[U-Boot] FDT placement in OpenSBI + U-Boot + Linux kernel issue for RISC-V

[Atish Patra]

Hi All,

I noticed following issues around U-Boot fdt location in Unleashed and
Qemu virt machine.

OpenSBI copies the FDT to following addresses for respective platforms

Qemu: 		0x82200000
Unleashed:	0x88000000


As CONFIG_PRIOR_STAGE is set for both platforms, fdt is first copied to
gd->fdt_blob and then gets relocated to a different address(gd-
>new_fdt) in function reloc_fdt.

As a result, FDT is present at two locations.

1. 0x88000000(Unleashed)/0x82200000(Qemu) : OpenSBI copied FDT to this
address.
2. gd->new_fdt: U-Boot relocated the fdt to this address.
fdtcontroladdr will also point to this address.

However, commit ac12c6190927 (riscv: set CONFIG_SYS_BOOTM_LEN to
SZ_64M) in U-boot changed Qemu config so that fdt_addr_r points to
0x88000000 which is wrong as nobody copies fdt to above address in
Qemu.
Also, 5.3-rc+ kernels overwrite the address pointed by fdtcontroladdr. 

As a result, fdtcontroladdr won't work on any platform and fdt_addr_r
will work only for Unleashed but not for Qemu. 

I am not sure what should be the ideal solution to avoid these kind of
fdt placement issues in future. Here are the few possible ones.

1. Change the FDT_JUMP_ADDR in OpenSBI to 0x88000000(RAM+128MB) for
Qemu as well. This won't work as Qemu copies initrd to that address. I
guess best next option is to copy to 0x84000000(RAM+64MB) and U-boot
config for Qemu accordingly.

2. Change fdt_addr_r to 0x82200000 in Qemu. Update documentation to use
fdt only from fdt_addr_r not fdtcontroladdr.

@david: What was the reason behind changing it for Qemu ?

3. Fix gd->new_fdt computation. This may affect every board which is
not a very good idea either.

4. Mandate loading fdt only from tftp or sdcard which is the safest
option and will avoid these kind of complications. However, I think a
default booting method without tftp server should at least work. Let me
know if that is not a sane request. In that case, we should update
documentation to clearly say that only tftpboot or sdcard loading
method works.


-- 
Regards,
Atish