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.
>
> 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
>
>
>
