Messages in this thread | | | Date | Tue, 28 Apr 2009 14:10:28 +1000 | From | David Gibson <> | Subject | Re: microblaze: Statically linking device tree blobs into the kernel |
| |
On Mon, Apr 27, 2009 at 02:24:42PM +1000, John Williams wrote: > To MicroBlazers and other interested parties: > > Currently the MicroBlaze kernel boot-time ABI requires r7 to point to > a valid DTB, whereupon in early kernel setup the DTB is copied to a > statically allocated 16k memory region inside the kernel. From there > it is later queried by the platform startup code. > > For simple boot scenarios the ability to statically bind a DTB into > the kernel image would clearly be useful. In PPC land, this is > achieved through the simpleboot bootloader that lives in > arch/powerpc/boot. The DTB becomes part of the simpleboot payload, > and is passed to the kernel through the normal means. > > I'm not convinced duplicating this for MicroBlaze is a good idea, I > think it would be overkill. However, the make syntax that PPC uses to > achieve DTB binding is quite nice: > > $ make simpleImage.<board> > > which binds arch/powerpc/boot/dts/<board>.dts into the boot payload. > > My feeling is that we should make use of the fact that the DTB region > is statically allocated in the MicroBlaze kernel anyway. From > arch/microblaze/kernel/vmlinux.lds.S: > > . = ALIGN (4) ; > _fdt_start = . ; /* place for fdt blob */ > . = . + 0x4000; > _fdt_end = . ; > > and in head.S, the DTB at r7 is copied into place at _fdt_start > > /* save fdt to kernel location */ > /* r7 stores pointer to fdt blob */ > beqi r7, no_fdt_arg > or r11, r0, r0 /* incremment */ > ori r4, r0, TOPHYS(_fdt_start) /* save bram context */ > ori r3, r0, (0x4000 - 4) > _copy_fdt: > <simple copy loop> > no_fdt_arg: > > Since this memory is already allocated in the kernel image but is > normally just zeros, to bind a DTB to the kernel we could just store > it in-situ. This way, if a non-zero r7 is passed in at boot time, > head.S will naturally overwrite (and thus override) the "default" DTB > that was inside the kernel image. > > What I'm not so sure about is how best to achieve this in the kbuild > sequence. I see two options: > > (a) use .incbin in a .S file, similar to how the initramfs gets > linked in in /usr/Makefile etc, or
Or you could build the .dts directly into a .S file (which dtc supports) and #include or link that in. It's possible the dtc asm output mode would need some work, but that's easily enough done.
-- David Gibson | I'll have my music baroque, and my code david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_ | _way_ _around_! http://www.ozlabs.org/~dgibson
| |