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

Marek Vasut
2021-07-13T14:35:45Z

On 7/13/21 3:47 PM, Tom Rini wrote:
> On Mon, Jul 12, 2021 at 11:01:24AM -0500, Alex G. wrote:
>> On 7/12/21 10:15 AM, 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.
>>
>> It's true that the flat devicetree spec requires an 8-byte alignment, even
>> on 32-bit. The issues here are specific to u-boot.
>>
>> SPL and u-boot have to agree where u-boot's FDT is located. We'll look at
>> two cases:
>> 1) u-boot as a FIT (binary and FDT separately loaded)
>> 2) u-boot with embedded FDT
>>
>> In case (1) SPL must place the FDT at a location where u-boot will find it.
>> The current logic is
>> SPL: fdt = ALIGN_4(u_boot + u_boot_size)
>> u-boot: fdt = ALIGN_4(u_boot + u_boot_size)
>>
>> In case (2), SPL's view of the FDT is not relevant, but instead the build
>> system must place the FDT correctly:
>> build: fdt >> u-boot.bin
>> u-boot: fdt = ALIGN_4(u_boot + u_boot_size)
>>
>> We have 3 places that must agree. A correct and complete patch could change
>> all three, but one has to consider compatibility issues when crossing u-boot
>> and SPL versions.
>>
>> I had proposed in the revert discussion that SPL use r2 or similar mechanism
>> to pass the location of the FDT to u-boot.
>
> I'm not sure that we need to worry too much about mix-and-match
> SPL/U-Boot, but documenting what to go change if you must do it
> somewhere under doc/ would be good. I think we can just switch to
> ALIGN(8) not ALIGN(4) and be done with it?

Remember, there is also falcon boot. And we definitely have to be able
to have old u-boot (SPL) boot new fitImage and vice versa.