[PATCH] asm-generic: allow generic unaligned access if the arch supports it

Russell King - ARM Linux linux at arm.linux.org.uk
Tue Apr 8 02:49:42 PDT 2014

On Mon, Mar 31, 2014 at 08:51:54PM +0200, Ard Biesheuvel wrote:
> On 31 March 2014 20:36, Russell King - ARM Linux <linux at arm.linux.org.uk> wrote:
> > On Mon, Mar 31, 2014 at 08:22:43PM +0200, Ard Biesheuvel wrote:
> >> On 31 March 2014 19:24, Catalin Marinas <catalin.marinas at arm.com> wrote:
> >> > It actually affects arch/arm as well which selects
> >> > HAVE_EFFICIENT_UNALIGNED_ACCESS for v6/v7 and uses the generic
> >> > unaligned.h
> >> >
> >>
> >> Oops, my bad. I did a quick 'find -name unaligned.h' but misread 'arc'
> >> for 'arm'.
> >>
> >> @Russell: ARM is a user of asm-generic/unaligned.h, and I proposed a
> >> patch to asm-generic that switches to unaligned accesses if
> >> HAVE_EFFICIENT_UNALIGNED_ACCESS is set by the arch. This should affect
> >> a fair amount of code living under net/ and drivers/ that don't test
> >> for the Kconfig symbol but expect get_unaligned_xx/put_unaligned_xx to
> >> turn into something suitable depending on the arch. Any objections?
> >
> > I'm not sure what you're talking about here, or what change you're
> > proposing.  Without any kind of frame of reference, I can't comment.
> > Sorry.
> >
> My apologies. I assumed the mail thread including a link to the patch
> would be clear enough.
> Currently, asm-generic's version of unaligned.h (defining stuff like
> get_unaligned() and put_unaligned()) unconditionally defaults to the
> most cautious method of performing unaligned memory accesses, even if
> the Kconfig symbol HAVE_EFFICIENT_UNALIGNED_ACCESS is defined.
> However, in that case, it makes sense to use a definition for
> get_unaligned()/put_unaligned() that lets the hardware perform the
> unaligned accesses instead.
> So that is what I proposed here
> http://marc.info/?l=linux-arch&m=139594237116121&w=2
> and in the mail exchange that followed, I stated that arm64 should be
> the only arch affected by this change. However, I didn't look
> carefully enough and, as Catalin pointed out, ARM will also be
> affected by it.
> So that is why I redirected this thread to you: would you object to
> changing asm-generic/unaligned.h so that calls to
> get_aligned()/put_aligned() on ARM v6/v7 will let the CPU perform the
> unaligned access?

The change looks fine to me, but given where we are in the cycle, I'd
rather hold off until after the merge window before queuing this up.

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