On Mon, Jul 16 2007, Bartlomiej Zolnierkiewicz wrote: > 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 <jens.axboe@xxxxxxxxxx> > > --- > > 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? Good spotting! This function is only used for libata atapi, which explains why my testing today didn't find any errors. Thanks Bart, I'll rebase with that fix. -- Jens Axboe - To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html