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Subject: Re: RFC: Copying Device Tree File into reserved area of VMLINUX
 before deployment
To:     Rob Herring <robh+dt@kernel.org>,
        Frank Rowand <frowand.list@gmail.com>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        "devicetree@vger.kernel.org" <devicetree@vger.kernel.org>
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From:   Ulf Samuelsson <linux-kernel@emagii.com>
Organization: eMagii
Message-ID: <4ce82f7e-a785-c152-3c48-8700534881d1@emagii.com>
Date:   Tue, 21 Nov 2017 21:03:09 +0100
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On 2017-11-21 18:09, Rob Herring wrote:
> On Mon, Nov 20, 2017 at 3:39 PM, Frank Rowand <frowand.list@gmail.com> wrote:
>> Hi Ulf, Rob,
>>
>> On 11/20/17 15:19, Ulf Samuelsson wrote:
>>>
>>>
>>> On 2017-11-20 05:32, Frank Rowand wrote:
>>>> Hi Ulf,
>>>>
>>>>
>>>> On 11/19/17 23:23, Frank Rowand wrote:
>>>>> adding devicetree list, devicetree maintainers
>>>>>
>>>>> On 11/18/17 12:59, Ulf Samuelsson wrote:
>>>>>> I noticed when checking out the OpenWRT support for the board that they have a method to avoid having to pass the device tree address to the kernel, and can thus boot device tree based kernels with U-boots that
>>>>>> does not support device trees.
>>>>>>
>>>>>> Is this something that would be considered useful for including in mainstream:
>>>>>>
>>>>>> BACKGROUND:
>>>>>> Trying to load a yocto kernel into a MIPS target (MT7620A based),
>>>>>> and the U-Boot is more than stupid.
>>>>>> Does not support the "run" command as an example.
>>>>>> They modified the U-Boot MAGIC Word to complicate things.
>>>>>> The U-Boot is not configured to use device tree files.
>>>>>> The board runs a 2.6 kernel right now.
>>>>>>
>>>>>> Several attempts by me a and others to rebuild U-Boot according to
>>>>>> the H/W vendors source code and build instructions results in a
>>>>>> bricked unit. Bricked units cannot be recovered.
>>>>
>>>> Hopefully you have brought this to the attention of the vendor.  U-Boot
>>>> is GPL v2 (or in some ways possibly GPL v2 or later), so if you can not
>>>> build U-Boot that is equivalent to the binary U-Boot they shipped, the
>>>> vendor may want to ensure that they are shipping the proper source and
>>>> build instructions.
>>>>
>>>
>>> I am not the one in contact with the H/W vendor.
>>> The U-boot is pretty old, and from comments from those
>>> in contact with them, the U-Boot knowledge at the H/W vendor
>>> is minimal at best.
>>> It might even be that they program an U-boot where the upgrade of the U-boot is broken...
>>>
>>>
>>>>
>>>>>> Not my choice of H/W, so I cannot change it.
>>>>>>
>>>>>>
>>>>>> ===================================================================
>>>>>> OPENWRT:
>>>>>> I noticed when checking out the OpenWRT support for the board that
>>>>>> they have a method to avoid having to pass the device tree address
>>>>>> to the kernel, and can thus boot device tree based kernels with
>>>>>> U-boots that does not support device trees.
>>>>>>
>>>>>> What they do is to reserve 16 kB of kernel space, and tag it with
>>>>>> an ASCII string "OWRTDTB:". After the kernel and dtb is built, a
>>>>>> utility "patch-dtb" will update the vmlinux binary, copying in the
>>>>>> device tree file.
>>>>>>
>>>>>> ===================================================================
>>>>>> It would be useful to me, and I could of course patch the
>>>>>> mainstream kernel, but first I would like to check if this is of
>>>>>> interest for mainstream.
>>>>
>>>> Not in this form.  Hard coding a fixed size area in the boot image
>>>> to contain the FDT (aka DTB) is a non-starter.
>>>
>>> OK, Is it the fixed size, which is a problem?
>>
>> Yes, it is the fixed size which is a problem.
>>
>>> Is generally combining an image with a DTB into a single file also a non-starter?
>>
>> Can you jump in here Rob?  My understanding is that CONFIG_ARM_APPENDED_DTB,
>> which is the ARM based solution that Mark mentioned, was envisioned as a
>> temporary stop gap until boot loaders could add devicetree support.  I don't
>> know if there is a desire to limit this approach or to remove it in the
>> future.
> 
> Yes, but I doubt we'll ever remove it. Most android devices use it (or
> the multiple appended dtb stuff Qcom did) and there's out of tree
> patches to do appended dtb on arm64. However, arm64 is a done a bit
> differently in that the bootloader has to find the dtb rather than the
> kernel (or really the decompressor). The purpose for arm64 is people
> like the single kernel+dtb image, not legacy bootloader support as DT
> support was there from day 1.
> 
> Another option is to do what's called an impedance matcher like this[1].
> 
>> I'm not sure why this feature should not be permanently supported.  I'm being
>> cautious, just in case I'm overlooking or missing an important issue, thus
>> asking for Rob's input.  I do know that this feature does not advance the
>> desires of people who want a single kernel (single boot image?) that runs on
>> many different systems, instead of a boot image that is unique to each
>> target platform.  But I don't see why that desire precludes also having
>> an option to have a target specific boot image.
> 
> MIPS already supports built-in dtbs which is an in-kernel way to get
> the dtb rather than the decompressor. The addition in this case is
> just putting in a dummy dtb and putting in the real dtb later. I'm not
> sure what MIPS image header looks like, but seems like it should be
> possible to have the "built-in dtb" point to an appended dtb and just
> update sizes in the image file. Then you don't have the fixed size.
> 
> But now that I finished writing this, I see there is already appended
> DTB support in MIPS.

I just managed to complete a build using yocto where I made the 
following changes.

I build the dtbs before linking the uImage.

I add some configuration variables, which define
* if the dtb should be included
* which dtb file should be included

uImage always gziped

Checking the resulting uImage, shows that the ".appended_dtb" section 
now contains the dtb file.

I have not loaded it into a board yet, but the default init seems to 
check if there is a U-boot header in the ".appended_dtb" section.

My bootloader has a custom MAGIC so loading still won't work.


> 
> Rob
> 
> [1] https://github.com/zonque/pxa-impedance-matcher
> 

-- 
Best Regards
Ulf Samuelsson

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