Re: [PATCH 7/12] i386 sg: add support for chaining scatterlists

["Satyam Sharma" <satyam.sharma@gmail.com>]

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.