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Reply-To: linux-kernel@emagii.com
Subject: Re: RFC: Copying Device Tree File into reserved area of VMLINUX
 before deployment
From:   Ulf Samuelsson <linux-kernel@emagii.com>
To:     Frank Rowand <frowand.list@gmail.com>,
        LKML <linux-kernel@vger.kernel.org>,
        "devicetree@vger.kernel.org" <devicetree@vger.kernel.org>,
        Rob Herring <robh+dt@kernel.org>
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Date:   Tue, 21 Nov 2017 07:19:22 +0100
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On 2017-11-21 00:09, Ulf Samuelsson wrote:
> 
> 
> On 2017-11-20 22:39, Frank Rowand 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.
> 
> The size can of course be changed, by setting the size configuration 
> option (DTB_SIZE).
> OpenWRT does not support that, but I think it needs to be there for a 
> generic option (but You have to recompile the kernel to increase the size).
> 
> One problem is that you normally compile and link the kernel before you
> compile the dtbs, so you do not know what size is until afterwards.
> 
Found this link:	https://csl.name/post/embedding-binary-data/

=======================================
...
Let's say you have an image target.dtb and want to embed it into your 
application. You can create an object file with


image_dtb.o:	<target dtb>
	mv <target dtb>   image_dtb
	ld -r -b binary image_dtb -o image_dtb.o

The object file will have three symbols in it,

$ nm cat.o
0000000000000512 D _binary_image_dtb_end
0000000000000512 A _binary_image_dtb_size
0000000000000000 D _binary_image_dtb_start
=======================================

This assumes that the dtbs are built before the kernel is linked.
The copy step is neccessary, since the generated names are
taken from the name of the "in file".
(Would have been better, if they used the "out file")

Otherwise you can create an assembler file which "incbin's" the dtb file.


> 
>>
>>> 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.
>>
>> 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.
> The main reason to keep it is when you are really constrained for memory.
> The U-Boot on the board is 96 kB, which is just a fraction of a more 
> normal U-Boot.
> Also, the u-boot is old.
> 
> 
>>
>> -Frank
>>
>>
>>>>
>>>> And again, I would first approach the H/W vendor before trying to
>>>> come up with a work around like this.
>>>>
>>>>
>>>>>> I envisage the support would look something like:
>>>>>>
>>>>>> ============
>>>>>> Kconfig.
>>>>>> config MIPS
>>>>>>       select    HAVE_IMAGE_DTB
>>>>>>
>>>>>> config    HAVE_IMAGE_DTB
>>>>>>       bool
>>>>>>
>>>>>> if HAVE_IMAGE_DTB
>>>>>> config     IMAGE_DTB
>>>>>>       bool    "Allocated space for DTB within image
>>>>>>
>>>>>> config    DTB_SIZE
>>>>>>       int    "DTB space (kB)
>>>>>>
>>>>>> config    DTB_TAG
>>>>>>       string    "DTB space tag"
>>>>>>       default    "OWRTDTB:"
>>>>>> endif
>>>>>>
>>>>>> ============
>>>>>> Some Makefile
>>>>>> obj-$(CONFIG_INCLUDE_DTB) += image_dtb.o
>>>>>>
>>>>>> ============
>>>>>> image_dtb.S:
>>>>>>       .text
>>>>>>       .align    5
>>>>>>       .ascii    CONFIG_DTB_TAG
>>>>>>       EXPORT(__image_dtb)
>>>>>>       .fill    DTB_SIZE * 1024
>>>>>>
>>>>>> ===================
>>>>>> arch/mips/xxx/of.c:
>>>>>>
>>>>>> #if    defined(CONFIG_IMAGE_DTB)
>>>>>>       if (<conditions to boot from dtb_space>)
>>>>>>           __dt_setup_arch(__dtb_start);
>>>>>>       else
>>>>>>           __dt_setup_arch(&__image_dtb);
>>>>>> #else
>>>>>>       __dt_setup_arch(__dtb_start);
>>>>>> #endif
>>>>>>
>>>>>> I imagine that if the support is enabled for a target, it should
>>>>>> be possible to override it with a CMDLINE argument
>>>>>>            They do something similar for the CMDLINE; copying it 
>>>>>> into the vmlinux, to allow a smaller boot
>>>
>>
> 

-- 
Best Regards
Ulf Samuelsson

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