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

[Alex G]

On 7/13/21 3:35 PM, Marek Vasut wrote:
> On 7/13/21 8:11 PM, Tom Rini wrote:
>> On Tue, Jul 13, 2021 at 07:50:49PM +0200, Marek Vasut wrote:
>>> 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.
>>
>> This is why I've asked you to make up some patch to perhaps highlight
>> the problem.  Ensuring that the device tree, which is small, is also
>> 8-byte aligned, shouldn't be a big problem nor performance hit.  I'm not
>> sure where the problem case is that isn't "user put things they control
>> in a bad spot, fail and tell them why" but I could just be missing a
>> case.
> 
> The fail case is this:
> - you update SPL with this 8 byte alignment change
> - you have older kernel fitImage with embedded DT for falcon mode
> - system no longer boots because there is off-by-4 error in the DT
>    address passed to the kernel

I'm not sure how falcon mode would break the kernel. It passes to the 
kernel the load address of the fdt. The concern here is loading u-boot.


> I hope this is clear now.