rc.conf: Add and document the missing root_rw_mount=YES

[dragonfly.git] / sys / dev / sound / clone.c

/*-
 * Copyright (c) 2007 Ariff Abdullah <ariff@FreeBSD.org>
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 *
 * $FreeBSD: head/sys/dev/sound/clone.c 193640 2009-06-07 19:12:08Z ariff $
 */

#include <sys/param.h>
#include <sys/systm.h>
#include <sys/conf.h>
#include <sys/kernel.h>
#include <sys/malloc.h>
#include <sys/proc.h>
#include <sys/devfs.h>

#ifdef HAVE_KERNEL_OPTION_HEADERS
#include "opt_snd.h"
#endif

#include <dev/sound/pcm/sound.h>
#include <dev/sound/clone.h>

DEVFS_DECLARE_CLONE_BITMAP(dsp);

/*
 * So here we go again, another clonedevs manager. Unlike default clonedevs,
 * this clone manager is designed to withstand various abusive behavior
 * (such as 'while : ; do ls /dev/whatever ; done', etc.), reusable object
 * after reaching certain expiration threshold, aggressive garbage collector,
 * transparent device allocator and concurrency handling across multiple
 * thread/proc. Due to limited information given by dev_clone EVENTHANDLER,
 * we don't have much clues whether the caller wants a real open() or simply
 * making fun of us with things like stat(), mtime() etc. Assuming that:
 * 1) Time window between dev_clone EH <-> real open() should be small
 * enough and 2) mtime()/stat() etc. always looks like a half way / stalled
 * operation, we can decide whether a new cdev must be created, old
 * (expired) cdev can be reused or an existing cdev can be shared.
 *
 * Most of the operations and logics are generic enough and can be applied
 * on other places (such as if_tap, snp, etc).  Perhaps this can be
 * rearranged to complement clone_*(). However, due to this still being
 * specific to the sound driver (and as a proof of concept on how it can be
 * done), si_drv2 is used to keep the pointer of the clone list entry to
 * avoid expensive lookup.
 */

/* clone entry */
struct snd_clone_entry {
        TAILQ_ENTRY(snd_clone_entry) link;
        struct snd_clone *parent;
        struct cdev *devt;
        struct timespec tsp;
        uint32_t flags;
        pid_t pid;
        int unit;
};

/* clone manager */
struct snd_clone {
        TAILQ_HEAD(link_head, snd_clone_entry) head;
        struct timespec tsp;
        int refcount;
        int size;
        int typemask;
        int maxunit;
        int deadline;
        uint32_t flags;
};

#ifdef SND_DIAGNOSTIC
#define SND_CLONE_ASSERT(x, y)          do {                    \
        if (!(x))                                               \
                panic y;                                        \
} while (0)
#else
#define SND_CLONE_ASSERT(...)           KASSERT(__VA_ARGS__)
#endif

/*
 * Shamelessly ripped off from vfs_subr.c
 * We need at least 1/HZ precision as default timestamping.
 */
enum { SND_TSP_SEC, SND_TSP_HZ, SND_TSP_USEC, SND_TSP_NSEC };

static int snd_timestamp_precision = SND_TSP_HZ;
TUNABLE_INT("hw.snd.timestamp_precision", &snd_timestamp_precision);

void
snd_timestamp(struct timespec *tsp)
{
        struct timeval tv;

        switch (snd_timestamp_precision) {
        case SND_TSP_SEC:
                tsp->tv_sec = time_second;
                tsp->tv_nsec = 0;
                break;
        case SND_TSP_HZ:
                getnanouptime(tsp);
                break;
        case SND_TSP_USEC:
                microuptime(&tv);
                TIMEVAL_TO_TIMESPEC(&tv, tsp);
                break;
        case SND_TSP_NSEC:
                nanouptime(tsp);
                break;
        default:
                snd_timestamp_precision = SND_TSP_HZ;
                getnanouptime(tsp);
                break;
        }
}

#if defined(SND_DIAGNOSTIC) || defined(SND_DEBUG)
static int
sysctl_hw_snd_timestamp_precision(SYSCTL_HANDLER_ARGS)
{
        int err, val;

        val = snd_timestamp_precision;
        err = sysctl_handle_int(oidp, &val, 0, req);
        if (err == 0 && req->newptr != NULL) {
                switch (val) {
                case SND_TSP_SEC:
                case SND_TSP_HZ:
                case SND_TSP_USEC:
                case SND_TSP_NSEC:
                        snd_timestamp_precision = val;
                        break;
                default:
                        break;
                }
        }

        return (err);
}
SYSCTL_PROC(_hw_snd, OID_AUTO, timestamp_precision, CTLTYPE_INT | CTLFLAG_RW,
    0, sizeof(int), sysctl_hw_snd_timestamp_precision, "I",
    "timestamp precision (0=s 1=hz 2=us 3=ns)");
#endif

/*
 * snd_clone_create() : Return opaque allocated clone manager.
 */
struct snd_clone *
snd_clone_create(int typemask, int maxunit, int deadline, uint32_t flags)
{
        struct snd_clone *c;

        SND_CLONE_ASSERT(!(typemask & ~SND_CLONE_MAXUNIT),
            ("invalid typemask: 0x%08x", typemask));
        SND_CLONE_ASSERT(!(flags & ~SND_CLONE_MASK),
            ("invalid clone flags=0x%08x", flags));

        c = kmalloc(sizeof(*c), M_DEVBUF, M_WAITOK | M_ZERO);
        c->refcount = 0;
        c->size = 0;
        c->typemask = typemask;
        c->maxunit = (maxunit == -1) ? (~typemask & SND_CLONE_MAXUNIT) :
            maxunit;
        c->deadline = deadline;
        c->flags = flags;
        snd_timestamp(&c->tsp);
        TAILQ_INIT(&c->head);

        return (c);
}

int
snd_clone_busy(struct snd_clone *c)
{
        struct snd_clone_entry *ce;

        SND_CLONE_ASSERT(c != NULL, ("NULL snd_clone"));

        if (c->size == 0)
                return (0);

        TAILQ_FOREACH(ce, &c->head, link) {
                if (ce->flags & SND_CLONE_BUSY)
                        return (EBUSY);
        }

        return (0);
}

/*
 * snd_clone_enable()/disable() : Suspend/resume clone allocation through
 * snd_clone_alloc(). Everything else will not be affected by this.
 */
int
snd_clone_enable(struct snd_clone *c)
{
        SND_CLONE_ASSERT(c != NULL, ("NULL snd_clone"));

        if (c->flags & SND_CLONE_ENABLE)
                return (EINVAL);

        c->flags |= SND_CLONE_ENABLE;

        return (0);
}

int
snd_clone_disable(struct snd_clone *c)
{
        SND_CLONE_ASSERT(c != NULL, ("NULL snd_clone"));

        if (!(c->flags & SND_CLONE_ENABLE))
                return (EINVAL);

        c->flags &= ~SND_CLONE_ENABLE;

        return (0);
}

/*
 * Getters / Setters. Not worth explaining :)
 */
int
snd_clone_getsize(struct snd_clone *c)
{
        SND_CLONE_ASSERT(c != NULL, ("NULL snd_clone"));

        return (c->size);
}

int
snd_clone_getmaxunit(struct snd_clone *c)
{
        SND_CLONE_ASSERT(c != NULL, ("NULL snd_clone"));

        return (c->maxunit);
}

int
snd_clone_setmaxunit(struct snd_clone *c, int maxunit)
{
        SND_CLONE_ASSERT(c != NULL, ("NULL snd_clone"));

        c->maxunit = (maxunit == -1) ? (~c->typemask & SND_CLONE_MAXUNIT) :
            maxunit;

        return (c->maxunit);
}

int
snd_clone_getdeadline(struct snd_clone *c)
{
        SND_CLONE_ASSERT(c != NULL, ("NULL snd_clone"));

        return (c->deadline);
}

int
snd_clone_setdeadline(struct snd_clone *c, int deadline)
{
        SND_CLONE_ASSERT(c != NULL, ("NULL snd_clone"));

        c->deadline = deadline;

        return (c->deadline);
}

int
snd_clone_gettime(struct snd_clone *c, struct timespec *tsp)
{
        SND_CLONE_ASSERT(c != NULL, ("NULL snd_clone"));
        SND_CLONE_ASSERT(tsp != NULL, ("NULL timespec"));

        *tsp = c->tsp;

        return (0);
}

uint32_t
snd_clone_getflags(struct snd_clone *c)
{
        SND_CLONE_ASSERT(c != NULL, ("NULL snd_clone"));

        return (c->flags);
}

uint32_t
snd_clone_setflags(struct snd_clone *c, uint32_t flags)
{
        SND_CLONE_ASSERT(c != NULL, ("NULL snd_clone"));
        SND_CLONE_ASSERT(!(flags & ~SND_CLONE_MASK),
            ("invalid clone flags=0x%08x", flags));

        c->flags = flags;

        return (c->flags);
}

int
snd_clone_getdevtime(struct cdev *dev, struct timespec *tsp)
{
        struct snd_clone_entry *ce;

        SND_CLONE_ASSERT(dev != NULL, ("NULL dev"));
        SND_CLONE_ASSERT(tsp != NULL, ("NULL timespec"));

        ce = dev->si_drv2;
        if (ce == NULL)
                return (ENODEV);

        SND_CLONE_ASSERT(ce->parent != NULL, ("NULL parent"));

        *tsp = ce->tsp;

        return (0);
}

uint32_t
snd_clone_getdevflags(struct cdev *dev)
{
        struct snd_clone_entry *ce;

        SND_CLONE_ASSERT(dev != NULL, ("NULL dev"));

        ce = dev->si_drv2;
        if (ce == NULL)
                return (0xffffffff);

        SND_CLONE_ASSERT(ce->parent != NULL, ("NULL parent"));

        return (ce->flags);
}

uint32_t
snd_clone_setdevflags(struct cdev *dev, uint32_t flags)
{
        struct snd_clone_entry *ce;

        SND_CLONE_ASSERT(dev != NULL, ("NULL dev"));
        SND_CLONE_ASSERT(!(flags & ~SND_CLONE_DEVMASK),
            ("invalid clone dev flags=0x%08x", flags));

        ce = dev->si_drv2;
        if (ce == NULL)
                return (0xffffffff);

        SND_CLONE_ASSERT(ce->parent != NULL, ("NULL parent"));

        ce->flags = flags;

        return (ce->flags);
}

/* Elapsed time conversion to ms */
#define SND_CLONE_ELAPSED(x, y)                                         \
        ((((x)->tv_sec - (y)->tv_sec) * 1000) +                         \
        (((y)->tv_nsec > (x)->tv_nsec) ?                                \
        (((1000000000L + (x)->tv_nsec -                                 \
        (y)->tv_nsec) / 1000000) - 1000) :                              \
        (((x)->tv_nsec - (y)->tv_nsec) / 1000000)))

#define SND_CLONE_EXPIRED(x, y, z)                                      \
        ((x)->deadline < 1 ||                                           \
        ((y)->tv_sec - (z)->tv_sec) > ((x)->deadline / 1000) ||         \
        SND_CLONE_ELAPSED(y, z) > (x)->deadline)

/*
 * snd_clone_gc() : Garbage collector for stalled, expired objects. Refer to
 * clone.h for explanations on GC settings.
 */
int
snd_clone_gc(struct snd_clone *c)
{
        struct snd_clone_entry *ce, *tce;
        struct timespec now;
        int pruned;
        int subunit;

        SND_CLONE_ASSERT(c != NULL, ("NULL snd_clone"));

        if (!(c->flags & SND_CLONE_GC_ENABLE) || c->size == 0)
                return (0);

        snd_timestamp(&now);

        /*
         * Bail out if the last clone handler was invoked below the deadline
         * threshold.
         */
        if ((c->flags & SND_CLONE_GC_EXPIRED) &&
            !SND_CLONE_EXPIRED(c, &now, &c->tsp))
                return (0);

        pruned = 0;

        /*
         * Visit each object in reverse order. If the object is still being
         * referenced by a valid open(), skip it. Look for expired objects
         * and either revoke its clone invocation status or mercilessly
         * throw it away.
         */
        TAILQ_FOREACH_REVERSE_MUTABLE(ce, &c->head, link_head, link, tce) {
                if (!(ce->flags & SND_CLONE_BUSY) &&
                    (!(ce->flags & SND_CLONE_INVOKE) ||
                    SND_CLONE_EXPIRED(c, &now, &ce->tsp))) {
                        if (c->flags & SND_CLONE_GC_REVOKE) {
                                ce->flags &= ~SND_CLONE_INVOKE;
                                ce->pid = -1;
                        } else {
                                TAILQ_REMOVE(&c->head, ce, link);
                                subunit = PCMSUBUNIT(ce->devt);
                                destroy_dev(ce->devt);
                                devfs_clone_bitmap_put(&DEVFS_CLONE_BITMAP(dsp), subunit);
                                kfree(ce, M_DEVBUF);
                                c->size--;
                        }
                        pruned++;
                }
        }

        /* return total pruned objects */
        return (pruned);
}

void
snd_clone_destroy(struct snd_clone *c)
{
        struct snd_clone_entry *ce, *tmp;
        int subunit;

        SND_CLONE_ASSERT(c != NULL, ("NULL snd_clone"));

        ce = TAILQ_FIRST(&c->head);
        while (ce != NULL) {
                tmp = TAILQ_NEXT(ce, link);
                if (ce->devt != NULL) {
                        subunit = PCMSUBUNIT(ce->devt);
                        destroy_dev(ce->devt);
                        devfs_clone_bitmap_put(&DEVFS_CLONE_BITMAP(dsp), subunit);
                }
                kfree(ce, M_DEVBUF);
                ce = tmp;
        }

        kfree(c, M_DEVBUF);
}

/*
 * snd_clone_acquire() : The vital part of concurrency management. Must be
 * called somewhere at the beginning of open() handler. ENODEV is not really
 * fatal since it just tell the caller that this is not cloned stuff.
 * EBUSY is *real*, don't forget that!
 */
int
snd_clone_acquire(struct cdev *dev)
{
        struct snd_clone_entry *ce;

        SND_CLONE_ASSERT(dev != NULL, ("NULL dev"));

        ce = dev->si_drv2;
        if (ce == NULL)
                return (ENODEV);

        SND_CLONE_ASSERT(ce->parent != NULL, ("NULL parent"));

        ce->flags &= ~SND_CLONE_INVOKE;

        if (ce->flags & SND_CLONE_BUSY)
                return (EBUSY);

        ce->flags |= SND_CLONE_BUSY;

        return (0);
}

/*
 * snd_clone_release() : Release busy status. Must be called somewhere at
 * the end of close() handler, or somewhere after fail open().
 */
int
snd_clone_release(struct cdev *dev)
{
        struct snd_clone_entry *ce;

        SND_CLONE_ASSERT(dev != NULL, ("NULL dev"));

        ce = dev->si_drv2;
        if (ce == NULL)
                return (ENODEV);

        SND_CLONE_ASSERT(ce->parent != NULL, ("NULL parent"));

        ce->flags &= ~SND_CLONE_INVOKE;

        if (!(ce->flags & SND_CLONE_BUSY))
                return (EBADF);

        ce->flags &= ~SND_CLONE_BUSY;
        ce->pid = -1;

        return (0);
}

/*
 * snd_clone_ref/unref() : Garbage collector reference counter. To make
 * garbage collector run automatically, the sequence must be something like
 * this (both in open() and close() handlers):
 *
 *  open() - 1) snd_clone_acquire()
 *           2) .... check check ... if failed, snd_clone_release()
 *           3) Success. Call snd_clone_ref()
 *
 * close() - 1) .... check check check ....
 *           2) Success. snd_clone_release()
 *           3) snd_clone_unref() . Garbage collector will run at this point
 *              if this is the last referenced object.
 */
int
snd_clone_ref(struct cdev *dev)
{
        struct snd_clone_entry *ce;
        struct snd_clone *c;

        SND_CLONE_ASSERT(dev != NULL, ("NULL dev"));

        ce = dev->si_drv2;
        if (ce == NULL)
                return (0);

        c = ce->parent;
        SND_CLONE_ASSERT(c != NULL, ("NULL parent"));
        SND_CLONE_ASSERT(c->refcount >= 0, ("refcount < 0"));

        return (++c->refcount);
}

int
snd_clone_unref(struct cdev *dev)
{
        struct snd_clone_entry *ce;
        struct snd_clone *c;

        SND_CLONE_ASSERT(dev != NULL, ("NULL dev"));

        ce = dev->si_drv2;
        if (ce == NULL)
                return (0);

        c = ce->parent;
        SND_CLONE_ASSERT(c != NULL, ("NULL parent"));
        SND_CLONE_ASSERT(c->refcount > 0, ("refcount <= 0"));

        c->refcount--;

        /* 
         * Run automatic garbage collector, if needed.
         */
        if ((c->flags & SND_CLONE_GC_UNREF) &&
            (!(c->flags & SND_CLONE_GC_LASTREF) ||
            (c->refcount == 0 && (c->flags & SND_CLONE_GC_LASTREF))))
                (void)snd_clone_gc(c);

        return (c->refcount);
}

void
snd_clone_register(struct snd_clone_entry *ce, struct cdev *dev)
{
        SND_CLONE_ASSERT(ce != NULL, ("NULL snd_clone_entry"));
        SND_CLONE_ASSERT(dev != NULL, ("NULL dev"));
        SND_CLONE_ASSERT(dev->si_drv2 == NULL, ("dev->si_drv2 not NULL"));
        SND_CLONE_ASSERT((ce->flags & SND_CLONE_ALLOC) == SND_CLONE_ALLOC,
            ("invalid clone alloc flags=0x%08x", ce->flags));
        SND_CLONE_ASSERT(ce->devt == NULL, ("ce->devt not NULL"));
#if 0   /* dev2unit doesn't make any sense on DragonFly */
        SND_CLONE_ASSERT(ce->unit == dev2unit(dev),
            ("invalid unit ce->unit=0x%08x dev2unit=0x%08x",
            ce->unit, dev2unit(dev)));
#endif

        SND_CLONE_ASSERT(ce->parent != NULL, ("NULL parent"));

        dev->si_drv2 = ce;
        ce->devt = dev;
        ce->flags &= ~SND_CLONE_ALLOC;
        ce->flags |= SND_CLONE_INVOKE;
}

struct snd_clone_entry *
snd_clone_alloc(struct snd_clone *c, struct cdev **dev, int *unit, int tmask)
{
        struct snd_clone_entry *ce, *after, *bce, *cce, *nce, *tce;
        struct timespec now;
        int cunit, allocunit;
        pid_t curpid;

        SND_CLONE_ASSERT(c != NULL, ("NULL snd_clone"));
        SND_CLONE_ASSERT(dev != NULL, ("NULL dev pointer"));
        SND_CLONE_ASSERT((c->typemask & tmask) == tmask,
            ("invalid tmask: typemask=0x%08x tmask=0x%08x",
            c->typemask, tmask));
        SND_CLONE_ASSERT(unit != NULL, ("NULL unit pointer"));
        SND_CLONE_ASSERT(*unit == -1 || !(*unit & (c->typemask | tmask)),
            ("typemask collision: typemask=0x%08x tmask=0x%08x *unit=%d",
            c->typemask, tmask, *unit));

        if (!(c->flags & SND_CLONE_ENABLE) ||
            (*unit != -1 && *unit > c->maxunit))
                return (NULL);

        ce = NULL;
        after = NULL;
        bce = NULL;     /* "b"usy candidate */
        cce = NULL;     /* "c"urthread/proc candidate */
        nce = NULL;     /* "n"ull, totally unbusy candidate */
        tce = NULL;     /* Last "t"ry candidate */
        cunit = 0;
        allocunit = (*unit == -1) ? 0 : *unit;
        curpid = curthread->td_proc->p_pid;

        snd_timestamp(&now);

        TAILQ_FOREACH(ce, &c->head, link) {
                /*
                 * Sort incrementally according to device type.
                 */
                if (tmask > (ce->unit & c->typemask)) {
                        if (cunit == 0)
                                after = ce;
                        continue;
                } else if (tmask < (ce->unit & c->typemask))
                        break;

                /*
                 * Shoot.. this is where the grumpiness begin. Just
                 * return immediately.
                 */
                if (*unit != -1 && *unit == (ce->unit & ~tmask))
                        goto snd_clone_alloc_out;

                cunit++;
                /*
                 * Simmilar device type. Sort incrementally according
                 * to allocation unit. While here, look for free slot
                 * and possible collision for new / future allocation.
                 */
                if (*unit == -1 && (ce->unit & ~tmask) == allocunit)
                        allocunit++;
                if ((ce->unit & ~tmask) < allocunit)
                        after = ce;
                /*
                 * Clone logic:
                 *   1. Look for non busy, but keep track of the best
                 *      possible busy cdev.
                 *   2. Look for the best (oldest referenced) entry that is
                 *      in a same process / thread.
                 *   3. Look for the best (oldest referenced), absolute free
                 *      entry.
                 *   4. Lastly, look for the best (oldest referenced)
                 *      any entries that doesn't fit with anything above.
                 */
                if (ce->flags & SND_CLONE_BUSY) {
                        if (ce->devt != NULL && (bce == NULL ||
                            timespeccmp(&ce->tsp, &bce->tsp, <)))
                                bce = ce;
                        continue;
                }
                if (ce->pid == curpid &&
                    (cce == NULL || timespeccmp(&ce->tsp, &cce->tsp, <)))
                        cce = ce;
                else if (!(ce->flags & SND_CLONE_INVOKE) &&
                    (nce == NULL || timespeccmp(&ce->tsp, &nce->tsp, <)))
                        nce = ce;
                else if (tce == NULL || timespeccmp(&ce->tsp, &tce->tsp, <))
                        tce = ce;
        }
        if (*unit != -1)
                goto snd_clone_alloc_new;
        else if (cce != NULL) {
                /* Same proc entry found, go for it */
                ce = cce;
                goto snd_clone_alloc_out;
        } else if (nce != NULL) {
                /*
                 * Next, try absolute free entry. If the calculated
                 * allocunit is smaller, create new entry instead.
                 */
                if (allocunit < (nce->unit & ~tmask))
                        goto snd_clone_alloc_new;
                ce = nce;
                goto snd_clone_alloc_out;
        } else if (allocunit > c->maxunit) {
                /*
                 * Maximum allowable unit reached. Try returning any
                 * available cdev and hope for the best. If the lookup is
                 * done for things like stat(), mtime() etc. , things should
                 * be ok. Otherwise, open() handler should do further checks
                 * and decide whether to return correct error code or not.
                 */
                if (tce != NULL) {
                        ce = tce;
                        goto snd_clone_alloc_out;
                } else if (bce != NULL) {
                        ce = bce;
                        goto snd_clone_alloc_out;
                }
                return (NULL);
        }

snd_clone_alloc_new:
        /*
         * No free entries found, and we still haven't reached maximum
         * allowable units. Allocate, setup a minimal unique entry with busy
         * status so nobody will monkey on this new entry. Unit magic is set
         * right here to avoid collision with other contesting handler.
         * The caller must be carefull here to maintain its own
         * synchronization, as long as it will not conflict with malloc(9)
         * operations.
         *
         * That said, go figure.
         */
        ce = kmalloc(sizeof(*ce), M_DEVBUF, M_WAITOK | M_ZERO);
        if (ce == NULL) {
                if (*unit != -1)
                        return (NULL);
                /*
                 * We're being dense, ignorance is bliss,
                 * Super Regulatory Measure (TM).. TRY AGAIN!
                 */
                if (nce != NULL) {
                        ce = nce;
                        goto snd_clone_alloc_out;
                } else if (tce != NULL) {
                        ce = tce;
                        goto snd_clone_alloc_out;
                } else if (bce != NULL) {
                        ce = bce;
                        goto snd_clone_alloc_out;
                }
                return (NULL);
        }
        /* Setup new entry */
        ce->parent = c;
        ce->unit = tmask | allocunit;
        ce->pid = curpid;
        ce->tsp = now;
        ce->flags |= SND_CLONE_ALLOC;
        if (after != NULL) {
                TAILQ_INSERT_AFTER(&c->head, after, ce, link);
        } else {
                TAILQ_INSERT_HEAD(&c->head, ce, link);
        }
        c->size++;
        c->tsp = now;
        /*
         * Save new allocation unit for caller which will be used
         * by make_dev().
         */
        *unit = allocunit;

        return (ce);

snd_clone_alloc_out:
        /*
         * Set, mark, timestamp the entry if this is a truly free entry.
         * Leave busy entry alone.
         */
        if (!(ce->flags & SND_CLONE_BUSY)) {
                ce->pid = curpid;
                ce->tsp = now;
                ce->flags |= SND_CLONE_INVOKE;
        }
        c->tsp = now;
        *dev = ce->devt;

        return (NULL);
}