Re: [PATCH 7/13] i386 sg: add support for chaining scatterlists

Jens Axboe
2007-05-10T10:45:31Z

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