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

[Atish Patra]

On Mon, 2019-09-09 at 13:22 +0300, David Abdurachmanov wrote:
> On Mon, Sep 9, 2019 at 8:05 AM Anup Patel <Anup.Patel at wdc.com> wrote:
> > Hi,
> > 
> > I think keeping FDT placement in-sync between U-Boot and OpenSBI
> > across platforms is going to be painful.
> > 
> > I suggest that for all platforms U-Boot explicitly load FDT from
> > somewhere
> > so:
> > 1. U-Boot ${fdt_addr_r} default value will be recommended location
> > of FDT
> > 2. U-Boot ${fdtcontroladdr} will always point to copy of FDT passed
> > by OpenSBI
> > 3. To forward FDT passed by OpenSBI to Linux, U-Boot users can
> > always explicitly
> > copy FDT from ${fdtcontroladdr} to ${fdt_addr_r} using U-Boot copy
> > command
> > 
> > I also suspect that in-future for certain platforms FDT passed to
> > U-Boot and
> > FDT passed to Linux might be little different due to U-Boot
> > specific changes
> > in DTS.
> > 
> > Thoughts ??
> 
> Do not forget PXE and EXTLINUX boot options. These options must
> always be able to override DTB from previous stages. See below what
> PXE/EXT use. For Fedora/RISCV we end up in scenario #2 and thus
> fdt_addr needs to be set if DTB is coming from somewhere in the
> firmware.
> This is why we had CONFIG_PREBOOT to set it.
> 
> [..]
> * Scenario 1: If fdt_addr_r specified and "fdt" label is defined in
> * pxe file, retrieve fdt blob from server. Pass fdt_addr_r to bootm,
> * and adjust argc appropriately.
> *
> * Scenario 2: If there is an fdt_addr specified, pass it along to
> * bootm, and adjust argc appropriately.
> *

How about if we do this in PREBOOT ? 

1. copy fdt from fdtcontroladdr to fdt_addr_r
2. setenv fdt_addr $fdt_addr_r

In this way, U-Boot will not have any direct dependancies on OpenSBI.
As long as U-Boot is configured with a fdt_addr_r, it should work. It
will be still valid for loading fdt from tftp server to fdt_addr_r as
well.

> * Scenario 3: fdt blob is not available.
> [..]
> 
> david
> 
> > Regards,
> > Anup
> > 
> > > -----Original Message-----
> > > From: Atish Patra
> > > Sent: Sunday, September 8, 2019 5:40 PM
> > > To: david.abdurachmanov at sifive.com; Alistair Francis
> > > <Alistair.Francis at wdc.com>; Anup Patel <Anup.Patel at wdc.com>
> > > Cc: u-boot at lists.denx.de; opensbi at lists.infradead.org
> > > Subject: FDT placement in OpenSBI + U-Boot + Linux kernel issue
> > > for RISC-V
> > > 
> > > 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
> > _______________________________________________
> > opensbi mailing list
> > opensbi at lists.infradead.org
> > http://lists.infradead.org/mailman/listinfo/opensbi

-- 
Regards,
Atish