/*
 * Copyright (C) 2007 Jens Axboe <jens.axboe@oracle.com>
 *
 * Scatterlist handling helpers.
 *
 * This source code is licensed under the GNU General Public License,
 * Version 2. See the file COPYING for more details.
 */
#include <linux/export.h>
#include <linux/slab.h>
#include <linux/scatterlist.h>
#include <linux/highmem.h>
#include <linux/kmemleak.h>

/**
 * sg_next - return the next scatterlist entry in a list
 * @sg:         The current sg entry
 *
 * Description:
 *   Usually the next entry will be @sg@ + 1, but if this sg element is part
 *   of a chained scatterlist, it could jump to the start of a new
 *   scatterlist array.
 *
 **/
struct scatterlist *sg_next(struct scatterlist *sg)
{
#ifdef CONFIG_DEBUG_SG
        BUG_ON(sg->sg_magic != SG_MAGIC);
#endif
        if (sg_is_last(sg))
                return NULL;

        sg++;
        if (unlikely(sg_is_chain(sg)))
                sg = sg_chain_ptr(sg);

        return sg;
}
EXPORT_SYMBOL(sg_next);

/**
 * sg_nents - return total count of entries in scatterlist
 * @sg:         The scatterlist
 *
 * Description:
 * Allows to know how many entries are in sg, taking into acount
 * chaining as well
 *
 **/
int sg_nents(struct scatterlist *sg)
{
        int nents;
        for (nents = 0; sg; sg = sg_next(sg))
                nents++;
        return nents;
}
EXPORT_SYMBOL(sg_nents);

/**
 * sg_nents_for_len - return total count of entries in scatterlist
 *                    needed to satisfy the supplied length
 * @sg:         The scatterlist
 * @len:        The total required length
 *
 * Description:
 * Determines the number of entries in sg that are required to meet
 * the supplied length, taking into acount chaining as well
 *
 * Returns:
 *   the number of sg entries needed, negative error on failure
 *
 **/
int sg_nents_for_len(struct scatterlist *sg, u64 len)
{
        int nents;
        u64 total;

        if (!len)
                return 0;

        for (nents = 0, total = 0; sg; sg = sg_next(sg)) {
                nents++;
                total += sg->length;
                if (total >= len)
                        return nents;
        }

        return -EINVAL;
}
EXPORT_SYMBOL(sg_nents_for_len);

/**
 * sg_last - return the last scatterlist entry in a list
 * @sgl:        First entry in the scatterlist
 * @nents:      Number of entries in the scatterlist
 *
 * Description:
 *   Should only be used casually, it (currently) scans the entire list
 *   to get the last entry.
 *
 *   Note that the @sgl@ pointer passed in need not be the first one,
 *   the important bit is that @nents@ denotes the number of entries that
 *   exist from @sgl@.
 *
 **/
struct scatterlist *sg_last(struct scatterlist *sgl, unsigned int nents)
{
        struct scatterlist *sg, *ret = NULL;
        unsigned int i;

        for_each_sg(sgl, sg, nents, i)
                ret = sg;

#ifdef CONFIG_DEBUG_SG
        BUG_ON(sgl[0].sg_magic != SG_MAGIC);
        BUG_ON(!sg_is_last(ret));
#endif
        return ret;
}
EXPORT_SYMBOL(sg_last);

/**
 * sg_init_table - Initialize SG table
 * @sgl:           The SG table
 * @nents:         Number of entries in table
 *
 * Notes:
 *   If this is part of a chained sg table, sg_mark_end() should be
 *   used only on the last table part.
 *
 **/
void sg_init_table(struct scatterlist *sgl, unsigned int nents)
{
        memset(sgl, 0, sizeof(*sgl) * nents);
#ifdef CONFIG_DEBUG_SG
        {
                unsigned int i;
                for (i = 0; i < nents; i++)
                        sgl[i].sg_magic = SG_MAGIC;
        }
#endif
        sg_mark_end(&sgl[nents - 1]);
}
EXPORT_SYMBOL(sg_init_table);

/**
 * sg_init_one - Initialize a single entry sg list
 * @sg:          SG entry
 * @buf:         Virtual address for IO
 * @buflen:      IO length
 *
 **/
void sg_init_one(struct scatterlist *sg, const void *buf, unsigned int buflen)
{
        sg_init_table(sg, 1);
        sg_set_buf(sg, buf, buflen);
}
EXPORT_SYMBOL(sg_init_one);

/*
 * The default behaviour of sg_alloc_table() is to use these kmalloc/kfree
 * helpers.
 */
static struct scatterlist *sg_kmalloc(unsigned int nents, gfp_t gfp_mask)
{
        if (nents == SG_MAX_SINGLE_ALLOC) {
                /*
                 * Kmemleak doesn't track page allocations as they are not
                 * commonly used (in a raw form) for kernel data structures.
                 * As we chain together a list of pages and then a normal
                 * kmalloc (tracked by kmemleak), in order to for that last
                 * allocation not to become decoupled (and thus a
                 * false-positive) we need to inform kmemleak of all the
                 * intermediate allocations.
                 */
                void *ptr = (void *) __get_free_page(gfp_mask);
                kmemleak_alloc(ptr, PAGE_SIZE, 1, gfp_mask);
                return ptr;
        } else
                return kmalloc(nents * sizeof(struct scatterlist), gfp_mask);
}

static void sg_kfree(struct scatterlist *sg, unsigned int nents)
{
        if (nents == SG_MAX_SINGLE_ALLOC) {
                kmemleak_free(sg);
                free_page((unsigned long) sg);
        } else
                kfree(sg);
}

/**
 * __sg_free_table - Free a previously mapped sg table
 * @table:      The sg table header to use
 * @max_ents:   The maximum number of entries per single scatterlist
 * @skip_first_chunk: don't free the (preallocated) first scatterlist chunk
 * @free_fn:    Free function
 *
 *  Description:
 *    Free an sg table previously allocated and setup with
 *    __sg_alloc_table().  The @max_ents value must be identical to
 *    that previously used with __sg_alloc_table().
 *
 **/
void __sg_free_table(struct sg_table *table, unsigned int max_ents,
                     bool skip_first_chunk, sg_free_fn *free_fn)
{
        struct scatterlist *sgl, *next;

        if (unlikely(!table->sgl))
                return;

        sgl = table->sgl;
        while (table->orig_nents) {
                unsigned int alloc_size = table->orig_nents;
                unsigned int sg_size;

                /*
                 * If we have more than max_ents segments left,
                 * then assign 'next' to the sg table after the current one.
                 * sg_size is then one less than alloc size, since the last
                 * element is the chain pointer.
                 */
                if (alloc_size > max_ents) {
                        next = sg_chain_ptr(&sgl[max_ents - 1]);
                        alloc_size = max_ents;
                        sg_size = alloc_size - 1;
                } else {
                        sg_size = alloc_size;
                        next = NULL;
                }

                table->orig_nents -= sg_size;
                if (skip_first_chunk)
                        skip_first_chunk = false;
                else
                        free_fn(sgl, alloc_size);
                sgl = next;
        }

        table->sgl = NULL;
}
EXPORT_SYMBOL(__sg_free_table);

/**
 * sg_free_table - Free a previously allocated sg table
 * @table:      The mapped sg table header
 *
 **/
void sg_free_table(struct sg_table *table)
{
        __sg_free_table(table, SG_MAX_SINGLE_ALLOC, false, sg_kfree);
}
EXPORT_SYMBOL(sg_free_table);

/**
 * __sg_alloc_table - Allocate and initialize an sg table with given allocator
 * @table:      The sg table header to use
 * @nents:      Number of entries in sg list
 * @max_ents:   The maximum number of entries the allocator returns per call
 * @gfp_mask:   GFP allocation mask
 * @alloc_fn:   Allocator to use
 *
 * Description:
 *   This function returns a @table @nents long. The allocator is
 *   defined to return scatterlist chunks of maximum size @max_ents.
 *   Thus if @nents is bigger than @max_ents, the scatterlists will be
 *   chained in units of @max_ents.
 *
 * Notes:
 *   If this function returns non-0 (eg failure), the caller must call
 *   __sg_free_table() to cleanup any leftover allocations.
 *
 **/
int __sg_alloc_table(struct sg_table *table, unsigned int nents,
                     unsigned int max_ents, struct scatterlist *first_chunk,
                     gfp_t gfp_mask, sg_alloc_fn *alloc_fn)
{
        struct scatterlist *sg, *prv;
        unsigned int left;

        memset(table, 0, sizeof(*table));

        if (nents == 0)
                return -EINVAL;
#ifndef CONFIG_ARCH_HAS_SG_CHAIN
        if (WARN_ON_ONCE(nents > max_ents))
                return -EINVAL;
#endif

        left = nents;
        prv = NULL;
        do {
                unsigned int sg_size, alloc_size = left;

                if (alloc_size > max_ents) {
                        alloc_size = max_ents;
                        sg_size = alloc_size - 1;
                } else
                        sg_size = alloc_size;

                left -= sg_size;

                if (first_chunk) {
                        sg = first_chunk;
                        first_chunk = NULL;
                } else {
                        sg = alloc_fn(alloc_size, gfp_mask);
                }
                if (unlikely(!sg)) {
                        /*
                         * Adjust entry count to reflect that the last
                         * entry of the previous table won't be used for
                         * linkage.  Without this, sg_kfree() may get
                         * confused.
                         */
                        if (prv)
                                table->nents = ++table->orig_nents;

                        return -ENOMEM;
                }

                sg_init_table(sg, alloc_size);
                table->nents = table->orig_nents += sg_size;

                /*
                 * If this is the first mapping, assign the sg table header.
                 * If this is not the first mapping, chain previous part.
                 */
                if (prv)
                        sg_chain(prv, max_ents, sg);
                else
                        table->sgl = sg;

                /*
                 * If no more entries after this one, mark the end
                 */
                if (!left)
                        sg_mark_end(&sg[sg_size - 1]);

                prv = sg;
        } while (left);

        return 0;
}
EXPORT_SYMBOL(__sg_alloc_table);

/**
 * sg_alloc_table - Allocate and initialize an sg table
 * @table:      The sg table header to use
 * @nents:      Number of entries in sg list
 * @gfp_mask:   GFP allocation mask
 *
 *  Description:
 *    Allocate and initialize an sg table. If @nents@ is larger than
 *    SG_MAX_SINGLE_ALLOC a chained sg table will be setup.
 *
 **/
int sg_alloc_table(struct sg_table *table, unsigned int nents, gfp_t gfp_mask)
{
        int ret;

        ret = __sg_alloc_table(table, nents, SG_MAX_SINGLE_ALLOC,
                               NULL, gfp_mask, sg_kmalloc);
        if (unlikely(ret))
                __sg_free_table(table, SG_MAX_SINGLE_ALLOC, false, sg_kfree);

        return ret;
}
EXPORT_SYMBOL(sg_alloc_table);

/**
 * __sg_alloc_table_from_pages - Allocate and initialize an sg table from
 *                               an array of pages
 * @sgt:         The sg table header to use
 * @pages:       Pointer to an array of page pointers
 * @n_pages:     Number of pages in the pages array
 * @offset:      Offset from start of the first page to the start of a buffer
 * @size:        Number of valid bytes in the buffer (after offset)
 * @max_segment: Maximum size of a scatterlist node in bytes (page aligned)
 * @gfp_mask:    GFP allocation mask
 *
 *  Description:
 *    Allocate and initialize an sg table from a list of pages. Contiguous
 *    ranges of the pages are squashed into a single scatterlist node up to the
 *    maximum size specified in @max_segment. An user may provide an offset at a
 *    start and a size of valid data in a buffer specified by the page array.
 *    The returned sg table is released by sg_free_table.
 *
 * Returns:
 *   0 on success, negative error on failure
 */
int __sg_alloc_table_from_pages(struct sg_table *sgt, struct page **pages,
                                unsigned int n_pages, unsigned int offset,
                                unsigned long size, unsigned int max_segment,
                                gfp_t gfp_mask)
{
        unsigned int chunks, cur_page, seg_len, i;
        int ret;
        struct scatterlist *s;

        if (WARN_ON(!max_segment || offset_in_page(max_segment)))
                return -EINVAL;

        /* compute number of contiguous chunks */
        chunks = 1;
        seg_len = 0;
        for (i = 1; i < n_pages; i++) {
                seg_len += PAGE_SIZE;
                if (seg_len >= max_segment ||
                    page_to_pfn(pages[i]) != page_to_pfn(pages[i - 1]) + 1) {
                        chunks++;
                        seg_len = 0;
                }
        }

        ret = sg_alloc_table(sgt, chunks, gfp_mask);
        if (unlikely(ret))
                return ret;

        /* merging chunks and putting them into the scatterlist */
        cur_page = 0;
        for_each_sg(sgt->sgl, s, sgt->orig_nents, i) {
                unsigned int j, chunk_size;

                /* look for the end of the current chunk */
                seg_len = 0;
                for (j = cur_page + 1; j < n_pages; j++) {
                        seg_len += PAGE_SIZE;
                        if (seg_len >= max_segment ||
                            page_to_pfn(pages[j]) !=
                            page_to_pfn(pages[j - 1]) + 1)
                                break;
                }

                chunk_size = ((j - cur_page) << PAGE_SHIFT) - offset;
                sg_set_page(s, pages[cur_page],
                            min_t(unsigned long, size, chunk_size), offset);
                size -= chunk_size;
                offset = 0;
                cur_page = j;
        }

        return 0;
}
EXPORT_SYMBOL(__sg_alloc_table_from_pages);

/**
 * sg_alloc_table_from_pages - Allocate and initialize an sg table from
 *                             an array of pages
 * @sgt:         The sg table header to use
 * @pages:       Pointer to an array of page pointers
 * @n_pages:     Number of pages in the pages array
 * @offset:      Offset from start of the first page to the start of a buffer
 * @size:        Number of valid bytes in the buffer (after offset)
 * @gfp_mask:    GFP allocation mask
 *
 *  Description:
 *    Allocate and initialize an sg table from a list of pages. Contiguous
 *    ranges of the pages are squashed into a single scatterlist node. A user
 *    may provide an offset at a start and a size of valid data in a buffer
 *    specified by the page array. The returned sg table is released by
 *    sg_free_table.
 *
 * Returns:
 *   0 on success, negative error on failure
 */
int sg_alloc_table_from_pages(struct sg_table *sgt, struct page **pages,
                              unsigned int n_pages, unsigned int offset,
                              unsigned long size, gfp_t gfp_mask)
{
        return __sg_alloc_table_from_pages(sgt, pages, n_pages, offset, size,
                                           SCATTERLIST_MAX_SEGMENT, gfp_mask);
}
EXPORT_SYMBOL(sg_alloc_table_from_pages);

void __sg_page_iter_start(struct sg_page_iter *piter,
                          struct scatterlist *sglist, unsigned int nents,
                          unsigned long pgoffset)
{
        piter->__pg_advance = 0;
        piter->__nents = nents;

        piter->sg = sglist;
        piter->sg_pgoffset = pgoffset;
}
EXPORT_SYMBOL(__sg_page_iter_start);

static int sg_page_count(struct scatterlist *sg)
{
        return PAGE_ALIGN(sg->offset + sg->length) >> PAGE_SHIFT;
}

bool __sg_page_iter_next(struct sg_page_iter *piter)
{
        if (!piter->__nents || !piter->sg)
                return false;

        piter->sg_pgoffset += piter->__pg_advance;
        piter->__pg_advance = 1;

        while (piter->sg_pgoffset >= sg_page_count(piter->sg)) {
                piter->sg_pgoffset -= sg_page_count(piter->sg);
                piter->sg = sg_next(piter->sg);
                if (!--piter->__nents || !piter->sg)
                        return false;
        }

        return true;
}
EXPORT_SYMBOL(__sg_page_iter_next);

/**
 * sg_miter_start - start mapping iteration over a sg list
 * @miter: sg mapping iter to be started
 * @sgl: sg list to iterate over
 * @nents: number of sg entries
 *
 * Description:
 *   Starts mapping iterator @miter.
 *
 * Context:
 *   Don't care.
 */
void sg_miter_start(struct sg_mapping_iter *miter, struct scatterlist *sgl,
                    unsigned int nents, unsigned int flags)
{
        memset(miter, 0, sizeof(struct sg_mapping_iter));

        __sg_page_iter_start(&miter->piter, sgl, nents, 0);
        WARN_ON(!(flags & (SG_MITER_TO_SG | SG_MITER_FROM_SG)));
        miter->__flags = flags;
}
EXPORT_SYMBOL(sg_miter_start);

static bool sg_miter_get_next_page(struct sg_mapping_iter *miter)
{
        if (!miter->__remaining) {
                struct scatterlist *sg;
                unsigned long pgoffset;

                if (!__sg_page_iter_next(&miter->piter))
                        return false;

                sg = miter->piter.sg;
                pgoffset = miter->piter.sg_pgoffset;

                miter->__offset = pgoffset ? 0 : sg->offset;
                miter->__remaining = sg->offset + sg->length -
                                (pgoffset << PAGE_SHIFT) - miter->__offset;
                miter->__remaining = min_t(unsigned long, miter->__remaining,
                                           PAGE_SIZE - miter->__offset);
        }

        return true;
}

/**
 * sg_miter_skip - reposition mapping iterator
 * @miter: sg mapping iter to be skipped
 * @offset: number of bytes to plus the current location
 *
 * Description:
 *   Sets the offset of @miter to its current location plus @offset bytes.
 *   If mapping iterator @miter has been proceeded by sg_miter_next(), this
 *   stops @miter.
 *
 * Context:
 *   Don't care if @miter is stopped, or not proceeded yet.
 *   Otherwise, preemption disabled if the SG_MITER_ATOMIC is set.
 *
 * Returns:
 *   true if @miter contains the valid mapping.  false if end of sg
 *   list is reached.
 */
bool sg_miter_skip(struct sg_mapping_iter *miter, off_t offset)
{
        sg_miter_stop(miter);

        while (offset) {
                off_t consumed;

                if (!sg_miter_get_next_page(miter))
                        return false;

                consumed = min_t(off_t, offset, miter->__remaining);
                miter->__offset += consumed;
                miter->__remaining -= consumed;
                offset -= consumed;
        }

        return true;
}
EXPORT_SYMBOL(sg_miter_skip);

/**
 * sg_miter_next - proceed mapping iterator to the next mapping
 * @miter: sg mapping iter to proceed
 *
 * Description:
 *   Proceeds @miter to the next mapping.  @miter should have been started
 *   using sg_miter_start().  On successful return, @miter->page,
 *   @miter->addr and @miter->length point to the current mapping.
 *
 * Context:
 *   Preemption disabled if SG_MITER_ATOMIC.  Preemption must stay disabled
 *   till @miter is stopped.  May sleep if !SG_MITER_ATOMIC.
 *
 * Returns:
 *   true if @miter contains the next mapping.  false if end of sg
 *   list is reached.
 */
bool sg_miter_next(struct sg_mapping_iter *miter)
{
        sg_miter_stop(miter);

        /*
         * Get to the next page if necessary.
         * __remaining, __offset is adjusted by sg_miter_stop
         */
        if (!sg_miter_get_next_page(miter))
                return false;

        miter->page = sg_page_iter_page(&miter->piter);
        miter->consumed = miter->length = miter->__remaining;

        if (miter->__flags & SG_MITER_ATOMIC)
                miter->addr = kmap_atomic(miter->page) + miter->__offset;
        else
                miter->addr = kmap(miter->page) + miter->__offset;

        return true;
}
EXPORT_SYMBOL(sg_miter_next);

/**
 * sg_miter_stop - stop mapping iteration
 * @miter: sg mapping iter to be stopped
 *
 * Description:
 *   Stops mapping iterator @miter.  @miter should have been started
 *   using sg_miter_start().  A stopped iteration can be resumed by
 *   calling sg_miter_next() on it.  This is useful when resources (kmap)
 *   need to be released during iteration.
 *
 * Context:
 *   Preemption disabled if the SG_MITER_ATOMIC is set.  Don't care
 *   otherwise.
 */
void sg_miter_stop(struct sg_mapping_iter *miter)
{
        WARN_ON(miter->consumed > miter->length);

        /* drop resources from the last iteration */
        if (miter->addr) {
                miter->__offset += miter->consumed;
                miter->__remaining -= miter->consumed;

                if ((miter->__flags & SG_MITER_TO_SG) &&
                    !PageSlab(miter->page))
                        flush_kernel_dcache_page(miter->page);

                if (miter->__flags & SG_MITER_ATOMIC) {
                        WARN_ON_ONCE(preemptible());
                        kunmap_atomic(miter->addr);
                } else
                        kunmap(miter->page);

                miter->page = NULL;
                miter->addr = NULL;
                miter->length = 0;
                miter->consumed = 0;
        }
}
EXPORT_SYMBOL(sg_miter_stop);

/**
 * sg_copy_buffer - Copy data between a linear buffer and an SG list
 * @sgl:                 The SG list
 * @nents:               Number of SG entries
 * @buf:                 Where to copy from
 * @buflen:              The number of bytes to copy
 * @skip:                Number of bytes to skip before copying
 * @to_buffer:           transfer direction (true == from an sg list to a
 *                       buffer, false == from a buffer to an sg list
 *
 * Returns the number of copied bytes.
 *
 **/
size_t sg_copy_buffer(struct scatterlist *sgl, unsigned int nents, void *buf,
                      size_t buflen, off_t skip, bool to_buffer)
{
        unsigned int offset = 0;
        struct sg_mapping_iter miter;
        unsigned int sg_flags = SG_MITER_ATOMIC;

        if (to_buffer)
                sg_flags |= SG_MITER_FROM_SG;
        else
                sg_flags |= SG_MITER_TO_SG;

        sg_miter_start(&miter, sgl, nents, sg_flags);

        if (!sg_miter_skip(&miter, skip))
                return false;

        while ((offset < buflen) && sg_miter_next(&miter)) {
                unsigned int len;

                len = min(miter.length, buflen - offset);

                if (to_buffer)
                        memcpy(buf + offset, miter.addr, len);
                else
                        memcpy(miter.addr, buf + offset, len);

                offset += len;
        }

        sg_miter_stop(&miter);

        return offset;
}
EXPORT_SYMBOL(sg_copy_buffer);

/**
 * sg_copy_from_buffer - Copy from a linear buffer to an SG list
 * @sgl:                 The SG list
 * @nents:               Number of SG entries
 * @buf:                 Where to copy from
 * @buflen:              The number of bytes to copy
 *
 * Returns the number of copied bytes.
 *
 **/
size_t sg_copy_from_buffer(struct scatterlist *sgl, unsigned int nents,
                           const void *buf, size_t buflen)
{
        return sg_copy_buffer(sgl, nents, (void *)buf, buflen, 0, false);
}
EXPORT_SYMBOL(sg_copy_from_buffer);

/**
 * sg_copy_to_buffer - Copy from an SG list to a linear buffer
 * @sgl:                 The SG list
 * @nents:               Number of SG entries
 * @buf:                 Where to copy to
 * @buflen:              The number of bytes to copy
 *
 * Returns the number of copied bytes.
 *
 **/
size_t sg_copy_to_buffer(struct scatterlist *sgl, unsigned int nents,
                         void *buf, size_t buflen)
{
        return sg_copy_buffer(sgl, nents, buf, buflen, 0, true);
}
EXPORT_SYMBOL(sg_copy_to_buffer);

/**
 * sg_pcopy_from_buffer - Copy from a linear buffer to an SG list
 * @sgl:                 The SG list
 * @nents:               Number of SG entries
 * @buf:                 Where to copy from
 * @buflen:              The number of bytes to copy
 * @skip:                Number of bytes to skip before copying
 *
 * Returns the number of copied bytes.
 *
 **/
size_t sg_pcopy_from_buffer(struct scatterlist *sgl, unsigned int nents,
                            const void *buf, size_t buflen, off_t skip)
{
        return sg_copy_buffer(sgl, nents, (void *)buf, buflen, skip, false);
}
EXPORT_SYMBOL(sg_pcopy_from_buffer);

/**
 * sg_pcopy_to_buffer - Copy from an SG list to a linear buffer
 * @sgl:                 The SG list
 * @nents:               Number of SG entries
 * @buf:                 Where to copy to
 * @buflen:              The number of bytes to copy
 * @skip:                Number of bytes to skip before copying
 *
 * Returns the number of copied bytes.
 *
 **/
size_t sg_pcopy_to_buffer(struct scatterlist *sgl, unsigned int nents,
                          void *buf, size_t buflen, off_t skip)
{
        return sg_copy_buffer(sgl, nents, buf, buflen, skip, true);
}
EXPORT_SYMBOL(sg_pcopy_to_buffer);

/**
 * sg_zero_buffer - Zero-out a part of a SG list
 * @sgl:                 The SG list
 * @nents:               Number of SG entries
 * @buflen:              The number of bytes to zero out
 * @skip:                Number of bytes to skip before zeroing
 *
 * Returns the number of bytes zeroed.
 **/
size_t sg_zero_buffer(struct scatterlist *sgl, unsigned int nents,
                       size_t buflen, off_t skip)
{
        unsigned int offset = 0;
        struct sg_mapping_iter miter;
        unsigned int sg_flags = SG_MITER_ATOMIC | SG_MITER_TO_SG;

        sg_miter_start(&miter, sgl, nents, sg_flags);

        if (!sg_miter_skip(&miter, skip))
                return false;

        while (offset < buflen && sg_miter_next(&miter)) {
                unsigned int len;

                len = min(miter.length, buflen - offset);
                memset(miter.addr, 0, len);

                offset += len;
        }

        sg_miter_stop(&miter);
        return offset;
}
EXPORT_SYMBOL(sg_zero_buffer);