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.