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

[Tom Rini <trini@konsulko.com>]

On Thu, Apr 14, 2016 at 12:42:41AM +0200, Alexander Graf wrote:
> 
> 
> On 14.04.16 00:34, Tom Rini wrote:
> > 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?
> 
> No, it does the same. To handle not naturally memory accesses you need
> to have dcache enabled and to enable the dcache the MMU needs to be
> turned on. ARMv7 is the same for all I'm aware of.

Ah, OK, so we just need to get the MMU on for ARMv8, and that's more
complex?  Or am I just flat out missing something?

-- 
Tom
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