/*-
 * SPDX-License-Identifier: BSD-3-Clause
 *
 * Copyright (c) 2008 Yahoo!, Inc.
 * All rights reserved.
 * Written by: John Baldwin <jhb@FreeBSD.org>
 *
 * 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. Neither the name of the author nor the names of any co-contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * 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.
 */

#include <sys/param.h>
#include <sys/kernel.h>
#include <sys/bio.h>
#include <sys/malloc.h>
#include <sys/mbuf.h>
#include <sys/proc.h>
#include <sys/sglist.h>
#include <sys/uio.h>

#include <vm/vm.h>
#include <vm/vm_page.h>
#include <vm/pmap.h>
#include <vm/vm_map.h>

#include <sys/ktr.h>

static MALLOC_DEFINE(M_SGLIST, "sglist", "scatter/gather lists");

/*
 * Convenience macros to save the state of an sglist so it can be restored
 * if an append attempt fails.  Since sglist's only grow we only need to
 * save the current count of segments and the length of the ending segment.
 * Earlier segments will not be changed by an append, and the only change
 * that can occur to the ending segment is that it can be extended.
 */
struct sgsave {
        u_short sg_nseg;
        size_t ss_len;
};

#define SGLIST_SAVE(sg, sgsave) do {                                    \
        (sgsave).sg_nseg = (sg)->sg_nseg;                               \
        if ((sgsave).sg_nseg > 0)                                       \
                (sgsave).ss_len = (sg)->sg_segs[(sgsave).sg_nseg - 1].ss_len; \
        else                                                            \
                (sgsave).ss_len = 0;                                    \
} while (0)

#define SGLIST_RESTORE(sg, sgsave) do {                                 \
        (sg)->sg_nseg = (sgsave).sg_nseg;                               \
        if ((sgsave).sg_nseg > 0)                                       \
                (sg)->sg_segs[(sgsave).sg_nseg - 1].ss_len = (sgsave).ss_len; \
} while (0)

/*
 * Append a single (paddr, len) to a sglist.  sg is the list and ss is
 * the current segment in the list.  If we run out of segments then
 * EFBIG will be returned.
 */
static __inline int
_sglist_append_range(struct sglist *sg, struct sglist_seg **ssp,
    vm_paddr_t paddr, size_t len)
{
        struct sglist_seg *ss;

        ss = *ssp;
        if (ss->ss_paddr + ss->ss_len == paddr)
                ss->ss_len += len;
        else {
                if (sg->sg_nseg == sg->sg_maxseg)
                        return (EFBIG);
                ss++;
                ss->ss_paddr = paddr;
                ss->ss_len = len;
                sg->sg_nseg++;
                *ssp = ss;
        }
        return (0);
}

/*
 * Worker routine to append a virtual address range (either kernel or
 * user) to a scatter/gather list.
 */
static __inline int
_sglist_append_buf(struct sglist *sg, void *buf, size_t len, pmap_t pmap,
    size_t *donep)
{
        struct sglist_seg *ss;
        vm_offset_t vaddr, offset;
        vm_paddr_t paddr;
        size_t seglen;
        int error;

        if (donep)
                *donep = 0;
        if (len == 0)
                return (0);

        /* Do the first page.  It may have an offset. */
        vaddr = (vm_offset_t)buf;
        offset = vaddr & PAGE_MASK;
        if (pmap != NULL)
                paddr = pmap_extract(pmap, vaddr);
        else
                paddr = pmap_kextract(vaddr);
        seglen = MIN(len, PAGE_SIZE - offset);
        if (sg->sg_nseg == 0) {
                ss = sg->sg_segs;
                ss->ss_paddr = paddr;
                ss->ss_len = seglen;
                sg->sg_nseg = 1;
        } else {
                ss = &sg->sg_segs[sg->sg_nseg - 1];
                error = _sglist_append_range(sg, &ss, paddr, seglen);
                if (error)
                        return (error);
        }
        vaddr += seglen;
        len -= seglen;
        if (donep)
                *donep += seglen;

        while (len > 0) {
                seglen = MIN(len, PAGE_SIZE);
                if (pmap != NULL)
                        paddr = pmap_extract(pmap, vaddr);
                else
                        paddr = pmap_kextract(vaddr);
                error = _sglist_append_range(sg, &ss, paddr, seglen);
                if (error)
                        return (error);
                vaddr += seglen;
                len -= seglen;
                if (donep)
                        *donep += seglen;
        }

        return (0);
}

/*
 * Determine the number of scatter/gather list elements needed to
 * describe a kernel virtual address range.
 */
int
sglist_count(void *buf, size_t len)
{
        vm_offset_t vaddr, vendaddr;
        vm_paddr_t lastaddr, paddr;
        int nsegs;

        if (len == 0)
                return (0);

        vaddr = trunc_page((vm_offset_t)buf);
        vendaddr = (vm_offset_t)buf + len;
        nsegs = 1;
        lastaddr = pmap_kextract(vaddr);
        vaddr += PAGE_SIZE;
        while (vaddr < vendaddr) {
                paddr = pmap_kextract(vaddr);
                if (lastaddr + PAGE_SIZE != paddr)
                        nsegs++;
                lastaddr = paddr;
                vaddr += PAGE_SIZE;
        }
        return (nsegs);
}

/*
 * Determine the number of scatter/gather list elements needed to
 * describe a buffer backed by an array of VM pages.
 */
int
sglist_count_vmpages(vm_page_t *m, size_t pgoff, size_t len)
{
        vm_paddr_t lastaddr, paddr;
        int i, nsegs;

        if (len == 0)
                return (0);

        len += pgoff;
        nsegs = 1;
        lastaddr = VM_PAGE_TO_PHYS(m[0]);
        for (i = 1; len > PAGE_SIZE; len -= PAGE_SIZE, i++) {
                paddr = VM_PAGE_TO_PHYS(m[i]);
                if (lastaddr + PAGE_SIZE != paddr)
                        nsegs++;
                lastaddr = paddr;
        }
        return (nsegs);
}

/*
 * Determine the number of scatter/gather list elements needed to
 * describe an M_EXTPG mbuf.
 */
int
sglist_count_mbuf_epg(struct mbuf *m, size_t off, size_t len)
{
        vm_paddr_t nextaddr, paddr;
        size_t seglen, segoff;
        int i, nsegs, pglen, pgoff;

        if (len == 0)
                return (0);

        nsegs = 0;
        if (m->m_epg_hdrlen != 0) {
                if (off >= m->m_epg_hdrlen) {
                        off -= m->m_epg_hdrlen;
                } else {
                        seglen = m->m_epg_hdrlen - off;
                        segoff = off;
                        seglen = MIN(seglen, len);
                        off = 0;
                        len -= seglen;
                        nsegs += sglist_count(&m->m_epg_hdr[segoff],
                            seglen);
                }
        }
        nextaddr = 0;
        pgoff = m->m_epg_1st_off;
        for (i = 0; i < m->m_epg_npgs && len > 0; i++) {
                pglen = m_epg_pagelen(m, i, pgoff);
                if (off >= pglen) {
                        off -= pglen;
                        pgoff = 0;
                        continue;
                }
                seglen = pglen - off;
                segoff = pgoff + off;
                off = 0;
                seglen = MIN(seglen, len);
                len -= seglen;
                paddr = m->m_epg_pa[i] + segoff;
                if (paddr != nextaddr)
                        nsegs++;
                nextaddr = paddr + seglen;
                pgoff = 0;
        };
        if (len != 0) {
                seglen = MIN(len, m->m_epg_trllen - off);
                len -= seglen;
                nsegs += sglist_count(&m->m_epg_trail[off], seglen);
        }
        KASSERT(len == 0, ("len != 0"));
        return (nsegs);
}

/*
 * Allocate a scatter/gather list along with 'nsegs' segments.  The
 * 'mflags' parameters are the same as passed to malloc(9).  The caller
 * should use sglist_free() to free this list.
 */
struct sglist *
sglist_alloc(int nsegs, int mflags)
{
        struct sglist *sg;

        sg = malloc(sizeof(struct sglist) + nsegs * sizeof(struct sglist_seg),
            M_SGLIST, mflags);
        if (sg == NULL)
                return (NULL);
        sglist_init(sg, nsegs, (struct sglist_seg *)(sg + 1));
        return (sg);
}

/*
 * Free a scatter/gather list allocated via sglist_allc().
 */
void
sglist_free(struct sglist *sg)
{

        if (sg == NULL)
                return;

        if (refcount_release(&sg->sg_refs))
                free(sg, M_SGLIST);
}

/*
 * Append the segments to describe a single kernel virtual address
 * range to a scatter/gather list.  If there are insufficient
 * segments, then this fails with EFBIG.
 */
int
sglist_append(struct sglist *sg, void *buf, size_t len)
{
        struct sgsave save;
        int error;

        if (sg->sg_maxseg == 0)
                return (EINVAL);
        SGLIST_SAVE(sg, save);
        error = _sglist_append_buf(sg, buf, len, NULL, NULL);
        if (error)
                SGLIST_RESTORE(sg, save);
        return (error);
}

/*
 * Append the segments to describe a bio's data to a scatter/gather list.
 * If there are insufficient segments, then this fails with EFBIG.
 *
 * NOTE: This function expects bio_bcount to be initialized.
 */
int
sglist_append_bio(struct sglist *sg, struct bio *bp)
{
        int error;

        if ((bp->bio_flags & BIO_UNMAPPED) == 0)
                error = sglist_append(sg, bp->bio_data, bp->bio_bcount);
        else
                error = sglist_append_vmpages(sg, bp->bio_ma,
                    bp->bio_ma_offset, bp->bio_bcount);
        return (error);
}

/*
 * Append a single physical address range to a scatter/gather list.
 * If there are insufficient segments, then this fails with EFBIG.
 */
int
sglist_append_phys(struct sglist *sg, vm_paddr_t paddr, size_t len)
{
        struct sglist_seg *ss;
        struct sgsave save;
        int error;

        if (sg->sg_maxseg == 0)
                return (EINVAL);
        if (len == 0)
                return (0);

        if (sg->sg_nseg == 0) {
                sg->sg_segs[0].ss_paddr = paddr;
                sg->sg_segs[0].ss_len = len;
                sg->sg_nseg = 1;
                return (0);
        }
        ss = &sg->sg_segs[sg->sg_nseg - 1];
        SGLIST_SAVE(sg, save);
        error = _sglist_append_range(sg, &ss, paddr, len);
        if (error)
                SGLIST_RESTORE(sg, save);
        return (error);
}

/*
 * Append the segments of single multi-page mbuf.
 * If there are insufficient segments, then this fails with EFBIG.
 */
int
sglist_append_mbuf_epg(struct sglist *sg, struct mbuf *m, size_t off,
    size_t len)
{
        size_t seglen, segoff;
        vm_paddr_t paddr;
        int error, i, pglen, pgoff;

        M_ASSERTEXTPG(m);

        error = 0;
        if (m->m_epg_hdrlen != 0) {
                if (off >= m->m_epg_hdrlen) {
                        off -= m->m_epg_hdrlen;
                } else {
                        seglen = m->m_epg_hdrlen - off;
                        segoff = off;
                        seglen = MIN(seglen, len);
                        off = 0;
                        len -= seglen;
                        error = sglist_append(sg,
                            &m->m_epg_hdr[segoff], seglen);
                }
        }
        pgoff = m->m_epg_1st_off;
        for (i = 0; i < m->m_epg_npgs && error == 0 && len > 0; i++) {
                pglen = m_epg_pagelen(m, i, pgoff);
                if (off >= pglen) {
                        off -= pglen;
                        pgoff = 0;
                        continue;
                }
                seglen = pglen - off;
                segoff = pgoff + off;
                off = 0;
                seglen = MIN(seglen, len);
                len -= seglen;
                paddr = m->m_epg_pa[i] + segoff;
                error = sglist_append_phys(sg, paddr, seglen);
                pgoff = 0;
        };
        if (error == 0 && len > 0) {
                seglen = MIN(len, m->m_epg_trllen - off);
                len -= seglen;
                error = sglist_append(sg,
                    &m->m_epg_trail[off], seglen);
        }
        if (error == 0)
                KASSERT(len == 0, ("len != 0"));
        return (error);
}

/*
 * Append the segments that describe a single mbuf chain to a
 * scatter/gather list.  If there are insufficient segments, then this
 * fails with EFBIG.
 */
int
sglist_append_mbuf(struct sglist *sg, struct mbuf *m0)
{
        struct sgsave save;
        struct mbuf *m;
        int error;

        if (sg->sg_maxseg == 0)
                return (EINVAL);

        error = 0;
        SGLIST_SAVE(sg, save);
        for (m = m0; m != NULL; m = m->m_next) {
                if (m->m_len > 0) {
                        if ((m->m_flags & M_EXTPG) != 0)
                                error = sglist_append_mbuf_epg(sg, m,
                                    mtod(m, vm_offset_t), m->m_len);
                        else
                                error = sglist_append(sg, m->m_data,
                                    m->m_len);
                        if (error) {
                                SGLIST_RESTORE(sg, save);
                                return (error);
                        }
                }
        }
        return (0);
}

/*
 * Append the segments that describe a single mbuf to a scatter/gather
 * list.  If there are insufficient segments, then this fails with
 * EFBIG.
 */
int
sglist_append_single_mbuf(struct sglist *sg, struct mbuf *m)
{
        if ((m->m_flags & M_EXTPG) != 0)
                return (sglist_append_mbuf_epg(sg, m,
                    mtod(m, vm_offset_t), m->m_len));
        else
                return (sglist_append(sg, m->m_data, m->m_len));
}

/*
 * Append the segments that describe a buffer spanning an array of VM
 * pages.  The buffer begins at an offset of 'pgoff' in the first
 * page.
 */
int
sglist_append_vmpages(struct sglist *sg, vm_page_t *m, size_t pgoff,
    size_t len)
{
        struct sgsave save;
        struct sglist_seg *ss;
        vm_paddr_t paddr;
        size_t seglen;
        int error, i;

        if (sg->sg_maxseg == 0)
                return (EINVAL);
        if (len == 0)
                return (0);

        SGLIST_SAVE(sg, save);
        i = 0;
        if (sg->sg_nseg == 0) {
                seglen = min(PAGE_SIZE - pgoff, len);
                sg->sg_segs[0].ss_paddr = VM_PAGE_TO_PHYS(m[0]) + pgoff;
                sg->sg_segs[0].ss_len = seglen;
                sg->sg_nseg = 1;
                pgoff = 0;
                len -= seglen;
                i++;
        }
        ss = &sg->sg_segs[sg->sg_nseg - 1];
        for (; len > 0; i++, len -= seglen) {
                seglen = min(PAGE_SIZE - pgoff, len);
                paddr = VM_PAGE_TO_PHYS(m[i]) + pgoff;
                error = _sglist_append_range(sg, &ss, paddr, seglen);
                if (error) {
                        SGLIST_RESTORE(sg, save);
                        return (error);
                }
                pgoff = 0;
        }
        return (0);
}

/*
 * Append the segments that describe a single user address range to a
 * scatter/gather list.  If there are insufficient segments, then this
 * fails with EFBIG.
 */
int
sglist_append_user(struct sglist *sg, void *buf, size_t len, struct thread *td)
{
        struct sgsave save;
        int error;

        if (sg->sg_maxseg == 0)
                return (EINVAL);
        SGLIST_SAVE(sg, save);
        error = _sglist_append_buf(sg, buf, len,
            vmspace_pmap(td->td_proc->p_vmspace), NULL);
        if (error)
                SGLIST_RESTORE(sg, save);
        return (error);
}

/*
 * Append a subset of an existing scatter/gather list 'source' to a
 * the scatter/gather list 'sg'.  If there are insufficient segments,
 * then this fails with EFBIG.
 */
int
sglist_append_sglist(struct sglist *sg, struct sglist *source, size_t offset,
    size_t length)
{
        struct sgsave save;
        struct sglist_seg *ss;
        size_t seglen;
        int error, i;

        if (sg->sg_maxseg == 0 || length == 0)
                return (EINVAL);
        SGLIST_SAVE(sg, save);
        error = EINVAL;
        ss = &sg->sg_segs[sg->sg_nseg - 1];
        for (i = 0; i < source->sg_nseg; i++) {
                if (offset >= source->sg_segs[i].ss_len) {
                        offset -= source->sg_segs[i].ss_len;
                        continue;
                }
                seglen = source->sg_segs[i].ss_len - offset;
                if (seglen > length)
                        seglen = length;
                error = _sglist_append_range(sg, &ss,
                    source->sg_segs[i].ss_paddr + offset, seglen);
                if (error)
                        break;
                offset = 0;
                length -= seglen;
                if (length == 0)
                        break;
        }
        if (length != 0)
                error = EINVAL;
        if (error)
                SGLIST_RESTORE(sg, save);
        return (error);
}

/*
 * Append the segments that describe a single uio to a scatter/gather
 * list.  If there are insufficient segments, then this fails with
 * EFBIG.
 */
int
sglist_append_uio(struct sglist *sg, struct uio *uio)
{
        struct iovec *iov;
        struct sgsave save;
        size_t resid, minlen;
        pmap_t pmap;
        int error, i;

        if (sg->sg_maxseg == 0)
                return (EINVAL);

        resid = uio->uio_resid;
        iov = uio->uio_iov;

        if (uio->uio_segflg == UIO_USERSPACE) {
                KASSERT(uio->uio_td != NULL,
                    ("sglist_append_uio: USERSPACE but no thread"));
                pmap = vmspace_pmap(uio->uio_td->td_proc->p_vmspace);
        } else
                pmap = NULL;

        error = 0;
        SGLIST_SAVE(sg, save);
        for (i = 0; i < uio->uio_iovcnt && resid != 0; i++) {
                /*
                 * Now at the first iovec to load.  Load each iovec
                 * until we have exhausted the residual count.
                 */
                minlen = MIN(resid, iov[i].iov_len);
                if (minlen > 0) {
                        error = _sglist_append_buf(sg, iov[i].iov_base, minlen,
                            pmap, NULL);
                        if (error) {
                                SGLIST_RESTORE(sg, save);
                                return (error);
                        }
                        resid -= minlen;
                }
        }
        return (0);
}

/*
 * Append the segments that describe at most 'resid' bytes from a
 * single uio to a scatter/gather list.  If there are insufficient
 * segments, then only the amount that fits is appended.
 */
int
sglist_consume_uio(struct sglist *sg, struct uio *uio, size_t resid)
{
        struct iovec *iov;
        size_t done;
        pmap_t pmap;
        int error, len;

        if (sg->sg_maxseg == 0)
                return (EINVAL);

        if (uio->uio_segflg == UIO_USERSPACE) {
                KASSERT(uio->uio_td != NULL,
                    ("sglist_consume_uio: USERSPACE but no thread"));
                pmap = vmspace_pmap(uio->uio_td->td_proc->p_vmspace);
        } else
                pmap = NULL;

        error = 0;
        while (resid > 0 && uio->uio_resid) {
                iov = uio->uio_iov;
                len = iov->iov_len;
                if (len == 0) {
                        uio->uio_iov++;
                        uio->uio_iovcnt--;
                        continue;
                }
                if (len > resid)
                        len = resid;

                /*
                 * Try to append this iovec.  If we run out of room,
                 * then break out of the loop.
                 */
                error = _sglist_append_buf(sg, iov->iov_base, len, pmap, &done);
                iov->iov_base = (char *)iov->iov_base + done;
                iov->iov_len -= done;
                uio->uio_resid -= done;
                uio->uio_offset += done;
                resid -= done;
                if (error)
                        break;
        }
        return (0);
}

/*
 * Allocate and populate a scatter/gather list to describe a single
 * kernel virtual address range.
 */
struct sglist *
sglist_build(void *buf, size_t len, int mflags)
{
        struct sglist *sg;
        int nsegs;

        if (len == 0)
                return (NULL);

        nsegs = sglist_count(buf, len);
        sg = sglist_alloc(nsegs, mflags);
        if (sg == NULL)
                return (NULL);
        if (sglist_append(sg, buf, len) != 0) {
                sglist_free(sg);
                return (NULL);
        }
        return (sg);
}

/*
 * Clone a new copy of a scatter/gather list.
 */
struct sglist *
sglist_clone(struct sglist *sg, int mflags)
{
        struct sglist *new;

        if (sg == NULL)
                return (NULL);
        new = sglist_alloc(sg->sg_maxseg, mflags);
        if (new == NULL)
                return (NULL);
        new->sg_nseg = sg->sg_nseg;
        bcopy(sg->sg_segs, new->sg_segs, sizeof(struct sglist_seg) *
            sg->sg_nseg);
        return (new);
}

/*
 * Calculate the total length of the segments described in a
 * scatter/gather list.
 */
size_t
sglist_length(struct sglist *sg)
{
        size_t space;
        int i;

        space = 0;
        for (i = 0; i < sg->sg_nseg; i++)
                space += sg->sg_segs[i].ss_len;
        return (space);
}

/*
 * Split a scatter/gather list into two lists.  The scatter/gather
 * entries for the first 'length' bytes of the 'original' list are
 * stored in the '*head' list and are removed from 'original'.
 *
 * If '*head' is NULL, then a new list will be allocated using
 * 'mflags'.  If M_NOWAIT is specified and the allocation fails,
 * ENOMEM will be returned.
 *
 * If '*head' is not NULL, it should point to an empty sglist.  If it
 * does not have enough room for the remaining space, then EFBIG will
 * be returned.  If '*head' is not empty, then EINVAL will be
 * returned.
 *
 * If 'original' is shared (refcount > 1), then EDOOFUS will be
 * returned.
 */
int
sglist_split(struct sglist *original, struct sglist **head, size_t length,
    int mflags)
{
        struct sglist *sg;
        size_t space, split;
        int count, i;

        if (original->sg_refs > 1)
                return (EDOOFUS);

        /* Figure out how big of a sglist '*head' has to hold. */
        count = 0;
        space = 0;
        split = 0;
        for (i = 0; i < original->sg_nseg; i++) {
                space += original->sg_segs[i].ss_len;
                count++;
                if (space >= length) {
                        /*
                         * If 'length' falls in the middle of a
                         * scatter/gather list entry, then 'split'
                         * holds how much of that entry will remain in
                         * 'original'.
                         */
                        split = space - length;
                        break;
                }
        }

        /* Nothing to do, so leave head empty. */
        if (count == 0)
                return (0);

        if (*head == NULL) {
                sg = sglist_alloc(count, mflags);
                if (sg == NULL)
                        return (ENOMEM);
                *head = sg;
        } else {
                sg = *head;
                if (sg->sg_maxseg < count)
                        return (EFBIG);
                if (sg->sg_nseg != 0)
                        return (EINVAL);
        }

        /* Copy 'count' entries to 'sg' from 'original'. */
        bcopy(original->sg_segs, sg->sg_segs, count *
            sizeof(struct sglist_seg));
        sg->sg_nseg = count;

        /*
         * If we had to split a list entry, fixup the last entry in
         * 'sg' and the new first entry in 'original'.  We also
         * decrement 'count' by 1 since we will only be removing
         * 'count - 1' segments from 'original' now.
         */
        if (split != 0) {
                count--;
                sg->sg_segs[count].ss_len -= split;
                original->sg_segs[count].ss_paddr =
                    sg->sg_segs[count].ss_paddr + split;
                original->sg_segs[count].ss_len = split;
        }

        /* Trim 'count' entries from the front of 'original'. */
        original->sg_nseg -= count;
        bcopy(original->sg_segs + count, original->sg_segs, count *
            sizeof(struct sglist_seg));
        return (0);
}

/*
 * Append the scatter/gather list elements in 'second' to the
 * scatter/gather list 'first'.  If there is not enough space in
 * 'first', EFBIG is returned.
 */
int
sglist_join(struct sglist *first, struct sglist *second)
{
        struct sglist_seg *flast, *sfirst;
        int append;

        /* If 'second' is empty, there is nothing to do. */
        if (second->sg_nseg == 0)
                return (0);

        /*
         * If the first entry in 'second' can be appended to the last entry
         * in 'first' then set append to '1'.
         */
        append = 0;
        flast = &first->sg_segs[first->sg_nseg - 1];
        sfirst = &second->sg_segs[0];
        if (first->sg_nseg != 0 &&
            flast->ss_paddr + flast->ss_len == sfirst->ss_paddr)
                append = 1;

        /* Make sure 'first' has enough room. */
        if (first->sg_nseg + second->sg_nseg - append > first->sg_maxseg)
                return (EFBIG);

        /* Merge last in 'first' and first in 'second' if needed. */
        if (append)
                flast->ss_len += sfirst->ss_len;

        /* Append new segments from 'second' to 'first'. */
        bcopy(first->sg_segs + first->sg_nseg, second->sg_segs + append,
            (second->sg_nseg - append) * sizeof(struct sglist_seg));
        first->sg_nseg += second->sg_nseg - append;
        sglist_reset(second);
        return (0);
}

/*
 * Generate a new scatter/gather list from a range of an existing
 * scatter/gather list.  The 'offset' and 'length' parameters specify
 * the logical range of the 'original' list to extract.  If that range
 * is not a subset of the length of 'original', then EINVAL is
 * returned.  The new scatter/gather list is stored in '*slice'.
 *
 * If '*slice' is NULL, then a new list will be allocated using
 * 'mflags'.  If M_NOWAIT is specified and the allocation fails,
 * ENOMEM will be returned.
 *
 * If '*slice' is not NULL, it should point to an empty sglist.  If it
 * does not have enough room for the remaining space, then EFBIG will
 * be returned.  If '*slice' is not empty, then EINVAL will be
 * returned.
 */
int
sglist_slice(struct sglist *original, struct sglist **slice, size_t offset,
    size_t length, int mflags)
{
        struct sglist *sg;
        size_t space, end, foffs, loffs;
        int count, i, fseg;

        /* Nothing to do. */
        if (length == 0)
                return (0);

        /* Figure out how many segments '*slice' needs to have. */
        end = offset + length;
        space = 0;
        count = 0;
        fseg = 0;
        foffs = loffs = 0;
        for (i = 0; i < original->sg_nseg; i++) {
                space += original->sg_segs[i].ss_len;
                if (space > offset) {
                        /*
                         * When we hit the first segment, store its index
                         * in 'fseg' and the offset into the first segment
                         * of 'offset' in 'foffs'.
                         */
                        if (count == 0) {
                                fseg = i;
                                foffs = offset - (space -
                                    original->sg_segs[i].ss_len);
                                CTR1(KTR_DEV, "sglist_slice: foffs = %08lx",
                                    foffs);
                        }
                        count++;

                        /*
                         * When we hit the last segment, break out of
                         * the loop.  Store the amount of extra space
                         * at the end of this segment in 'loffs'.
                         */
                        if (space >= end) {
                                loffs = space - end;
                                CTR1(KTR_DEV, "sglist_slice: loffs = %08lx",
                                    loffs);
                                break;
                        }
                }
        }

        /* If we never hit 'end', then 'length' ran off the end, so fail. */
        if (space < end)
                return (EINVAL);

        if (*slice == NULL) {
                sg = sglist_alloc(count, mflags);
                if (sg == NULL)
                        return (ENOMEM);
                *slice = sg;
        } else {
                sg = *slice;
                if (sg->sg_maxseg < count)
                        return (EFBIG);
                if (sg->sg_nseg != 0)
                        return (EINVAL);
        }

        /*
         * Copy over 'count' segments from 'original' starting at
         * 'fseg' to 'sg'.
         */
        bcopy(original->sg_segs + fseg, sg->sg_segs,
            count * sizeof(struct sglist_seg));
        sg->sg_nseg = count;

        /* Fixup first and last segments if needed. */
        if (foffs != 0) {
                sg->sg_segs[0].ss_paddr += foffs;
                sg->sg_segs[0].ss_len -= foffs;
                CTR2(KTR_DEV, "sglist_slice seg[0]: %08lx:%08lx",
                    (long)sg->sg_segs[0].ss_paddr, sg->sg_segs[0].ss_len);
        }
        if (loffs != 0) {
                sg->sg_segs[count - 1].ss_len -= loffs;
                CTR2(KTR_DEV, "sglist_slice seg[%d]: len %08x", count - 1,
                    sg->sg_segs[count - 1].ss_len);
        }
        return (0);
}