Re: [PATCH] spl: Align device tree blob address at 8-byte boundary

Tom Rini
2021-07-12T15:43:28Z

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On Mon, Jul 12, 2021 at 05:38:33PM +0200, Marek Vasut wrote:
> On 7/12/21 5:15 PM, Tom Rini wrote:
> > On Mon, Jul 12, 2021 at 01:36:14PM +0800, Bin Meng wrote:
> > > On Mon, Jul 12, 2021 at 1:21 PM Reuben Dowle <reuben.dowle@4rf.com> wrote:
> > > >
> > > > I submitted an almost identical patch. See https://github.com/u-boot/u-boot/commit/eb39d8ba5f0d1468b01b89a2a464d18612d3ea76
> > > >
> > > > This patch eventually had to be reverted (https://github.com/u-boot/u-boot/commit/5675ed7cb645f5ec13958726992daeeed16fd114), because it was causing issues on some platforms that had FIT on 32 bit boundary. However I continue to use it in production code, as without it the boot on my platform aborts.
> > > >
> > > > I don't have time to investigate why this was happening, but you need to check this code won't just cause exactly the same faults.
> > >
> > > Thanks for your information.
> > >
> > > +Marek who did the revert
> > >
> > > The revert commit message says:
> > >
> > > "The commit breaks booting of fitImage by SPL, the system simply
> > > hangs. This is because on arm32, the fitImage and all of its content
> > > can be aligned to 4 bytes and U-Boot expects just that."
> > >
> > > I don't understand this. If an address is aligned to 8, it is already
> > > aligned to 4, so how did this commit make the system hang on arm32?
> >
> > I think this had something to do with embedding contents somewhere in
> > the image? There is a thread on the ML from then but I don't know how
> > informative it will end up being.
>
> If I recall this correctly, DT node alignment is 4 byte and that is what DTC
> emits. If you have fitImage with embedded data, you basically end up with
>
> / {
> prop1 = "string1";
> prop2 = "string2";
> };
>
> where the "string2" is aligned to 4 bytes. And that is what U-Boot expects
> when it tries to access those data in-place in SPL.
>
> The problem with the reverted patch was that it made U-Boot assume the
> alignment is 8 bytes, and that actually works only if you use fitImage with
> external data (mkimage -E), but with embedded data (mkimage default) not so
> much. That caused off-by-4 error in some cases and that made the SPL hang.
>
> > > Note, as I indicated in this patch, now with libfdt 1.6.1, the
> > > alignment to 8 byte is a must-have. So we have to do such alignment
> > > anyway.
> > >
> > > @Tom may fill in why libfdt commit commit 5e735860c478 ("libfdt: Check
> > > for 8-byte address alignment in fdt_ro_probe_()") was made to have the
> > > 8-byte alignment requirement.
> >
> > Note that it's not so much since libfdt 1.6.1 but that since always the
> > device tree has required 8 byte alignment.
>
> DT alignment was always 4 byte , no ?

I'm pretty sure, no, 8 byte base alignment is a pretty much always
thing. I don't have a reference handy but I also know I couldn't have
convinced dgibson to add the check otherwise.

--
Tom

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