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    SubjectRe: [PATCH 7/13] i386 sg: add support for chaining scatterlists
    On Thu, May 10 2007, Andrew Morton wrote:
    > On Thu, 10 May 2007 12:21:49 +0200 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, 51 insertions(+), 3 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 c5bffde..bad6b9e 100644
    > > --- a/include/linux/scatterlist.h
    > > +++ b/include/linux/scatterlist.h
    > > @@ -20,13 +20,59 @@ static inline void sg_init_one(struct scatterlist *sg, const void *buf,
    > > sg_set_buf(sg, buf, buflen);
    > > }
    > >
    > > ...
    > >
    > > /*
    > > * Loop over each sg element, following the pointer to a new list if necessary
    > > */
    > > #define for_each_sg(sglist, sg, nr, __i) \
    > > - for (__i = 0, sg = (sglist); __i < nr; __i++, sg = sg_next(sg))
    > > + for (__i = 0, sg = (sglist); __i < (nr); __i++, sg = sg_next(sg))
    > > +
    > > +/*
    > > + * 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;
    > > +}
    >
    > Looks too large to be inlined.

    Yeah, I'm inclined to agree. Perhaps it would be better to put this
    stuff in lib/scatterlist.c or something like that instead?

    > > +/*
    > > + * 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
    >
    > Can use BUILD_BUG_ON here. Or just #error.

    Good idea, thanks!

    --
    Jens Axboe

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