From: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Date: Mon, 22 Oct 2007 14:47:38 -0700 (PDT)
On Mon, 22 Oct 2007, Alan Cox wrote:
Still doesn't answer the rather more important question - why not just
stick a NULL on the end instead of all the nutty hacks ?
You still do need one bit for the discontiguous case, so it's not like you
can avoid the hacks anyway (unless you just blow up the structure
entirely) and make it a separate member). So once you have that
bit+pointer, using a separate NULL entry isn't exactly prettier.
Especially as we actally want to see the difference between
"end-of-allocation" and "not yet filled in", so you shouldn't use NULL
anyway, you should probably use something like "all-ones".
Indeed that's the crux of the matter, we need to express a trinary
state "end of scatterlist, next entry is linear, next entry is
indirect" plus a pointer for the indirect case.
Generally, Jens was doing a good job cooking up the patch that
implemented this fully and I took care of making sure tricky
ports like sparc64 built cleanly etc.
He went away for a few days, but when he gets back we should seriously
work on integrating his work.
I fully recognize Alan's m68k on-stack alignment concern. The on-stack
cases are troublesome in other ways as well, and I think therefore the
way to move forward is to convert those to some kind of dynamic scheme.
Usually such code is working in a locked context on some object
(crypto instance, for example) and thus the scatterlist chunk can
be embedded into that object for ensured alignment.
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