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

[Albert ARIBAUD]

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,
-- 
Albert.