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.
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