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: >> >> 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. 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. >> 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. Not in this form. Hard coding a fixed size area in the boot image to contain the FDT (aka DTB) is a non-starter. 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: >> >> ============ >> 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
