Re: [PATCH v4 04/20] arm: fdt: Ensure that an embedded fdt is word-aligned

[Albert ARIBAUD <albert.u.boot@aribaud.net>]

Hi Simon,

Le 19/02/2012 17:23, Simon Glass a écrit :

> Sorry I had that wrong - it's not the link step I am bothered by, it is the
> objcopy step.
>
> What I would like to do is mark the input section, created by objcopy, with
> a particular alignment so that the linker does the right thing. This
> feature appears to be present in objcopy but does not appear to work.
> Perhaps I should debug that first to see what is going on.

(Formally there are two "objcopy steps" in u-boot building. One happens 
after the link step, where from the u-boot ELF binary objcopy derives 
the u-boot.bin pure binary and u-boot.srec S-Record file. I suspect 
you're referring to the other one, which happens before the link stage 
and which turns the binary FDT data into an ELF-linkable object. Correct?)

Maybe one is allowed to specify alignment requirements when making an 
ELF object file from a pure binary with objcopy; but obviously it does 
not work as you want it to, whereas section alignment in linker scripts 
is a tried and proven method.

>>>> So I must be missing something. Can you clarify the scenario that gets
>>>
>>> fixed here?
>>>
>>> The fdt blob is in one of the object files. If during linking the file
>>> before it has a data segment whose size is not a multiple of 4, then the
>>> fdt object can be misaligned in the output.
>>
>>
>> Understood. This example of an alignment failure scenario shows that the
> failure cause is indeed that the FDT is embedded in the .data output
> section at link time.
>>
>>
>>>> Not that I am against the patch, mind you, quite the opposite. I am just
>>>
>>> wondering about the pros and cons of having a separated (and aligned) DTS
>>> data *output* section and placing that section after .code and .data
> rather
>>> than between them.
>>>
>>> Bear in mind that embedding the fdt in the image is really only for
>>> development.
>>
>>
>> Understood, and I think that's one more reason to make sure that the act
> of FDT embedding does not alter .data input section ordering.
>
> Well it is just a data section really.

So are the relocation tables in .rel.dyn and .dynsyms, but that does not 
mean they have to go to the "dot data" output section.

>>> We could have a separate output section if you like. Up to you.
>>
>>
>> I much prefer embedded FDT data to be explicitly mapped to a a dedicated
> .fdt output section in u-boot.lds, because it ensures that aligment
> constaints for .fdt and .data can be controlled independently.
>
> I will take a look at this.

Thanks. This should be fairly easy to do in the linker script by 
inserting an output section (e.g. ".fdt") after .data, aligning the 
start of that section to a word boundary, and explicitly mapping the 
.data section of the FDT object file into it.

> Regards,
> Simon

Amicalement,
-- 
Albert.