[U-Boot] __attribute__((__packed__)) switching to byte-access on ARM

[Guennadi Liakhovetski]

On Mon, 11 Aug 2008, Wolfgang Denk wrote:

> Dear Guennadi Liakhovetski,
> 
> In message <Pine.LNX.4.64.0808111737330.30591 at axis700.grange> you wrote:
> > 
> > As I remarked in the opening email to the SMDK6400 patch-series, I've got 
> > a strange problem, I cannot understand.
> 
> What exactly is your problem?

Right, sorry, the problem is simple: it doesn't work with the byte access.

> > which converts all accesses to the structure to 8-bit... Ideas? Toolchain 
> > eldk-4.1. U-Boot from nand/testing git with the patches I just sent to the 
> > list, configured for smdk6400_config.
> 
> Note that you did not exactly ask a question, you just describe  your
> observations so we have to guess what might be puzzeling you. I think
> that  some  people  with  more ARM experience than me will simply not
> understand what you are asking about, because they are  used  to  GCC
> turning  perfectly  valid  looking 32 bit accesses into a sequence of
> four 8 bit accesses.
> 
> They consider this "normal".

I cannot think of any such examples... I know access-types can be changed 
on the hardware, say, on a 16-bit bus a 32-bit access can be split into 
two 16-bit accesses, but I currently cannot think of any examples of the 
compiler doing such things. But, I think, I can imagine _why_ the compiler 
might decide to do that - it cannot guarantee, that the structure is 
32-bit aligned, so, it is opting for the "safe" variant... So, the 
question is: is the compiler right and the c-code was wrong, or the other 
way round?

Thanks
Guennadi
---
Guennadi Liakhovetski, Ph.D.

DENX Software Engineering GmbH,     MD: Wolfgang Denk & Detlev Zundel
HRB 165235 Munich, Office: Kirchenstr.5, D-82194 Groebenzell, Germany
Phone: +49-8142-66989-0 Fax: +49-8142-66989-80  Email: office at denx.de