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

Simon Glass sjg at chromium.org
Sat Dec 4 18:35:25 CET 2021

Hi François,

On Sat, 4 Dec 2021 at 09:55, François Ozog <francois.ozog at linaro.org> wrote:
> Hi Simon
> Le sam. 4 déc. 2021 à 16:21, Simon Glass <sjg at chromium.org> a écrit :
>> 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?
>> >
>> > 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.
>> >
>> > > > > 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.
>> >
>> > > > 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.
>> >
>> > > 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.
> there is no question that U-Boot can have its properties specified in Device Tree.

I am pretty sure you were on the other side of that fence at some
point. I know quite a few others that still are.

> What we may not agree in is how those properties make it to U-Boot.

Yes but that is just the next step along in my progression in that
email ('why can't we just...' to 'this is how U-Boot works'). From
memory there are 3 more steps.

>> >
>> > > 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.
> A V7 with empty device trees (except with comments to explain why they are empty and how to force one for dev purposes) for platforms that provide U-Boot with DT (RPI, Qemu, xen…) seem a good base unless someone can propose a better way forward. If we build consensus on the feature aspect of the patch set I will be able to dedicate some time on the documentation part as I thought it was useless to check those until we agree on the functional part.

That's the status quo, so it doesn't resolve any of my concerns,
sorry. I suggest:

- Check in all the DTs we can get (e.g. from Linux), and build them
- Use an empty one if we cannot find it, and ask the maintainer to add
docs and deal with it
- For QEMU arm and QEMU RISC-V (i.e. the 'virt' case), use a base one
that works with the base QEMU config

We then have the follow-ons that Tom and I have discussed, e.g. the
Kconfig option that Mark mentioned.

That will clear up all the confusion and provide a baseline for how DT
is dealt with in U-Boot.

We should then continue on the path towards upstreaming bindings,
syncing DT with Linux, validation, removing them from U-Boot if we can
automatically download them all from somewhere, etc.

The thing is, I think people are more aligned on the eventual goal
than on this series. My concern is that without this series, it will
continue to be crazy-town and no one will be able to find anything
without manual effort. For those of us who deal with more than one
platform, this is an important point.


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