[U-Boot] [PATCH v3 2/2] arm: add initial support for Amlogic Meson and ODROID-C2

[Tom Rini]

On Wed, Apr 13, 2016 at 11:38:51PM +0200, Alexander Graf wrote:
> On 13.04.16 13:52, Marek Vasut wrote:
> > On 04/13/2016 01:22 PM, Beniamino Galvani wrote:
> >> On Wed, Apr 13, 2016 at 12:26:43AM +0200, Marek Vasut wrote:
> >>>> So, after some investigation, the reason is that the code runs when
> >>>> caches are still disabled and thus all the memory is treated as
> >>>> Device-nGnRnE, requiring aligned accesses.
> >>>
> >>> You mean 8-byte aligned accesses, correct ?
> >>
> >> Yes.
> >>
> >>>> The return value of
> >>>> fdt_getprop() is guaranteed to be aligned to a 4 byte boundary (but
> >>>> not 8)
> >>>
> >>> The return value of fdt_getprop() is a pointer, thus 8byte long on
> >>> aarch64 and thus aligned to 8 bytes on the stack unless there is
> >>> some real problem.
> >>
> >> Right, however I'm not talking about the alignment of the pointer on
> >> the stack, but about the value of the pointer, which depends on the
> >> offset inside the device tree blob of the property. If I use this:
> >>
> >>     val = fdt_getprop(gd->fdt_blob, offset, "reg", &len)
> >>     gd->ram_size = fdt64_to_cpu(*(fdt64_t *)val)
> >>
> >> when the CPU tries to dereference val (which is something like
> >> 0x00000000010429e4) an alignment fault is generated for the reason
> >> stated above.
> > 
> > Oh, now it's clear what the problem is, thanks. But then, we'd need such
> > fixups all over the place I'm afraid. Isn't there some way to enable
> > support for "unaligned" accesses instead?
> 
> Yes, and it's called "enable the MMU". You could probably do this in the
> early dram init stage already, but I'm not sure it's worth it. The NXP
> people are the only ones doing it really early today FWIW.
> 
> Also, if you find it more readable, you could just use
> get_unaligned_be64(). It gets you down to byte accesses rather than
> 32bit fetches, but the function name makes it pretty obvious what we're
> looking at.

OK, now I'm starting to get nightmares back to our last unaligned access
discussion.  Is ARMv8 doing something radically different from ARMv7
here, wrt unaligned accesses?

-- 
Tom
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