are there PHYS_OFFSET alignment requirements for ARM Linux?

[Nicolas Pitre]

On Thu, 19 Jul 2012, John Linn wrote:

> On Thu, Jul 19, 2012 at 12:07 PM, Nicolas Pitre <nico at fluxnic.net> wrote:
> > On Wed, 18 Jul 2012, John Linn wrote:
> >
> >> I have seen some mention of 2 MB alignment requirements, but it's not
> >> clear to me if that's true or not (looks old)?
> >>
> >> My apologies if it's obvious somewhere, but are there any alignment
> >> requirements?
> >
> > There is a 2MB alignment requirement on PHYS_OFFSET.  You may play
> > tricks with memory reservation if a smaller offset is required, but I'd
> > recommend against that.  Even 2MB is too small an alignment if you want
> > to have CONFIG_ARM_PATCH_PHYS_VIRT=y (the default) which requires a 16MB
> > alignment.
> >
> 
> Thanks, it wasn't clear to me. If it's documented somewhere it would
> be great to know where I overlooked it.

I think this is not documented anywhere.  Only the code would tell.  And 
even then I'm not sure if a 1MB might be a problem or not, although I 
suspect it might.

With CONFIG_ARM_PATCH_PHYS_VIRT=y the requirement is really 16MB and 
that one is documented in the help text for that option.

> > It is very exceptional to see systems where RAM is not aligned to a much
> > bigger boundary.  Why would you need a 2MB alignment?
> 
> Our DDR can be from anywhere from 0 to 1 GB address. Our platform
> can't DMA to the lower 1MB and we have on chip memory (OCM) at 0.
> 
> We are currently starting DDR at 0, taking care of the DMA issue in
> the kernel (kind of a pain) and moving OCM to higher address, but we'd
> like to simplify our memory map so moving the start address of DDR to
> 1MB (or 2) would do that we were thinking.

Could you move it to 16MB?  That would be the simplest and most 
efficient location.


Nicolas