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

["Alex G." <mr.nuke.me@gmail.com>]

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.

Alex