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

[Frank Rowand <frowand.list@gmail.com>]

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
> 
> 
>