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Commit 70eb8040 authored by Jens Axboe's avatar Jens Axboe
Browse files

Add chained sg support to linux/scatterlist.h 



The core of the patch - allow the last sg element in a scatterlist
table to point to the start of a new table. We overload the LSB of
the page pointer to indicate whether this is a valid sg entry, or
merely a link to the next list.

Includes a fix from Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>
correcting the ifdef ARCH_HAS_SG_CHAIN guarding sg_last().

Signed-off-by: default avatarJens Axboe <jens.axboe@oracle.com>
parent c6132da1

include/linux/scatterlist.h

+77 −2
Original line number Diff line number Diff line
@@ -20,8 +20,36 @@ static inline void sg_init_one(struct scatterlist *sg, const void *buf, 
	sg_set_buf(sg, buf, buflen);

}



#define sg_next(sg)		((sg) + 1)

#define sg_last(sg, nents)	(&(sg[(nents) - 1]))

/*

 * We overload the LSB of the page pointer to indicate whether it's

 * a valid sg entry, or whether it points to the start of a new scatterlist.

 * Those low bits are there for everyone! (thanks mason :-)

 */

#define sg_is_chain(sg)		((unsigned long) (sg)->page & 0x01)

#define sg_chain_ptr(sg)	\

	((struct scatterlist *) ((unsigned long) (sg)->page & ~0x01))



/**

 * sg_next - return the next scatterlist entry in a list

 * @sg:		The current sg entry

 *

 * 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.

 *

 * Note that the caller must ensure that there are further entries after

 * the current entry, this function will NOT return NULL for an end-of-list.

 *

 */

static inline struct scatterlist *sg_next(struct scatterlist *sg)

{

	sg++;



	if (unlikely(sg_is_chain(sg)))

		sg = sg_chain_ptr(sg);



	return sg;

}



/*

 * Loop over each sg element, following the pointer to a new list if necessary
@@ -29,4 +57,51 @@ static inline void sg_init_one(struct scatterlist *sg, const void *buf, 
#define for_each_sg(sglist, sg, nr, __i)	\

	for (__i = 0, sg = (sglist); __i < (nr); __i++, sg = sg_next(sg))



/**

 * sg_last - return the last scatterlist entry in a list

 * @sgl:	First entry in the scatterlist

 * @nents:	Number of entries in the scatterlist

 *

 * Should only be used casually, it (currently) scan 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@.

 *

 */

static inline struct scatterlist *sg_last(struct scatterlist *sgl,

					  unsigned int nents)

{

#ifndef ARCH_HAS_SG_CHAIN

	struct scatterlist *ret = &sgl[nents - 1];

#else

	struct scatterlist *sg, *ret = NULL;

	int i;



	for_each_sg(sgl, sg, nents, i)

		ret = sg;



#endif

	return ret;

}



/**

 * sg_chain - Chain two sglists together

 * @prv:	First scatterlist

 * @prv_nents:	Number of entries in prv

 * @sgl:	Second scatterlist

 *

 * Links @prv@ and @sgl@ together, to form a longer scatterlist.

 *

 */

static inline void sg_chain(struct scatterlist *prv, unsigned int prv_nents,

			    struct scatterlist *sgl)

{

#ifndef ARCH_HAS_SG_CHAIN

	BUG();

#endif

	prv[prv_nents - 1].page = (struct page *) ((unsigned long) sgl | 0x01);

}



#endif /* _LINUX_SCATTERLIST_H */