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

[Marek Vasut]

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 at 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 ?

> It's just that on 32bit
> platforms 4-but-not-8 byte alignment tends to not be fatal but on 64bit
> platforms it is.

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