On 11/08/2016 10:47 AM, Cédric Schieli wrote:
Raspberry firmware used to pass a FDT blob at a fixed address (0x100), but this is not true anymore. The address now depends on both the memory size and the blob size [1].
If one wants to passthrough this FDT blob to the kernel, the most reliable way is to save its address from the r2/x0 register in the U-Boot entry point and expose it in a environment variable for further processing.
This patch just does this:
- save the provided address in the global variable fw_dtb_pointer
- expose it in ${fdt_addr} if it points to a a valid FDT blob
There are many different ways to use it. One can, for example, use the following script which will extract from the tree the command line built by the firmware, then hand over the blob to a previously loaded kernel:
fdt addr ${fdt_addr} fdt get value bootargs /chosen bootargs bootz ${kernel_addr_r} - ${fdt_addr}
Alternatively, users relying on sysboot/pxe can simply omit any FDT statement in their extlinux.conf file, U-Boot will automagically pick ${fdt_addr} and pass it to the kernel.
[1] https://www.raspberrypi.org/forums//viewtopic.php?f=107&t=134018
This looks fine for the ARMv7 and ARMv8 Pis, but I forgot to mention last time that it causes a build failure for the ARMv6-based original Pis:
board/raspberrypi/rpi/built-in.o: In function `save_boot_params': board/raspberrypi/rpi/lowlevel_init.S:36: undefined reference to `save_boot_params_ret'
That's because arch/arm/cpu/arm1176/start.S doesn't implement the save_boot_params hook; you can probably solve this quite easily by taking commit 0e2b5350d952 "ARM: Add save_boot_params for ARMv8" and porting that to the ARM1176 code first.
Assuming that fix is sent and applied first e.g. in a series with this patch, then this patch, Acked-by: Stephen Warren swarren@wwwdotorg.org