Re: [PATCH 1/3] treewide: Remove OF_PRIOR_STAGE from RISC-V boards

[Ilias Apalodimas <ilias.apalodimas@linaro.org>]

> > > > > -       if (IS_ENABLED(CONFIG_OF_SEPARATE)) {
> > > > > -               if (gd->arch.firmware_fdt_addr)
> > > > > -                       return (ulong *)gd->arch.firmware_fdt_addr;
> > > > > -               else
> > > > > -                       return (ulong *)&_end;
> > > > > -       }
> > > > > +       if (gd->arch.firmware_fdt_addr)
> > > > > +               return (void *)gd->arch.firmware_fdt_addr;
> > > > > +       else
> > > > > +               return (void *)&_end;
> > > > >  }
> > > >
> > > > I was wondering if we need to check CONFIG_OF_BOARD here? I'm not sure
> > > > whether we should distinguish the value of a1 register which is
> > > > meaningless. It means that if we don't expect the device tree to be
> > > > passed by prior stage, then the a1 register might be a trash value at
> > > > the beginning, so it would still return the arch.firmware_fdt_addr
> > > > here, rather than _end.
> > >
> > > I thought about it as well.  Those boards were configured up to now with
> > > 'CONFIG_OF_SEPARATE'.  Which means we are looking at an existing issue?
> > > IOW the device tree was passed as part of U-Boot,  which would mean a1 would
> > > have had thrash as well.  Maybe a1 always has a valid DT on those boards
> > > so we never noticed?
> > >
> > >
> > > > And do you think that we should enable the
> > > > CONFIG_OF_BOARD for unmatched and unleashed? Because it seems to me
> > > > that we actually pass the device tree by prior stage (i.e. OpenSBI).
> > >
> > > Yes in that case what you request makes sense for unmatched/unleashed.
> > > Return gd->arch.firmware_fdt_addr in OF_BOARD is selected otherwise return
> > > _end (instead of the current check).
> > > If that sounds good to you I'll send a v2
> >
> > Looking a bit more at it...
> > Apparently those boards boot from SPL.  So it's SPL->OpenSBI->U-Boot.
> > By having the config as OF_SEPARATE the *U-Boot* DTB is used. SPL passes it to
> > OpenSBI and OpenSBI passes it on a1 to U-Boot proper.  That's why the register
> > reading works for that config.
> >
> > In that case the pre-existing code is 'wrong' as well,  since the DTB is
> > not at _end,  but the bogus path is never taken...
> > (check the __weak board_fdt_blob_setup for details).
> >
> 
> If I remember correctly, the SPL would calculate the size of u-boot
> proper, and then put the DTB at the end of  u-boot proper, so the DTB
> would fortuitously be put at the _end location.

I haven't yet seen the creation part,  but looking into the default
board_fdt_blob_setup() the location seems to vary depending on
CONFIG_SPL_BUILD.  If that's selected (which is the case for those boards),
then it depends on yet another SPL config for a separate .bsdd section.

I don't have a board to verify my suspicion but I think reading the DTB
without looking into a1 is broken for these boards.

> 
> > So I think I'll send a v2, keeping the config as-is and fixing the return
> > address of the DTB in case OF_BOARD is ever selected.
> >
> 
> Yes, it seems to me that we could use a config to separate the case
> between the prior stage and the _end. 

Untangling OF_SEPARATE and OF_BOARD is part of a bigger revamp I wanted to 
do on the handover of a device tree from previous bootloaders,  since we do 
have similar 'problems' in Arm and TF-A.  But in principle OF_SEPARATE
shouldn't have per board code to overwrite it.  OF_BOARD should be used for
that.  OF_SEPARATE should merely mean "The dtb is concatenated to my U-Boot
binary.

Right now RISC-V uses OF_SEPARATE reads the DTB on SPL and then goes back
to using the a1 register for U-Boot proper.  We could instead read the 
U-Boot concatenated DTB always in that case.  OF_BOARD would then be used in
case OpenSBI is compiled with a *different* DTB and you'd want to use that.
Any idea if OpenSBI performs fixups before handing over the dtb in a1?

Unfortunately I don't have a board to test apart from QEMU.  Let me respin
this, with a potential fix I have in mind and we can discuss further.

> Just note that, there is a patch
> on the fly, it modifies the same snippet of code, you might need to
> update your code based on top of it.
> https://lists.denx.de/pipermail/u-boot/2021-September/460378.html

I'll reply to that and see if the _end is indeed a problem.

Thanks
/Ilias