linux-kernel - RFC: Copying Device Tree File into reserved area of VMLINUX before deployment

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Date:   Sat, 18 Nov 2017 18:59:17 +0100
From:   Ulf Samuelsson <linux-kernel@...gii.com>
To:     LKML <linux-kernel@...r.kernel.org>
Subject: RFC: Copying Device Tree File into reserved area of VMLINUX before
 deployment

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