/*
 *  scsi_queue.c Copyright (C) 1997 Eric Youngdale
 *
 *  generic mid-level SCSI queueing.
 *
 *  The point of this is that we need to track when hosts are unable to
 *  accept a command because they are busy.  In addition, we track devices
 *  that cannot accept a command because of a QUEUE_FULL condition.  In both
 *  of these cases, we enter the command in the queue.  At some later point,
 *  we attempt to remove commands from the queue and retry them.
 */

#define __NO_VERSION__
#include <linux/module.h>

#include <linux/sched.h>
#include <linux/timer.h>
#include <linux/string.h>
#include <linux/malloc.h>
#include <linux/ioport.h>
#include <linux/kernel.h>
#include <linux/stat.h>
#include <linux/blk.h>
#include <linux/interrupt.h>
#include <linux/delay.h>
#include <linux/smp_lock.h>

#define __KERNEL_SYSCALLS__

#include <linux/unistd.h>

#include <asm/system.h>
#include <asm/irq.h>
#include <asm/dma.h>

#include "scsi.h"
#include "hosts.h"
#include "constants.h"

/*
 * TODO:
 *      1) Prevent multiple traversals of list to look for commands to
 *         queue.
 *      2) Protect against multiple insertions of list at the same time.
 * DONE:
 *      1) Set state of scsi command to a new state value for ml queue.
 *      2) Insert into queue when host rejects command.
 *      3) Make sure status code is properly passed from low-level queue func
 *         so that internal_cmnd properly returns the right value.
 *      4) Insert into queue when QUEUE_FULL.
 *      5) Cull queue in bottom half handler.
 *      6) Check usage count prior to queue insertion.  Requeue if usage
 *         count is 0.
 *      7) Don't send down any more commands if the host/device is busy.
 */

static const char RCSid[] = "$Header: /mnt/ide/home/eric/CVSROOT/linux/drivers/scsi/scsi_queue.c,v 1.1 1997/10/21 11:16:38 eric Exp $";

/*
 * Lock used to prevent more than one process from frobbing the list at the
 * same time.  FIXME(eric) - there should be separate spinlocks for each host.
 * This will reduce contention.
 */

spinlock_t scsi_mlqueue_lock = SPIN_LOCK_UNLOCKED;
spinlock_t scsi_mlqueue_remove_lock = SPIN_LOCK_UNLOCKED;

/*
 * Function:    scsi_mlqueue_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.
 *
 * 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_mlqueue_insert(Scsi_Cmnd * cmd, int reason)
{
	Scsi_Cmnd *cpnt;
	unsigned long flags;
	struct Scsi_Host *host;

	SCSI_LOG_MLQUEUE(1, printk("Inserting command %p into mlqueue\n", cmd));

	/*
	 * We are inserting the command into the ml queue.  First, we
	 * cancel the timer, so it doesn't time out.
	 */
	scsi_delete_timer(cmd);

	host = cmd->host;

	/*
	 * Next, set the appropriate busy bit for the device/host.
	 */
	if (reason == SCSI_MLQUEUE_HOST_BUSY) {
		/*
		 * Protect against race conditions.  If the host isn't busy,
		 * assume that something actually completed, and that we should
		 * be able to queue a command now.  Note that there is an implicit
		 * assumption that every host can always queue at least one command.
		 * If a host is inactive and cannot queue any commands, I don't see
		 * how things could possibly work anyways.
		 */
		if (host->host_busy == 0) {
			if (scsi_retry_command(cmd) == 0) {
				return 0;
			}
		}
		host->host_blocked = TRUE;
		cmd->host_wait = TRUE;
	} else {
		/*
		 * Protect against race conditions.  If the device isn't busy,
		 * assume that something actually completed, and that we should
		 * be able to queue a command now.  Note that there is an implicit
		 * assumption that every host can always queue at least one command.
		 * If a host is inactive and cannot queue any commands, I don't see
		 * how things could possibly work anyways.
		 */
		if (cmd->device->device_busy == 0) {
			if (scsi_retry_command(cmd) == 0) {
				return 0;
			}
		}
		cmd->device->device_busy = TRUE;
		cmd->device_wait = TRUE;
	}

	/*
	 * Register the fact that we own the thing for now.
	 */
	cmd->state = SCSI_STATE_MLQUEUE;
	cmd->owner = SCSI_OWNER_MIDLEVEL;
	cmd->bh_next = NULL;

	/*
	 * As a performance enhancement, look to see whether the list is
	 * empty.  If it is, then we can just atomicly insert the command
	 * in the list and return without locking.
	 */
	if (host->pending_commands == NULL) {
		cpnt = xchg(&host->pending_commands, cmd);
		if (cpnt == NULL) {
			return 0;
		}
		/*
		 * Rats.  Something slipped in while we were exchanging.
		 * Swap it back and fall through to do it the hard way.
		 */
		cmd = xchg(&host->pending_commands, cpnt);

	}
	/*
	 * Next append the command to the list of pending commands.
	 */
	spin_lock_irqsave(&scsi_mlqueue_lock, flags);
	for (cpnt = host->pending_commands; cpnt && cpnt->bh_next;
	     cpnt = cpnt->bh_next) {
		continue;
	}
	if (cpnt != NULL) {
		cpnt->bh_next = cmd;
	} else {
		host->pending_commands = cmd;
	}

	spin_unlock_irqrestore(&scsi_mlqueue_lock, flags);
	return 0;
}

/*
 * Function:    scsi_mlqueue_finish()
 *
 * Purpose:     Try and queue commands from the midlevel queue.
 *
 * Arguments:   host    - host that just finished a command.
 *              device  - device that just finished a command.
 *
 * Returns:     Nothing.
 *
 * Notes:       This could be called either from an interrupt context or a
 *              normal process context.
 */
int scsi_mlqueue_finish(struct Scsi_Host *host, Scsi_Device * device)
{
	Scsi_Cmnd *cpnt;
	unsigned long flags;
	Scsi_Cmnd *next;
	Scsi_Cmnd *prev;
	int reason = 0;
	int rtn;

	SCSI_LOG_MLQUEUE(2, printk("scsi_mlqueue_finish starting\n"));
	/*
	 * First, clear the flag for the host/device.  We will then start
	 * pushing commands through until either something else blocks, or
	 * the queue is empty.
	 */
	if (host->host_blocked) {
		reason = SCSI_MLQUEUE_HOST_BUSY;
		host->host_blocked = FALSE;
	}
	if (device->device_busy) {
		reason = SCSI_MLQUEUE_DEVICE_BUSY;
		device->device_busy = FALSE;
	}
	/*
	 * Walk the list of commands to see if there is anything we can
	 * queue.  This probably needs to be optimized for performance at
	 * some point.
	 */
	prev = NULL;
	spin_lock_irqsave(&scsi_mlqueue_remove_lock, flags);
	for (cpnt = host->pending_commands; cpnt; cpnt = next) {
		next = cpnt->bh_next;
		/*
		 * First, see if this command is suitable for being retried now.
		 */
		if (reason == SCSI_MLQUEUE_HOST_BUSY) {
			/*
			 * The host was busy, but isn't any more.  Thus we may be
			 * able to queue the command now, but we were waiting for
			 * the device, then we should keep waiting.  Similarily, if
			 * the device is now busy, we should also keep waiting.
			 */
			if ((cpnt->host_wait == FALSE)
			    || (device->device_busy == TRUE)) {
				prev = cpnt;
				continue;
			}
		}
		if (reason == SCSI_MLQUEUE_DEVICE_BUSY) {
			/*
			 * The device was busy, but isn't any more.  Thus we may be
			 * able to queue the command now, but we were waiting for
			 * the host, then we should keep waiting.  Similarily, if
			 * the host is now busy, we should also keep waiting.
			 */
			if ((cpnt->device_wait == FALSE)
			    || (host->host_blocked == TRUE)) {
				prev = cpnt;
				continue;
			}
		}
		/*
		 * First, remove the command from the list.
		 */
		if (prev == NULL) {
			host->pending_commands = next;
		} else {
			prev->bh_next = next;
		}
		cpnt->bh_next = NULL;

		rtn = scsi_retry_command(cpnt);

		/*
		 * If we got a non-zero return value, it means that the host rejected
		 * the command.  The internal_cmnd function will have added the
		 * command back to the end of the list, so we don't have anything
		 * more to do here except return.
		 */
		if (rtn) {
			spin_unlock_irqrestore(&scsi_mlqueue_remove_lock, flags);
			SCSI_LOG_MLQUEUE(1, printk("Unable to remove command %p from mlqueue\n", cpnt));
			goto finish;
		}
		SCSI_LOG_MLQUEUE(1, printk("Removed command %p from mlqueue\n", cpnt));
	}

	spin_unlock_irqrestore(&scsi_mlqueue_remove_lock, flags);
finish:
	SCSI_LOG_MLQUEUE(2, printk("scsi_mlqueue_finish returning\n"));
	return 0;
}