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    SubjectRe: [PATCH 7/12] i386 sg: add support for chaining scatterlists
    Hi Jens,

    On 5/10/07, Jens Axboe <jens.axboe@oracle.com> 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 <jens.axboe@oracle.com>
    > ---
    > include/asm-i386/scatterlist.h | 2 +
    > include/linux/scatterlist.h | 52 ++++++++++++++++++++++++++++++++++++++-
    > 2 files changed, 52 insertions(+), 2 deletions(-)
    >
    > diff --git a/include/asm-i386/scatterlist.h b/include/asm-i386/scatterlist.h
    > index d7e45a8..bd5164a 100644
    > --- a/include/asm-i386/scatterlist.h
    > +++ b/include/asm-i386/scatterlist.h
    > @@ -10,6 +10,8 @@ struct scatterlist {
    > unsigned int length;
    > };
    >
    > +#define ARCH_HAS_SG_CHAIN
    > +
    > /* These macros should be used after a pci_map_sg call has been done
    > * to get bus addresses of each of the SG entries and their lengths.
    > * You should only work with the number of sg entries pci_map_sg
    > diff --git a/include/linux/scatterlist.h b/include/linux/scatterlist.h
    > index bed5ab4..fa2dc1c 100644
    > --- a/include/linux/scatterlist.h
    > +++ b/include/linux/scatterlist.h
    > [...]
    > +/*
    > + * We could improve this by passing in the maximum size of an sglist, so
    > + * we could jump directly to the last table. That would eliminate this
    > + * (potentially) lengthy scan.
    > + */
    > +static inline struct scatterlist *sg_last(struct scatterlist *sgl,
    > + unsigned int nents)
    > +{
    > +#ifdef 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;
    > +}
    > +
    > +/*
    > + * Chain previous sglist to this one
    > + */
    > +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 */

    (Style triviality)

    In include/linux/scatterlist.h, why not simply:

    #ifdef ARCH_HAS_SG_CHAIN

    > +/*
    > + * We could improve this by passing in the maximum size of an sglist, so
    > + * we could jump directly to the last table. That would eliminate this
    > + * (potentially) lengthy scan.
    > + */
    > +static inline struct scatterlist *sg_last(struct scatterlist *sgl,
    > + unsigned int nents)
    > +{
    > + struct scatterlist *ret = &sgl[nents - 1];
    > + return ret;
    > +}
    > +
    > +/*
    > + * Chain previous sglist to this one
    > + */
    > +static inline void sg_chain(struct scatterlist *prv, unsigned int prv_nents,
    > + struct scatterlist *sgl)
    > +{
    > + prv[prv_nents - 1].page = (struct page *) ((unsigned long) sgl | 0x01);
    > +}

    #else

    > +/*
    > + * We could improve this by passing in the maximum size of an sglist, so
    > + * we could jump directly to the last table. That would eliminate this
    > + * (potentially) lengthy scan.
    > + */
    > +static inline struct scatterlist *sg_last(struct scatterlist *sgl,
    > + unsigned int nents)
    > +{
    > + struct scatterlist *sg, *ret = NULL;
    > + int i;
    > +
    > + for_each_sg(sgl, sg, nents, i)
    > + ret = sg;
    > +
    > + return ret;
    > +}
    > +
    > +/*
    > + * Chain previous sglist to this one
    > + */
    > +static inline void sg_chain(struct scatterlist *prv, unsigned int prv_nents,
    > + struct scatterlist *sgl)
    > +{
    > + BUG();
    > +}

    #endif /* ARCH_HAS_SG_CHAIN */

    Similar to most of the other headers I've seen, and avoids multiple
    usage if #ifdef / #ifndef.
    -
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