Re: RFC: Copying Device Tree File into reserved area of VMLINUX before deployment

From: Ulf Samuelsson
Date: Tue Nov 21 2017 - 07:02:19 EST

On 2017-11-21 07:19, Ulf Samuelsson wrote:

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:

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.

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:ÂÂÂ

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:ÂÂÂ <target dtb>
ÂÂÂÂmv <target dtb>ÂÂ image_dtb
ÂÂÂÂld -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.

Just checked the kernel source, and it appears that the discussion is somewhat redundant, since the support is already in the linux kernel
for some MIPS boards

bool "vmlinux"
With this option, the boot code will look for a device tree binary
DTB) included in the vmlinux ELF section .appended_dtb. By default
it is empty and the DTB can be appended using binutils command

objcopy --update-section .appended_dtb=<filename>.dtb vmlinux

This is meant as a backward compatiblity convenience for those
systems with a bootloader that can't be upgraded to accommodate
the documented boot protocol using a device tree.

const char __section(.appended_dtb) __appended_dtb[0x100000];


if (!fdt_check_header(&__appended_dtb))
dtb = (void *)&__appended_dtb;

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

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.


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:

config MIPS


ÂÂÂÂÂÂboolÂÂÂ "Allocated space for DTB within image

ÂÂÂÂÂÂintÂÂÂ "DTB space (kB)

ÂÂÂÂÂÂstringÂÂÂ "DTB space tag"

Some Makefile
obj-$(CONFIG_INCLUDE_DTB) += image_dtb.o



ÂÂÂÂÂÂif (<conditions to boot from dtb_space>)
ÂÂÂÂÂÂÂÂÂ __dt_setup_arch(__dtb_start);
ÂÂÂÂÂÂÂÂÂ __dt_setup_arch(&__image_dtb);

I imagine that if the support is enabled for a target, it should
be possible to override it with a CMDLINE argument
ÂÂÂÂÂ ÂÂÂÂ They do something similar for the CMDLINE; copying it into the vmlinux, to allow a smaller boot

Best Regards
Ulf Samuelsson