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
========================================================================
On Sat, 27 Oct 2007 07:39:40 -0400 (EDT)
"Robert P. J. Day" <[email protected]> wrote:
> 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?
If it increments sg and finds a "chain" entry, it will follow it to the
next sg array instead of just returning it. Which makes sense because
the chain entry itself isn't a valid entry in the sg list.
Håvard
On Sat, 27 Oct 2007, Haavard Skinnemoen wrote:
> On Sat, 27 Oct 2007 07:39:40 -0400 (EDT)
> "Robert P. J. Day" <[email protected]> wrote:
>
> > 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?
>
> If it increments sg and finds a "chain" entry, it will follow it to
> the next sg array instead of just returning it. Which makes sense
> because the chain entry itself isn't a valid entry in the sg list.
yes, i finally twigged on that after a few more minutes. it, of
course, makes sense, but it sure doesn't match the comment that's at
the top of that file.
rday
--
========================================================================
Robert P. J. Day
Linux Consulting, Training and Annoying Kernel Pedantry
Waterloo, Ontario, CANADA
http://crashcourse.ca
========================================================================