[U-Boot] [PATCH] usb: do explicit unaligned accesses

[Albert ARIBAUD]

Hi Marek,

(Apologies for the private mail)

On Sat, 1 Sep 2012 00:16:43 +0200, Marek Vasut <marex at denx.de> wrote:

> Dear Albert ARIBAUD,

> > I think you are talking about lumping small-sized accesses together
> > into a bigger access possibly aligned.
> 
> This is exactly what I mean.
> 
> > If I am correct, then I don't
> > think this is related to misaligned accesses.
> 
> Why won't it be? Such access can in the end turn out to be aligned,
> therefore leveraging all the penalty.

I have not expressed myself clearly. Yes, access lumping is related to
access alignment. What I meant is: disallowing misaligned native
accesses will not prevent access lumping. Misalignment restrictions do
indeed restrict how such lumpings will happen, but it does not prevent
lumping per se.

One place where lumping and misalignement prevention did clash was
raised in the previous discussion: a 7+1 bytes function-local char array
was allocated on a non-aligned address (which is possible and normal
because it is a char) and was initialized with some content. The
compiler lumped the initialization as two misaligned 32-byte native
accesses, despite misaligned native accesses being forbidden by
compiler command line options. This was a compiler bug.
 
> > If I am not correct, can
> > you please detail what you meant?
> > 
> > > Besides, right now, the code is much more readable. So I really
> > > don't like adding some strange macros to force crazy aligned
> > > access if the compiler can do it for us and can do it better.
> > 
> > I personally would let the compiler do it too, but I prefer it to be
> > clearly indicated to the reader of the code when an access is
> > known to be misaligned.
> 
> I'd enable enable the Alignment trapping in the CPU and die on an
> unaligned access at runtime -- to indicate the user that he should
> fix his bloody compiler.

... or fix his bloody structure, or fix his bloody f...ixing pointer
arithmetic, or... but I do agree with the trapping, and that's my plan.

However other architectures may need, or choose, another stance on
alignments, and it is best if they don't have to discover intended
misaligned accesses the hard way. Thus my opinion that any misaligned
access which cannot be fixed should not be sliently left for the
compiler to handle, but should (also) be clearly marked as such, if only
for humans to notice.

> Best regards,
> Marek Vasut

Amicalement,
-- 
Albert.