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SubjectRFC: Copying Device Tree File into reserved area of VMLINUX before deployment
Date
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.

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.

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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