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

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Message-ID: <4642FB1B.80702@panasas.com>
Date:	Thu, 10 May 2007 13:59:39 +0300
From:	Benny Halevy <bhalevy@...asas.com>
To:	Jens Axboe <jens.axboe@...cle.com>
CC:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 7/13] i386 sg: add support for chaining scatterlists

Jens Axboe wrote:
> +#define sg_is_chain(sg)		((unsigned long) (sg)->page & 0x01)
> +#define sg_chain_ptr(sg)	\
> +	((struct scatterlist *) ((unsigned long) (sg)->page & ~0x01))
> +
> +/*
> + * We overload the meaning of ->page for sg chaining. If the LSB is
> + * set, the page member contains a pointer to the next sgtable.
> + */
> +static inline struct scatterlist *sg_next(struct scatterlist *sg)
> +{
> +	if (sg_is_chain(sg))
> +		return sg_chain_ptr(sg);
> +
> +	return sg + 1;
> +}

Jens, should sg_next ever return the sg containing the chain link?
If sg points at the entry right before the link entry, don't we
want to skip it?

E.g.:

static inline struct scatterlist *sg_next(struct scatterlist *sg)
{
	struct scatterlist *next;

	/* just in case, shouldn't really ever be here */
	if (sg_is_chain(sg))
		return sg_chain_ptr(sg);

	next = sg + 1;

	if (sg_is_chain(next))
		return sg_chain_ptr(next);

	return next;
}
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