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

[Alexander Graf]

On 14.04.16 00:51, Tom Rini wrote:
> 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?
> 

It's not terribly complex, but the code in question runs in very early
init (it initializes dram). I guess people usually like their caches off
that early.


Alex