NetBSD/sys/dev/ld.c

/*	$NetBSD: ld.c,v 1.19 2003/01/27 02:43:30 thorpej Exp $	*/

/*-
 * Copyright (c) 1998, 2000 The NetBSD Foundation, Inc.
 * All rights reserved.
 *
 * This code is derived from software contributed to The NetBSD Foundation
 * by Andrew Doran and Charles M. Hannum.
 *
 * 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.
 * 3. All advertising materials mentioning features or use of this software
 *    must display the following acknowledgement:
 *        This product includes software developed by the NetBSD
 *        Foundation, Inc. and its contributors.
 * 4. Neither the name of The NetBSD Foundation nor the names of its
 *    contributors may be used to endorse or promote products derived
 *    from this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. 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 FOUNDATION 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.
 */

/*
 * Disk driver for use by RAID controllers.
 */

#include <sys/cdefs.h>
__KERNEL_RCSID(0, "$NetBSD: ld.c,v 1.19 2003/01/27 02:43:30 thorpej Exp $");

#include "rnd.h"

#include <sys/param.h>
#include <sys/systm.h>
#include <sys/kernel.h>
#include <sys/device.h>
#include <sys/queue.h>
#include <sys/proc.h>
#include <sys/buf.h>
#include <sys/endian.h>
#include <sys/disklabel.h>
#include <sys/disk.h>
#include <sys/dkio.h>
#include <sys/stat.h>
#include <sys/lock.h>
#include <sys/conf.h>
#include <sys/fcntl.h>
#include <sys/vnode.h>
#include <sys/syslog.h>
#if NRND > 0
#include <sys/rnd.h>
#endif

#include <dev/ldvar.h>

static void	ldgetdefaultlabel(struct ld_softc *, struct disklabel *);
static void	ldgetdisklabel(struct ld_softc *);
static int	ldlock(struct ld_softc *);
static void	ldminphys(struct buf *bp);
static void	ldshutdown(void *);
static int	ldstart(struct ld_softc *, struct buf *);
static void	ldunlock(struct ld_softc *);

extern struct	cfdriver ld_cd;

dev_type_open(ldopen);
dev_type_close(ldclose);
dev_type_read(ldread);
dev_type_write(ldwrite);
dev_type_ioctl(ldioctl);
dev_type_strategy(ldstrategy);
dev_type_dump(lddump);
dev_type_size(ldsize);

const struct bdevsw ld_bdevsw = {
	ldopen, ldclose, ldstrategy, ldioctl, lddump, ldsize, D_DISK
};

const struct cdevsw ld_cdevsw = {
	ldopen, ldclose, ldread, ldwrite, ldioctl,
	nostop, notty, nopoll, nommap, nokqfilter, D_DISK
};

static struct	dkdriver lddkdriver = { ldstrategy };
static void	*ld_sdh;

void
ldattach(struct ld_softc *sc)
{
	char buf[9];

	if ((sc->sc_flags & LDF_ENABLED) == 0) {
		printf("%s: disabled\n", sc->sc_dv.dv_xname);
		return;
	}

	/* Initialise and attach the disk structure. */
	sc->sc_dk.dk_driver = &lddkdriver;
	sc->sc_dk.dk_name = sc->sc_dv.dv_xname;
	disk_attach(&sc->sc_dk);

	if (sc->sc_maxxfer > MAXPHYS)
		sc->sc_maxxfer = MAXPHYS;

	/* Build synthetic geometry if necessary. */
	if (sc->sc_nheads == 0 || sc->sc_nsectors == 0 ||
	    sc->sc_ncylinders == 0) {
		if (sc->sc_secperunit <= 528 * 2048)		/* 528MB */
			sc->sc_nheads = 16;
		else if (sc->sc_secperunit <= 1024 * 2048)	/* 1GB */
			sc->sc_nheads = 32;
		else if (sc->sc_secperunit <= 21504 * 2048)	/* 21GB */
			sc->sc_nheads = 64;
		else if (sc->sc_secperunit <= 43008 * 2048)	/* 42GB */
			sc->sc_nheads = 128;
		else
			sc->sc_nheads = 255;

		sc->sc_nsectors = 63;
		sc->sc_ncylinders = sc->sc_secperunit / 
		    (sc->sc_nheads * sc->sc_nsectors);
	}

	format_bytes(buf, sizeof(buf), (u_int64_t)sc->sc_secperunit *
	    sc->sc_secsize);
	printf("%s: %s, %d cyl, %d head, %d sec, %d bytes/sect x %d sectors\n",
	    sc->sc_dv.dv_xname, buf, sc->sc_ncylinders, sc->sc_nheads,
	    sc->sc_nsectors, sc->sc_secsize, sc->sc_secperunit);

#if NRND > 0
	/* Attach the device into the rnd source list. */
	rnd_attach_source(&sc->sc_rnd_source, sc->sc_dv.dv_xname,
	    RND_TYPE_DISK, 0);
#endif

	/* Set the `shutdownhook'. */
	if (ld_sdh == NULL)
		ld_sdh = shutdownhook_establish(ldshutdown, NULL);
	bufq_alloc(&sc->sc_bufq, BUFQ_FCFS);
}

int
ldadjqparam(struct ld_softc *sc, int max)
{
	int s, rv;

	s = splbio();
	sc->sc_maxqueuecnt = max;
	if (sc->sc_queuecnt > max) {
		sc->sc_flags |= LDF_DRAIN;
		rv = tsleep(&sc->sc_queuecnt, PRIBIO, "lddrn", 30 * hz);
		sc->sc_flags &= ~LDF_DRAIN;
	} else
		rv = 0;
	splx(s);

	return (rv);
}

int
ldbegindetach(struct ld_softc *sc, int flags)
{
	int s, rv;

	if ((sc->sc_flags & LDF_ENABLED) == 0)
		return (0);

	if ((flags & DETACH_FORCE) == 0 && sc->sc_dk.dk_openmask != 0)
		return (EBUSY);

	s = splbio();
	sc->sc_flags |= LDF_DETACH;
	rv = ldadjqparam(sc, 0);
	splx(s);

	return (rv);
}

void
ldenddetach(struct ld_softc *sc)
{
	struct buf *bp;
	int s, bmaj, cmaj, i, mn;

	if ((sc->sc_flags & LDF_ENABLED) == 0)
		return;

	/* Wait for commands queued with the hardware to complete. */
	if (sc->sc_queuecnt != 0)
		if (tsleep(&sc->sc_queuecnt, PRIBIO, "lddtch", 30 * hz))
			printf("%s: not drained\n", sc->sc_dv.dv_xname);

	/* Locate the major numbers. */
	bmaj = bdevsw_lookup_major(&ld_bdevsw);
	cmaj = cdevsw_lookup_major(&ld_cdevsw);

	/* Kill off any queued buffers. */
	s = splbio();
	while ((bp = BUFQ_GET(&sc->sc_bufq)) != NULL) {
		bp->b_error = EIO;
		bp->b_flags |= B_ERROR;
		bp->b_resid = bp->b_bcount;
		biodone(bp);
	}
	bufq_free(&sc->sc_bufq);
	splx(s);

	/* Nuke the vnodes for any open instances. */
	for (i = 0; i < MAXPARTITIONS; i++) {
		mn = DISKMINOR(sc->sc_dv.dv_unit, i);
		vdevgone(bmaj, mn, mn, VBLK);
		vdevgone(cmaj, mn, mn, VCHR);
	}

	/* Detach from the disk list. */
	disk_detach(&sc->sc_dk);

#if NRND > 0
	/* Unhook the entropy source. */
	rnd_detach_source(&sc->sc_rnd_source);
#endif

	/* Flush the device's cache. */
	if (sc->sc_flush != NULL)
		if ((*sc->sc_flush)(sc) != 0)
			printf("%s: unable to flush cache\n",
			    sc->sc_dv.dv_xname);
}

/* ARGSUSED */
static void
ldshutdown(void *cookie)
{
	struct ld_softc *sc;
	int i;

	for (i = 0; i < ld_cd.cd_ndevs; i++) {
		if ((sc = device_lookup(&ld_cd, i)) == NULL)
			continue;
		if (sc->sc_flush != NULL && (*sc->sc_flush)(sc) != 0)
			printf("%s: unable to flush cache\n",
			    sc->sc_dv.dv_xname);
	}
}

/* ARGSUSED */
int
ldopen(dev_t dev, int flags, int fmt, struct proc *p)
{
	struct ld_softc *sc;
	int unit, part;

	unit = DISKUNIT(dev);
	if ((sc = device_lookup(&ld_cd, unit))== NULL)
		return (ENXIO);
	if ((sc->sc_flags & LDF_ENABLED) == 0)
		return (ENODEV);
	part = DISKPART(dev);
	ldlock(sc);

	if (sc->sc_dk.dk_openmask == 0)
		ldgetdisklabel(sc);

	/* Check that the partition exists. */
	if (part != RAW_PART && (part >= sc->sc_dk.dk_label->d_npartitions ||
	    sc->sc_dk.dk_label->d_partitions[part].p_fstype == FS_UNUSED)) {
	     	ldunlock(sc);
		return (ENXIO);
	}

	/* Ensure only one open at a time. */
	switch (fmt) {
	case S_IFCHR:
		sc->sc_dk.dk_copenmask |= (1 << part);
		break;
	case S_IFBLK:
		sc->sc_dk.dk_bopenmask |= (1 << part);
		break;
	}
	sc->sc_dk.dk_openmask =
	    sc->sc_dk.dk_copenmask | sc->sc_dk.dk_bopenmask;

	ldunlock(sc);
	return (0);
}

/* ARGSUSED */
int
ldclose(dev_t dev, int flags, int fmt, struct proc *p)
{
	struct ld_softc *sc;
	int part, unit;

	unit = DISKUNIT(dev);
	part = DISKPART(dev);
	sc = device_lookup(&ld_cd, unit);
	ldlock(sc);

	switch (fmt) {
	case S_IFCHR:
		sc->sc_dk.dk_copenmask &= ~(1 << part);
		break;
	case S_IFBLK:
		sc->sc_dk.dk_bopenmask &= ~(1 << part);
		break;
	}
	sc->sc_dk.dk_openmask =
	    sc->sc_dk.dk_copenmask | sc->sc_dk.dk_bopenmask;

	if (sc->sc_dk.dk_openmask == 0 && sc->sc_flush != NULL)
		if ((*sc->sc_flush)(sc) != 0)
			printf("%s: unable to flush cache\n",
			    sc->sc_dv.dv_xname);

	ldunlock(sc);
	return (0);
}

/* ARGSUSED */
int
ldread(dev_t dev, struct uio *uio, int ioflag)
{

	return (physio(ldstrategy, NULL, dev, B_READ, ldminphys, uio));
}

/* ARGSUSED */
int
ldwrite(dev_t dev, struct uio *uio, int ioflag)
{

	return (physio(ldstrategy, NULL, dev, B_WRITE, ldminphys, uio));
}

/* ARGSUSED */
int
ldioctl(dev_t dev, u_long cmd, caddr_t addr, int32_t flag, struct proc *p)
{
	struct ld_softc *sc;
	int part, unit, error;
#ifdef __HAVE_OLD_DISKLABEL
	struct disklabel newlabel;
#endif
	struct disklabel *lp;

	unit = DISKUNIT(dev);
	part = DISKPART(dev);
	sc = device_lookup(&ld_cd, unit);
	error = 0;

	switch (cmd) {
	case DIOCGDINFO:
		memcpy(addr, sc->sc_dk.dk_label, sizeof(struct disklabel));
		return (0);

#ifdef __HAVE_OLD_DISKLABEL
	case ODIOCGDINFO:
		newlabel = *(sc->sc_dk.dk_label);
		if (newlabel.d_npartitions > OLDMAXPARTITIONS)
			return ENOTTY;
		memcpy(addr, &newlabel, sizeof(struct olddisklabel));
		return (0);
#endif

	case DIOCGPART:
		((struct partinfo *)addr)->disklab = sc->sc_dk.dk_label;
		((struct partinfo *)addr)->part =
		    &sc->sc_dk.dk_label->d_partitions[part];
		break;

	case DIOCWDINFO:
	case DIOCSDINFO:
#ifdef __HAVE_OLD_DISKLABEL
	case ODIOCWDINFO:
	case ODIOCSDINFO:

		if (cmd == ODIOCSDINFO || cmd == ODIOCWDINFO) {
			memset(&newlabel, 0, sizeof newlabel);
			memcpy(&newlabel, addr, sizeof (struct olddisklabel));
			lp = &newlabel;
		} else
#endif
		lp = (struct disklabel *)addr;

		if ((flag & FWRITE) == 0)
			return (EBADF);

		if ((error = ldlock(sc)) != 0)
			return (error);
		sc->sc_flags |= LDF_LABELLING;

		error = setdisklabel(sc->sc_dk.dk_label,
		    lp, /*sc->sc_dk.dk_openmask : */0,
		    sc->sc_dk.dk_cpulabel);
		if (error == 0 && (cmd == DIOCWDINFO
#ifdef __HAVE_OLD_DISKLABEL
		    || cmd == ODIOCWDINFO
#endif
		    ))
			error = writedisklabel(
			    MAKEDISKDEV(major(dev), DISKUNIT(dev), RAW_PART), 
			    ldstrategy, sc->sc_dk.dk_label, 
			    sc->sc_dk.dk_cpulabel);

		sc->sc_flags &= ~LDF_LABELLING;
		ldunlock(sc);
		break;

	case DIOCWLABEL:
		if ((flag & FWRITE) == 0)
			return (EBADF);
		if (*(int *)addr)
			sc->sc_flags |= LDF_WLABEL;
		else
			sc->sc_flags &= ~LDF_WLABEL;
		break;

	case DIOCGDEFLABEL:
		ldgetdefaultlabel(sc, (struct disklabel *)addr);
		break;

#ifdef __HAVE_OLD_DISKLABEL
	case ODIOCGDEFLABEL:
		ldgetdefaultlabel(sc, &newlabel);
		if (newlabel.d_npartitions > OLDMAXPARTITIONS)
			return ENOTTY;
		memcpy(addr, &newlabel, sizeof (struct olddisklabel));
		break;
#endif

	default:
		error = ENOTTY;
		break;
	}

	return (error);
}

void
ldstrategy(struct buf *bp)
{
	struct ld_softc *sc;
	int s;

	sc = device_lookup(&ld_cd, DISKUNIT(bp->b_dev));

	s = splbio();
	if (sc->sc_queuecnt >= sc->sc_maxqueuecnt) {
		BUFQ_PUT(&sc->sc_bufq, bp);
		splx(s);
		return;
	}
	splx(s);
	ldstart(sc, bp);
}

static int
ldstart(struct ld_softc *sc, struct buf *bp)
{
	struct disklabel *lp;
	int part, s, rv;

	if ((sc->sc_flags & LDF_DETACH) != 0) {
		bp->b_error = EIO;
		bp->b_flags |= B_ERROR;
		bp->b_resid = bp->b_bcount;
		biodone(bp);
		return (-1);
	}

	part = DISKPART(bp->b_dev);
	lp = sc->sc_dk.dk_label;

	/*
	 * The transfer must be a whole number of blocks and the offset must
	 * not be negative.
	 */
	if ((bp->b_bcount % lp->d_secsize) != 0 || bp->b_blkno < 0) {
		bp->b_flags |= B_ERROR;
		biodone(bp);
		return (-1);
	}

	/*
	 * If it's a null transfer, return.
	 */
	if (bp->b_bcount == 0) {
		bp->b_resid = bp->b_bcount;
		biodone(bp);
		return (-1);
	}

	/*
	 * Do bounds checking and adjust the transfer.  If error, process.
	 * If past the end of partition, just return.
	 */
	if (part != RAW_PART &&
	    bounds_check_with_label(bp, lp,
	    (sc->sc_flags & (LDF_WLABEL | LDF_LABELLING)) != 0) <= 0) {
		bp->b_resid = bp->b_bcount;
		biodone(bp);
		return (-1);
	}

	/*
	 * Convert the logical block number to a physical one and put it in
	 * terms of the device's logical block size.
	 */
	if (lp->d_secsize >= DEV_BSIZE)
		bp->b_rawblkno = bp->b_blkno / (lp->d_secsize / DEV_BSIZE);
	else
		bp->b_rawblkno = bp->b_blkno * (DEV_BSIZE / lp->d_secsize);

	if (part != RAW_PART)
		bp->b_rawblkno += lp->d_partitions[part].p_offset;

	s = splbio();
	disk_busy(&sc->sc_dk);
	sc->sc_queuecnt++;
	splx(s);

	if ((rv = (*sc->sc_start)(sc, bp)) != 0) {
		bp->b_error = rv;
		bp->b_flags |= B_ERROR;
		bp->b_resid = bp->b_bcount;
		s = splbio();
		lddone(sc, bp);
		splx(s);
	}

	return (0);
}

void
lddone(struct ld_softc *sc, struct buf *bp)
{

	if ((bp->b_flags & B_ERROR) != 0) {
		diskerr(bp, "ld", "error", LOG_PRINTF, 0, sc->sc_dk.dk_label);
		printf("\n");
	}

	disk_unbusy(&sc->sc_dk, bp->b_bcount - bp->b_resid,
	    (bp->b_flags & B_READ));
#if NRND > 0
	rnd_add_uint32(&sc->sc_rnd_source, bp->b_rawblkno);
#endif
	biodone(bp);

	if (--sc->sc_queuecnt <= sc->sc_maxqueuecnt) {
		if ((sc->sc_flags & LDF_DRAIN) != 0)
			wakeup(&sc->sc_queuecnt);
		while ((bp = BUFQ_GET(&sc->sc_bufq)) != NULL) {
			if (!ldstart(sc, bp))
				break;
		}
	}
}

int
ldsize(dev_t dev)
{
	struct ld_softc *sc;
	int part, unit, omask, size;

	unit = DISKUNIT(dev);
	if ((sc = device_lookup(&ld_cd, unit)) == NULL)
		return (ENODEV);
	if ((sc->sc_flags & LDF_ENABLED) == 0)
		return (ENODEV);
	part = DISKPART(dev);

	omask = sc->sc_dk.dk_openmask & (1 << part);

	if (omask == 0 && ldopen(dev, 0, S_IFBLK, NULL) != 0)
		return (-1);
	else if (sc->sc_dk.dk_label->d_partitions[part].p_fstype != FS_SWAP)
		size = -1;
	else
		size = sc->sc_dk.dk_label->d_partitions[part].p_size *
		    (sc->sc_dk.dk_label->d_secsize / DEV_BSIZE);
	if (omask == 0 && ldclose(dev, 0, S_IFBLK, NULL) != 0)
		return (-1);

	return (size);
}

/*
 * Load the label information from the specified device.
 */
static void
ldgetdisklabel(struct ld_softc *sc)
{
	const char *errstring;

	ldgetdefaultlabel(sc, sc->sc_dk.dk_label);

	/* Call the generic disklabel extraction routine. */
	errstring = readdisklabel(MAKEDISKDEV(0, sc->sc_dv.dv_unit, RAW_PART),
	    ldstrategy, sc->sc_dk.dk_label, sc->sc_dk.dk_cpulabel);
	if (errstring != NULL)
		printf("%s: %s\n", sc->sc_dv.dv_xname, errstring);
}

/*
 * Construct a ficticious label.
 */
static void
ldgetdefaultlabel(struct ld_softc *sc, struct disklabel *lp)
{

	memset(lp, 0, sizeof(struct disklabel));

	lp->d_secsize = sc->sc_secsize;
	lp->d_ntracks = sc->sc_nheads;
	lp->d_nsectors = sc->sc_nsectors;
	lp->d_ncylinders = sc->sc_ncylinders;
	lp->d_secpercyl = lp->d_ntracks * lp->d_nsectors;
	lp->d_type = DTYPE_LD;
	strcpy(lp->d_typename, "unknown");
	strcpy(lp->d_packname, "fictitious");
	lp->d_secperunit = sc->sc_secperunit;
	lp->d_rpm = 7200;
	lp->d_interleave = 1;
	lp->d_flags = 0;

	lp->d_partitions[RAW_PART].p_offset = 0;
	lp->d_partitions[RAW_PART].p_size =
	    lp->d_secperunit * (lp->d_secsize / DEV_BSIZE);
	lp->d_partitions[RAW_PART].p_fstype = FS_UNUSED;
	lp->d_npartitions = RAW_PART + 1;

	lp->d_magic = DISKMAGIC;
	lp->d_magic2 = DISKMAGIC;
	lp->d_checksum = dkcksum(lp);
}

/*
 * Wait interruptibly for an exclusive lock.
 *
 * XXX Several drivers do this; it should be abstracted and made MP-safe.
 */
static int
ldlock(struct ld_softc *sc)
{
	int error;

	while ((sc->sc_flags & LDF_LKHELD) != 0) {
		sc->sc_flags |= LDF_LKWANTED;
		if ((error = tsleep(sc, PRIBIO | PCATCH, "ldlck", 0)) != 0)
			return (error);
	}
	sc->sc_flags |= LDF_LKHELD;
	return (0);
}

/*
 * Unlock and wake up any waiters.
 */
static void
ldunlock(struct ld_softc *sc)
{

	sc->sc_flags &= ~LDF_LKHELD;
	if ((sc->sc_flags & LDF_LKWANTED) != 0) {
		sc->sc_flags &= ~LDF_LKWANTED;
		wakeup(sc);
	}
}

/*
 * Take a dump.
 */
int
lddump(dev_t dev, daddr_t blkno, caddr_t va, size_t size)
{
	struct ld_softc *sc;
	struct disklabel *lp;
	int unit, part, nsects, sectoff, towrt, nblk, maxblkcnt, rv;
	static int dumping;

	unit = DISKUNIT(dev);
	if ((sc = device_lookup(&ld_cd, unit)) == NULL)
		return (ENXIO);
	if ((sc->sc_flags & LDF_ENABLED) == 0)
		return (ENODEV);
	if (sc->sc_dump == NULL)
		return (ENXIO);

	/* Check if recursive dump; if so, punt. */
	if (dumping)
		return (EFAULT);
	dumping = 1;

	/* Convert to disk sectors.  Request must be a multiple of size. */
	part = DISKPART(dev);
	lp = sc->sc_dk.dk_label;
	if ((size % lp->d_secsize) != 0)
		return (EFAULT);
	towrt = size / lp->d_secsize;
	blkno = dbtob(blkno) / lp->d_secsize;	/* blkno in DEV_BSIZE units */

	nsects = lp->d_partitions[part].p_size;
	sectoff = lp->d_partitions[part].p_offset;

	/* Check transfer bounds against partition size. */
	if ((blkno < 0) || ((blkno + towrt) > nsects))
		return (EINVAL);

	/* Offset block number to start of partition. */
	blkno += sectoff;

	/* Start dumping and return when done. */
	maxblkcnt = sc->sc_maxxfer / sc->sc_secsize - 1;
	while (towrt > 0) {
		nblk = min(maxblkcnt, towrt);

		if ((rv = (*sc->sc_dump)(sc, va, blkno, nblk)) != 0)
			return (rv);

		towrt -= nblk;
		blkno += nblk;
		va += nblk * sc->sc_secsize;
	}

	dumping = 0;
	return (0);
}

/*
 * Adjust the size of a transfer.
 */
static void
ldminphys(struct buf *bp)
{
	struct ld_softc *sc;

	sc = device_lookup(&ld_cd, DISKUNIT(bp->b_dev));

	if (bp->b_bcount > sc->sc_maxxfer)
		bp->b_bcount = sc->sc_maxxfer;
	minphys(bp);
}