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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: Jens Axboe <jens.axboe@oracle.com>
This commit is contained in:
Jens Axboe 2007-07-16 21:17:16 +02:00
parent c6132da170
commit 70eb8040dc
1 changed files with 77 additions and 2 deletions
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79
include/linux/scatterlist.h
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@ -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 */