[U-Boot] [PATCH] arm: Disable the strict alignment of data on armv7

[Mark Kettenis]

> From: Wolfgang Denk <wd at denx.de>
> Date: Fri, 09 Mar 2018 12:26:01 +0100
> 
> Dear Michal,
> 
> In message <f144e491-5ce5-2ad0-70a8-d36397752863 at xilinx.com> you wrote:
> > 
> > > Can you please add some comments what the consequences of this
> > > change are?  I guess there are advantages, but I also guess these
> > > come at a price?
> > 
> > That's something what I am expecting from this discussion if there are
> > any corner cases which we are not aware of.
> > 
> > We found this setting based on randomized testing where simply non
> > aligned access for memory read was causing exception.
> 
> Hm... One possible position one can take is that unaligned accesses
> are likely an indication for incorrect or sloppy programming.
> usually we design data structures, buffers etc. such that no
> unaligned accesses take place.  Most code is explicitly written in
> such a way that it also runs on architectures where unaligned access
> simply don't work.
> 
> I feel that providing such an option just opens the door for lousy
> programmers who don't think throroughly about the code they write.
> 
> Yes, in some cases this may be conveniendt.  But there are also
> cases where the problem is just papered over, and hits all the
> harder later like when you also enable caching.

That is pretty much why we run OpenBSD in strict alignment mode on as
much architectures as possible.  More strict-alignment architectures
means that bugs are found quicker.  So as long as U-Boot supports
hardware that cares about alignment (e.g. older ARM, MIPS, SuperH) I'd
say there is a clear benefit of running armv7 in strict alignment mode.

Also note that some armv7 instructions still require properly aligned
data.  And that really old ARM hardware silently fetches the wrong
data for a misaligned load instruction.