/*
   * 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)
   * @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 long offset, unsigned long size,
  	gfp_t gfp_mask)
  {
  	unsigned int chunks;
  	unsigned int i;
  	unsigned int cur_page;
  	int ret;
  	struct scatterlist *s;
  
  	/* compute number of contiguous chunks */
  	chunks = 1;
  	for (i = 1; i < n_pages; ++i)
  		if (page_to_pfn(pages[i]) != page_to_pfn(pages[i - 1]) + 1)
  			++chunks;
  
  	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 long chunk_size;
  		unsigned int j;
  
  		/* look for the end of the current chunk */
  		for (j = cur_page + 1; j < n_pages; ++j)
  			if (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(size, chunk_size), offset);
  		size -= chunk_size;
  		offset = 0;
  		cur_page = j;
  	}
  
  	return 0;
  }
  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);