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

[Marek Vasut]

On 7/13/21 6:47 PM, Simon Glass wrote:
> Hi Marek,
> 
> On Tue, 13 Jul 2021 at 08:53, Marek Vasut <marex at denx.de> wrote:
>>
>> On 7/13/21 4:41 PM, Tom Rini wrote:
>>> On Tue, Jul 13, 2021 at 04:35:38PM +0200, Marek Vasut wrote:
>>>> 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 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.
>>>>>>
>>>>>> 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.
>>>
>>> I don't follow you, sorry.  But since you seem to have the best
>>> understanding of where all of the cases something could go wrong here,
>>> can you perhaps post an RFC patch?  That is likely to be clearer than
>>> another long thread here.
>>
>> I don't follow you, sorry. I believe the revert did the right thing and
>> new systems should use mkimage -E when generating fitImages, to avoid
>> the string alignment problem. That is all.
> 
> Using -E should be optional and things really should work without it.

See the DTSpec, I don't think that is possible unless you relocate 
fitImage components, and if you want fast boot time esp. in SPL, that is 
not good.