linux-kernel - Re: [PATCH 05/33] Add chained sg support to linux/scatterlist.h

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Message-ID: <20070716191529.GL5195@kernel.dk>
Date:	Mon, 16 Jul 2007 21:15:34 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Bartlomiej Zolnierkiewicz <bzolnier@...il.com>
Cc:	linux-kernel@...r.kernel.org, linux-scsi@...r.kernel.org
Subject: Re: [PATCH 05/33] Add chained sg support to linux/scatterlist.h

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@...cle.com>
> > ---
> >  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

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