[U-Boot] [PATCH v2] rpi: passthrough of the firmware provided FDT blob

[Stephen Warren]

On 11/07/2016 07:44 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:
>
> if fdt addr ${fdt_addr}
> then
> 	fdt get value bootargs /chosen bootargs
> 	bootz ${kernel_addr_r} - ${fdt_addr}
> fi

FWIW, I'd just hard-code the commands into any script that I wrote, and 
avoid the if test completely. If the FW doesn't pass a DTB, the system 
isn't going to boot correctly anyway, and I'd only use this custom 
script in a situation I know that it'd work.

> 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.
>
> Please note that for this to work the U-Boot binary must be tagged
> with a recent version of the mkknlimg script found in the Rasperry
> Fundation's kernel tree:
>
> <kernel>/scripts/mkknlimg --dtok <uboot>/u-boot.bin /boot/u-boot.bin
>
> [1] https://www.raspberrypi.org/forums//viewtopic.php?f=107&t=134018

I believe the very latest firmware has been fixed to default to DTB 
rather than ATAGs, since essentially nothing uses ATAGs any more. At 
least, Phil mentioned that sometime; I haven't tracked the status of 
that change or tested it.

> diff --git a/board/raspberrypi/rpi/rpi.c b/board/raspberrypi/rpi/rpi.c

> +#ifdef CONFIG_ARM64
> +void save_boot_params(unsigned long dtb)
> +#else
> +void save_boot_params(unsigned long r0, unsigned long r1, unsigned long dtb)
> +#endif
> +{
> +	fw_dtb_pointer = dtb;
> +	save_boot_params_ret();
> +}

I think you need to write that function in assembly. This "function" is 
called very early during U-Boot startup, before any stack pointer is set 
up. You can't guarantee that a function written in C won't attempt to 
use the stack, although admittedly with the current code, Makefile, and 
the Ubuntu compilers, it just happens not to.

Aside from that issue, this version looks fine.