[U-Boot] [PATCH] arm: Allow u-boot to run from offset base address

[Steve Rae]

On 14-06-09 03:23 AM, Albert ARIBAUD wrote:
> Hi Darwin,
>
> On Mon, 2 Jun 2014 17:37:25 -0700, Darwin Rambo <drambo at broadcom.com>
> wrote:
>
>>
>>
>> On 14-06-02 12:26 AM, Albert ARIBAUD wrote:
>>> Hi Darwin,
>>>
>>> On Mon, 26 May 2014 09:11:35 -0700, Darwin Rambo <drambo at broadcom.com>
>>> wrote:
>>>
>>>> Hi Albert,
>>>>
>>>> The previous stage bootloader (which I had no control over) wanted it's
>>>> header to be aligned to a 512 byte MMC block boundary, presumably since
>>>> this allowed DMA operations without copy/shifting. At the same time, I
>>>> didn't want to hack a header into start.S because I didn't want to carry
>>>> another downstream patch. So I investigated if I could shift u-boot's
>>>> base address as a feature that would allow an aligned header to be used
>>>> without the start.S patch.
>>>>
>>>> I know that a custom header patch to start.S would work, and that a
>>>> header plus padding will also work. But I found out that you can align
>>>> the base on certain smaller offsets if you keep the relocation offset at
>>>> nice boundaries like 0x1000 and if the relocation offset is a multiple
>>>> of the maximum alignment requirements of the image.
>>>>
>>>> The original patch I submitted didn't handle an end condition properly,
>>>> was ARM64-specific (wasn't tested on other architectures), and because
>>>> the patch was NAK'd, I didn't bother to submit a v2 patch and consider
>>>> the idea to be dead. I'm happy to abandon the patch. I hope this helps.
>>>
>>> Thanks.
>>>
>>> If I understand correctly, your target has a requirement for storing
>>> the image on a 512-byte boundary. But how does this affect the loading
>>> of the image into RAM, where the requirement is only that the vectors
>>> table be 32-bytes aligned? I mean, if you store the image in MMC at
>>> offset 0x200 (thus satisfying the 512-byte boundary requirement) and
>>> load it to, say, offset 0x10020 in RAM, how is it a problem for
>>> your target?
>>>
>>> If my example above inadequately represents the issue, then can you
>>> please provide a similar but adequate example, a failure case scenario,
>>> so that I can hve a correct understanding of the problem?
>>
>> Hi Albert,
>>
>> The constraints I have that I can't change, are that
>> - the 32 byte header is postprocessed and prepended to the image after
>> the build is complete
>> - the header is at a 512 byte alignment in MMC
>> - the header and image are copied to SDRAM to an alignment like
>> 0x88000000. Thus the u-boot image is linked at and starts at 0x88000020.
>> - the vectors need to be 0x800 aligned for armv8 (.align 11 directive)
>
> So far, so good -- I understand that the link-time location of the
> vectors table is incorrect.
>
>> So the failure case is that when the relocation happens, it relocates to
>> a 0x1000 alignment, say something like 0xffffa000. The relocation offset
>> is not a multiple of 0x1000 (0xffffa000 - 0x88000020) and the relocation
>> fails.
>
> What does "relocation fails" mean exactly, i.e., where and how exactly
> does the relocation code behave differently from expected? I'm asking
> because I don't understand why the relocation offset should be a
> multiple of 0x1000.
>
>> Adjusting the relocation offset to a multiple of 0x1000 (by
>> making the relocation address end in 0xNNNNN020) fixes the issues and
>> allows u-boot to relocate and run from this address without failing. I
>> hope this helps explain it a bit better.
>
> I do understand, however, that if the relocation offset must indeed be a
> multiple of 0x1000, then obviously the vectors table will end up as
> misaligned as it was before relocation.
>
> Also, personally I would like it if the vectors table was always
> aligned as it should, and there are at least three other boards which
> require a prefix/header before their vectors table, as Masahiro (cc:)
> indicated recently, so that makes the problem a generic one: how to
> properly integrate a header in-image (as opposed to an out-of-image
> one, which is just a matter of doing a 'cat', so to speak.
>
> Therefore I'd like a generic solution to this, where the header is
> prepended *and* aligned properly without breaking the start symbol
> alignment constraints. This /might/ be possible by cleverly adding
> a '.header' or '.signature' section to the linker script, possibly
> doing a two-stage link; but this should not require the source code to
> contain ad hoc relocation tricks.
>
> Let me tinker with it in the next few days; I'll try and come up with a
> clean and generic solution to this "in-code header" question.
>
> Thanks again for your explanation!
>
>> Best regards,
>> Darwin
>
> Amicalement,
>

Perhaps an oversimplified example of the current code would help to 
explain this better:

scenario #1:
CONFIG_SYS_TEXT_BASE		0x88000000
vectors:	.align 11	/* exception vectors need to be on a 0x800 byte 
boundary */
compile/linker produces (before relocation):
_start	symbol is at		0x88000000
vectors	symbol is at		0x88000800
the relocation offset is:	0x77f9b000
therefore, after relocation:
_start	symbol is at		0xfff9b000 (0x88000000+0xfff9b000)
vectors	symbol is at		0xfff9b800 (0x88000800+0x77f9b000)

scenario #2:
CONFIG_SYS_TEXT_BASE		0x88000020
vectors:	.align 11	/* exception vectors need to be on a 0x800 byte 
boundary */
compiler/linker produces (before relocation):
_start	symbol is at		0x88000020
vectors	symbol is at		0x88000800
the relocation offset is:	0x77f9afe0
therefore, after relocation:
_start	symbol is at		0xfff9b000 (0x88000020+0x77f9afe0)
vectors	symbol is at		0xfff9b7e0 (0x88000800+0x77f9afe0)

Note that in scenario #2, after relocation, the vectors are not on a 
0x800 byte boundary anymore.

Thanks, Steve