr_sectors);

	/* release the command and kill it */
	scsi_release_buffers(cmd);
	scsi_put_command(cmd);
	return BLKPREP_KILL;
}

static int scsi_prepare_flush_fn(request_queue_t *q, struct request *rq)
{
	struct scsi_device *sdev = q->queuedata;
	struct scsi_driver *drv;

	if (sdev->sdev_state == SDEV_RUNNING) {
		drv = *(struct scsi_driver **) rq->rq_disk->private_data;

		if (drv->prepare_flush)
			return drv->prepare_flush(q, rq);
	}

	return 0;
}

static void scsi_end_flush_fn(request_queue_t *q, struct request *rq)
{
	struct scsi_device *sdev = q->queuedata;
	struct request *flush_rq = rq->end_io_data;
	struct scsi_driver *drv;

	if (flush_rq->errors) {
		printk("scsi: barrier error, disabling flush support\n");
		blk_queue_ordered(q, QUEUE_ORDERED_NONE);
	}

	if (sdev->sdev_state == SDEV_RUNNING) {
		drv = *(struct scsi_driver **) rq->rq_disk->private_data;
		drv->end_flush(q, rq);
	}
}

static int scsi_issue_flush_fn(request_queue_t *q, struct gendisk *disk,
			       sector_t *error_sector)
{
	struct scsi_device *sdev = q->queuedata;
	struct scsi_driver *drv;

	if (sdev->sdev_state != SDEV_RUNNING)
		return -ENXIO;

	drv = *(struct scsi_driver **) disk->private_data;
	if (drv->issue_flush)
		return drv->issue_flush(&sdev->sdev_gendev, error_sector);

	return -EOPNOTSUPP;
}

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static void scsi_generic_done(struct scsi_cmnd *cmd)
{
	BUG_ON(!blk_pc_request(cmd->request));
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	/*
	 * This will complete the whole command with uptodate=1 so
	 * as far as the block layer is concerned the command completed
	 * successfully. Since this is a REQ_BLOCK_PC command the
	 * caller should check the request's errors value
	 */
	scsi_io_completion(cmd, cmd->bufflen, 0);
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}

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static int scsi_prep_fn(struct request_queue *q, struct request *req)
{
	struct scsi_device *sdev = q->queuedata;
	struct scsi_cmnd *cmd;
	int specials_only = 0;

	/*
	 * Just check to see if the device is online.  If it isn't, we
	 * refuse to process any commands.  The device must be brought
	 * online before trying any recovery commands
	 */
	if (unlikely(!scsi_device_online(sdev))) {
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		sdev_printk(KERN_ERR, sdev,
			    "rejecting I/O to offline device\n");
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		goto kill;
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	}
	if (unlikely(sdev->sdev_state != SDEV_RUNNING)) {
		/* OK, we're not in a running state don't prep
		 * user commands */
		if (sdev->sdev_state == SDEV_DEL) {
			/* Device is fully deleted, no commands
			 * at all allowed down */
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			sdev_printk(KERN_ERR, sdev,
				    "rejecting I/O to dead device\n");
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			goto kill;
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		}
		/* OK, we only allow special commands (i.e. not
		 * user initiated ones */
		specials_only = sdev->sdev_state;
	}

	/*
	 * Find the actual device driver associated with this command.
	 * The SPECIAL requests are things like character device or
	 * ioctls, which did not originate from ll_rw_blk.  Note that
	 * the special field is also used to indicate the cmd for
	 * the remainder of a partially fulfilled request that can 
	 * come up when there is a medium error.  We have to treat
	 * these two cases differently.  We differentiate by looking
	 * at request->cmd, as this tells us the real story.
	 */
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	if (req->flags & REQ_SPECIAL && req->special) {
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		struct scsi_request *sreq = req->special;

		if (sreq->sr_magic == SCSI_REQ_MAGIC) {
			cmd = scsi_get_command(sreq->sr_device, GFP_ATOMIC);
			if (unlikely(!cmd))
				goto defer;
			scsi_init_cmd_from_req(cmd, sreq);
		} else
			cmd = req->special;
	} else if (req->flags & (REQ_CMD | REQ_BLOCK_PC)) {

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		if(unlikely(specials_only) && !(req->flags & REQ_SPECIAL)) {
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			if(specials_only == SDEV_QUIESCE ||
					specials_only == SDEV_BLOCK)
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				goto defer;
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			sdev_printk(KERN_ERR, sdev,
				    "rejecting I/O to device being removed\n");
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			goto kill;
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		}
			
			
		/*
		 * Now try and find a command block that we can use.
		 */
		if (!req->special) {
			cmd = scsi_get_command(sdev, GFP_ATOMIC);
			if (unlikely(!cmd))
				goto defer;
		} else
			cmd = req->special;
		
		/* pull a tag out of the request if we have one */
		cmd->tag = req->tag;
	} else {
		blk_dump_rq_flags(req, "SCSI bad req");
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		goto kill;
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	}
	
	/* note the overloading of req->special.  When the tag
	 * is active it always means cmd.  If the tag goes
	 * back for re-queueing, it may be reset */
	req->special = cmd;
	cmd->request = req;
	
	/*
	 * FIXME: drop the lock here because the functions below
	 * expect to be called without the queue lock held.  Also,
	 * previously, we dequeued the request before dropping the
	 * lock.  We hope REQ_STARTED prevents anything untoward from
	 * happening now.
	 */
	if (req->flags & (REQ_CMD | REQ_BLOCK_PC)) {
		struct scsi_driver *drv;
		int ret;

		/*
		 * This will do a couple of things:
		 *  1) Fill in the actual SCSI command.
		 *  2) Fill in any other upper-level specific fields
		 * (timeout).
		 *
		 * If this returns 0, it means that the request failed
		 * (reading past end of disk, reading offline device,
		 * etc).   This won't actually talk to the device, but
		 * some kinds of consistency checking may cause the	
		 * request to be rejected immediately.
		 */

		/* 
		 * This sets up the scatter-gather table (allocating if
		 * required).
		 */
		ret = scsi_init_io(cmd);
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		switch(ret) {
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			/* For BLKPREP_KILL/DEFER the cmd was released */
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		case BLKPREP_KILL:
			goto kill;
		case BLKPREP_DEFER:
			goto defer;
		}
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		/*
		 * Initialize the actual SCSI command for this request.
		 */
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		if (req->rq_disk) {
			drv = *(struct scsi_driver **)req->rq_disk->private_data;
			if (unlikely(!drv->init_command(cmd))) {
				scsi_release_buffers(cmd);
				scsi_put_command(cmd);
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				goto kill;
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			}
		} else {
			memcpy(cmd->cmnd, req->cmd, sizeof(cmd->cmnd));
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			cmd->cmd_len = req->cmd_len;
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			if (rq_data_dir(req) == WRITE)
				cmd->sc_data_direction = DMA_TO_DEVICE;
			else if (req->data_len)
				cmd->sc_data_direction = DMA_FROM_DEVICE;
			else
				cmd->sc_data_direction = DMA_NONE;
			
			cmd->transfersize = req->data_len;
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			cmd->allowed = req->retries;
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			cmd->timeout_per_command = req->timeout;
			cmd->done = scsi_generic_done;
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		}
	}

	/*
	 * The request is now prepped, no need to come back here
	 */
	req->flags |= REQ_DONTPREP;
	return BLKPREP_OK;

 defer:
	/* If we defer, the elv_next_request() returns NULL, but the
	 * queue must be restarted, so we plug here if no returning
	 * command will automatically do that. */
	if (sdev->device_busy == 0)
		blk_plug_device(q);
	return BLKPREP_DEFER;
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 kill:
	req->errors = DID_NO_CONNECT << 16;
	return BLKPREP_KILL;
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}

/*
 * scsi_dev_queue_ready: if we can send requests to sdev, return 1 else
 * return 0.
 *
 * Called with the queue_lock held.
 */
static inline int scsi_dev_queue_ready(struct request_queue *q,
				  struct scsi_device *sdev)
{
	if (sdev->device_busy >= sdev->queue_depth)
		return 0;
	if (sdev->device_busy == 0 && sdev->device_blocked) {
		/*
		 * unblock after device_blocked iterates to zero
		 */
		if (--sdev->device_blocked == 0) {
			SCSI_LOG_MLQUEUE(3,
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				   sdev_printk(KERN_INFO, sdev,
				   "unblocking device at zero depth\n"));
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		} else {
			blk_plug_device(q);
			return 0;
		}
	}
	if (sdev->device_blocked)
		return 0;

	return 1;
}

/*
 * scsi_host_queue_ready: if we can send requests to shost, return 1 else
 * return 0. We must end up running the queue again whenever 0 is
 * returned, else IO can hang.
 *
 * Called with host_lock held.
 */
static inline int scsi_host_queue_ready(struct request_queue *q,
				   struct Scsi_Host *shost,
				   struct scsi_device *sdev)
{
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	if (scsi_host_in_recovery(shost))
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		return 0;
	if (shost->host_busy == 0 && shost->host_blocked) {
		/*
		 * unblock after host_blocked iterates to zero
		 */
		if (--shost->host_blocked == 0) {
			SCSI_LOG_MLQUEUE(3,
				printk("scsi%d unblocking host at zero depth\n",
					shost->host_no));
		} else {
			blk_plug_device(q);
			return 0;
		}
	}
	if ((shost->can_queue > 0 && shost->host_busy >= shost->can_queue) ||
	    shost->host_blocked || shost->host_self_blocked) {
		if (list_empty(&sdev->starved_entry))
			list_add_tail(&sdev->starved_entry, &shost->starved_list);
		return 0;
	}

	/* We're OK to process the command, so we can't be starved */
	if (!list_empty(&sdev->starved_entry))
		list_del_init(&sdev->starved_entry);

	return 1;
}

/*
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 * Kill a request for a dead device
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 */
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static void scsi_kill_request(struct request *req, request_queue_t *q)
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{
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	struct scsi_cmnd *cmd = req->special;
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	blkdev_dequeue_request(req);

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	if (unlikely(cmd == NULL)) {
		printk(KERN_CRIT "impossible request in %s.\n",
				 __FUNCTION__);
		BUG();
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	}
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	scsi_init_cmd_errh(cmd);
	cmd->result = DID_NO_CONNECT << 16;
	atomic_inc(&cmd->device->iorequest_cnt);
	__scsi_done(cmd);
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}

/*
 * Function:    scsi_request_fn()
 *
 * Purpose:     Main strategy routine for SCSI.
 *
 * Arguments:   q       - Pointer to actual queue.
 *
 * Returns:     Nothing
 *
 * Lock status: IO request lock assumed to be held when called.
 */
static void scsi_request_fn(struct request_queue *q)
{
	struct scsi_device *sdev = q->queuedata;
	struct Scsi_Host *shost;
	struct scsi_cmnd *cmd;
	struct request *req;

	if (!sdev) {
		printk("scsi: killing requests for dead queue\n");
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		while ((req = elv_next_request(q)) != NULL)
			scsi_kill_request(req, q);
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		return;
	}

	if(!get_device(&sdev->sdev_gendev))
		/* We must be tearing the block queue down already */
		return;

	/*
	 * To start with, we keep looping until the queue is empty, or until
	 * the host is no longer able to accept any more requests.
	 */
	shost = sdev->host;
	while (!blk_queue_plugged(q)) {
		int rtn;
		/*
		 * get next queueable request.  We do this early to make sure
		 * that the request is fully prepared even if we cannot 
		 * accept it.
		 */
		req = elv_next_request(q);
		if (!req || !scsi_dev_queue_ready(q, sdev))
			break;

		if (unlikely(!scsi_device_online(sdev))) {
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			sdev_printk(KERN_ERR, sdev,
				    "rejecting I/O to offline device\n");
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			scsi_kill_request(req, q);
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			continue;
		}


		/*
		 * Remove the request from the request list.
		 */
		if (!(blk_queue_tagged(q) && !blk_queue_start_tag(q, req)))
			blkdev_dequeue_request(req);
		sdev->device_busy++;

		spin_unlock(q->queue_lock);
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		cmd = req->special;
		if (unlikely(cmd == NULL)) {
			printk(KERN_CRIT "impossible request in %s.\n"
					 "please mail a stack trace to "
					 "linux-scsi@vger.kernel.org",
					 __FUNCTION__);
			BUG();
		}
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		spin_lock(shost->host_lock);

		if (!scsi_host_queue_ready(q, shost, sdev))
			goto not_ready;
		if (sdev->single_lun) {
			if (scsi_target(sdev)->starget_sdev_user &&
			    scsi_target(sdev)->starget_sdev_user != sdev)
				goto not_ready;
			scsi_target(sdev)->starget_sdev_user = sdev;
		}
		shost->host_busy++;

		/*
		 * XXX(hch): This is rather suboptimal, scsi_dispatch_cmd will
		 *		take the lock again.
		 */
		spin_unlock_irq(shost->host_lock);

		/*
		 * Finally, initialize any error handling parameters, and set up
		 * the timers for timeouts.
		 */
		scsi_init_cmd_errh(cmd);

		/*
		 * Dispatch the command to the low-level driver.
		 */
		rtn = scsi_dispatch_cmd(cmd);
		spin_lock_irq(q->queue_lock);
		if(rtn) {
			/* we're refusing the command; because of
			 * the way locks get dropped, we need to 
			 * check here if plugging is required */
			if(sdev->device_busy == 0)
				blk_plug_device(q);

			break;
		}
	}

	goto out;