On Monday 16 July 2007, Jens Axboe wrote:
> 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.
>
> Signed-off-by: Jens Axboe <[email protected]>
> ---
> include/linux/scatterlist.h | 79 +++++++++++++++++++++++++++++++++++++++++-
> 1 files changed, 77 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/scatterlist.h b/include/linux/scatterlist.h
> index bed5ab4..10f6223 100644
> --- a/include/linux/scatterlist.h
> +++ b/include/linux/scatterlist.h
> @@ -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)
> +{
> +#ifdef ARCH_HAS_SG_CHAIN
Shouldn't this be #ifndef?
> + 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 */
-
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