linux-kernel - sg_next() for struct scatterlist is confusing

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Message-ID: <Pine.LNX.4.64.0710270734210.24293@localhost.localdomain>
Date:	Sat, 27 Oct 2007 07:39:40 -0400 (EDT)
From:	"Robert P. J. Day" <rpjday@...shcourse.ca>
To:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: sg_next() for struct scatterlist is confusing

  according to the inline comments in include/linux/scatterlist.h:

"* If bit 0 is set, then the page_link contains a pointer to the next sg
 * table list. Otherwise the next entry is at sg + 1."

but if that's the case, then the implementation of sg_next() seems a
bit weird:

=================================
/**
 * sg_next - return the next scatterlist entry in a list
 * @sg:         The current sg entry
 *
 * Description:
 *   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.
 *
 **/
static inline struct scatterlist *sg_next(struct scatterlist *sg)
{
#ifdef CONFIG_DEBUG_SG
        BUG_ON(sg->sg_magic != SG_MAGIC);
#endif
        if (sg_is_last(sg))
                return NULL;

        sg++;
        if (unlikely(sg_is_chain(sg)))
                sg = sg_chain_ptr(sg);

        return sg;
}
================================

  note how the comment says that the next entry will "usually" be
sg+1, "but" not if it's actually a pointer.

  however, as i read the code above, sg is *always* incremented before
that testing.  is that correct?  am i just misreading something?  or
could the comment have been a bit clearer?

rday

-- 
========================================================================
Robert P. J. Day
Linux Consulting, Training and Annoying Kernel Pedantry
Waterloo, Ontario, CANADA

http://crashcourse.ca
========================================================================
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