[PATCH v6 00/25] fdt: Make OF_BOARD a boolean option

Mark Kettenis mark.kettenis at xs4all.nl
Sat Dec 4 17:02:32 CET 2021

> From: Simon Glass <sjg at chromium.org>
> Date: Sat, 4 Dec 2021 08:20:55 -0700
> Hi Tom,
> On Sat, 4 Dec 2021 at 06:52, Tom Rini <trini at konsulko.com> wrote:
> >
> > On Fri, Dec 03, 2021 at 06:01:56PM -0700, Simon Glass wrote:
> >
> > [huge snip]
> > > > There's things that need to be cleaned up because we have some small
> > > > number of platforms that went off and did their own thing.  But largely
> > > > yes, things make sense to me.  We have:
> > > > - We embedded the device tree that will configure U-Boot, because there
> > > >   is no way for the hardware to have provided us one.
> > > > - We do not embed the device tree that will configure U-Boot, because
> > > >   there is already one present in memory for us to use.
> > > >
> > > > Then we have the developer option of:
> > > > - We embedded the device tree that will configure U-Boot, because we're
> > > >   developing something.
> > >
> > > Yes, agreed those are the cases. To me this needs to be a run-time choice.
> >
> > But it's not possible.  That's the problem we keep going around and
> > around about.  People keep raising real life examples where you cannot
> > make a run time choice between "device tree we're passed at run time"
> > and "device tree we're compiled with".
> I haven't seen one. The most extreme case is QEMU and it works fine. I
> even added a test with it. What am I missing?

Try it on your M1 Mac.

> > And it's not helpful.  It is ALWAYS the case that we know that we want
> > to override the run time device tree with our own, because it's a
> > developer developing things or it's a user / production case where we
> > must use the provided tree.  NOT doing that is what leads to madness
> > like we see for example on Pi where if we don't use the passed tree we
> > still need to copy X/Y/Z out of it.
> Aren't you talking about the distro DT there, rather than the the one
> on the boot disk? That is my reading of that patch. If we need to do
> that sort of thing, it doesn't matter where the the cointrol DT comes
> from. You are still going to have to do that sort of thing.
> It is not ALWAYS the case. I've shown you how easy it is to disable
> OF_BOARD and still boot / iterate.

You are cheating.  If you dump the device tree passed to U-Boot (or
the OS) and don't change anything in the configuration of whatever is
passing U-Boot that device tree this is likely to work fine.  But it
won't work in general.

> > > > > Are you looking to have an empty DT in u-boot.bin? Perhaps we should
> > > > > provide a way to do that? But what is driving that desire?
> > > >
> > > > I'm looking for ways to convince you that we do not need to include a
> > > > device tree in the binary.  There's a growing set of devices where the
> > > > device tree exists with the device.  If it's missing, that's a huge
> > > > fatal error we can't do all that much about.  If we need to do something
> > > > to that device tree for U-Boot, yes, fine, we should make it straight
> > > > forward for the developer to do that.  But that's not the common case!
> > >
> > > Well we could add another Kconfig which tells U-Boot not to include a
> > > devicetree in u-boot.bin, if that would resolve this?
> > >
> > > I just want to make sure that we always build the devicetrees and that
> > > it is easy for a knowledgeable dev to switch over to use them, without
> > > spelunking through dozens of other projects to discover the secret DT
> > > that no one will tell us about.
> >
> > Should we demand better documentation for boards?  Yes.  But it's still
> > a valid case to have zero device trees for a given platform in-tree.
> > Xen is an example of this.  QEMU is an example of this.  Platforms need
> > to work without adding special tweaks for us.  Maybe that means some
> > features can't be tested in QEMU-as-virtual-platform and only in
> > QEMU-faithfully-emulating-specific-physical-platforms.
> You mention QEMU (for ARM and RISC-V) and now XEN. They are a special
> case, I think. How about we create a special Kconfig for that case? We
> need to make some progress here.

No, Apple M1 and Raspberry Pi 4 also fall into that case.  And I
suspect at least some of the other boards that currently defined
OF_BOARD fall into this category.  So rather than changing the meaning
of OF_BOARD, how about introducing another Kconfig option that
indicates that switching to a device tree built into U-Boot is ok?
You could use that to implement the runtime switching that you desire.
This would prevent confusing users with options that result in making
their systems unusable.

> > > > I guess another part of the problem is that historically almost all
> > > > platforms were in the first case I list above, no run time provided
> > > > device tree, so we took the kernel one and added our bindings to it.
> > > > Now we're being bit by the growing number of platforms that are the
> > > > second case, and how do we get our properties in there, and which ones
> > > > even make sense to do that for.
> > >
> > > I think upstreaming the bindings is the solution there. I've made a
> > > start, but we need to make progress with this series and all the other
> > > things in flight. I think a lot of people want U-Boot to not have a
> > > devicetree source files in it for ARMv8 platforms. I am strongly
> > > opposed to that. I've laid out my reasons very clearly in the past. I
> > > think this is a good summary:
> > >
> > > https://lists.gnu.org/archive/html/qemu-devel/2021-10/msg03480.html
> >
> > Yes, there are some ARMv8 platforms we will have to have the source
> > files to, in tree, because they won't come to us at run time.  But
> > others we won't for practical reasons, namely that we can't statically
> > provide something that exists dynamically without massive duplication of
> > code or just taking things from that passed to us tree.
> So let's require that the static ones have the Linux DT in our tree
> for now. The dynamic ones are just QEMU for ARM and Xen, I think. If
> that's it then I can agree a special case for them, so long as we sort
> out the docs for Xen.

No Raspberry Pi (all models) and Apple M1 are also dynamic.  And there
are probably a few more that I'm not familliar with.  We've explain
how these are dynamic in this thread and earlier discussions many
times already, but I can do it again if you need me to.

> > > I believe I have been consistent in this although with all the
> > > discussion I'm really not sure anymore.
> >
> > Yes, everyone has been consistent in these discussions.
> I'd like to think more people accept that U-Boot is allowed its own
> properties than did at the start.
> >
> > > The problem is that various people have various views about how U-Boot
> > > should work with devicetree. I strongly believe that until we have
> > > bindings upstream, a central repo for DTs with easy downloading for
> > > builds, automated validation, among other things, we must maintain the
> > > devicetree in U-Boot. Just from the POV of energy expended, I do not
> > > want to be arguing with the Linaro folks about what U-Boot is allowed
> > > to do every month for the next two years. I'd rather set out the stall
> > > now and then deal with the problems it causes from that perspective.
> >
> > The problems of the last going on 12 years won't be solved instantly.
> > The conflict as I see it is that you're insisting that all platforms
> > must have statically usable device trees, and I (and I believe others)
> > are saying that's unreasonable in cases where the trees are dynamic at
> > heart, lets just ensure we have good enough documentation for them,
> > which we don't today.
> >
> > To be clear and pick an example, I don't want Pi dts files in U-Boot,
> > but, OK, it's an easy enough case to sync them up and so long as we
> > aren't yet at the "now we pick at run time between compiled in or passed
> > to us dtb", I can accept them in tree, but not in the resulting binary
> > for OF_CONTROL=y.  But as the Xen folks have also noted, there's no
> > reasonable tree to include there.  It does need to be better documented
> > how to fire it up however, in our sources.
> I'm OK with us copying in the Linux devicetree and using that. But
> OF_BOARD must be a run-time option and able to be disabled. The
> devicetree must be built, so it is actually real. We can have a
> separate OF_OMIT or something like that to omit the devicetree from
> the output image, perhaps.
> All of the other things need to wait until we make progress with
> devicetree bindings, validation,
> How can we make progress on this? We have different goals, as I have
> explained, so we are not going to agree on everything.
> Regards,
> Simon

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