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

[Andre Przywara]

On Thu, 2 Dec 2021 11:17:38 -0700
Simon Glass <sjg at chromium.org> wrote:

> Hi Tom,
> 
> On Thu, 2 Dec 2021 at 11:03, Tom Rini <trini at konsulko.com> wrote:
> >
> > On Thu, Dec 02, 2021 at 10:07:13AM -0700, Simon Glass wrote:  
> > > Hi Tom,
> > >
> > > On Thu, 2 Dec 2021 at 09:59, Tom Rini <trini at konsulko.com> wrote:  
> > > >
> > > > On Thu, Dec 02, 2021 at 09:49:51AM -0700, Simon Glass wrote:  
> > > > > Hi Tom,
> > > > >
> > > > > On Thu, 2 Dec 2021 at 09:38, Tom Rini <trini at konsulko.com> wrote:  
> > > > > >
> > > > > > On Thu, Dec 02, 2021 at 05:33:53PM +0100, François Ozog wrote:  
> > > > > > > Hi Simon
> > > > > > >
> > > > > > > Le jeu. 2 déc. 2021 à 17:00, Simon Glass <sjg at chromium.org> a écrit :
> > > > > > >  
> > > > > > > > With Ilias' efforts we have dropped OF_PRIOR_STAGE and OF_HOSTFILE so
> > > > > > > > there are only three ways to obtain a devicetree:
> > > > > > > >
> > > > > > > >    - OF_SEPARATE - the normal way, where the devicetree is built and
> > > > > > > >       appended to U-Boot
> > > > > > > >    - OF_EMBED - for development purposes, the devicetree is embedded in
> > > > > > > >       the ELF file (also used for EFI)
> > > > > > > >    - OF_BOARD - the board figures it out on its own
> > > > > > > >
> > > > > > > > The last one is currently set up so that no devicetree is needed at all
> > > > > > > > in the U-Boot tree. Most boards do provide one, but some don't. Some
> > > > > > > > don't even provide instructions on how to boot on the board.
> > > > > > > >
> > > > > > > > The problems with this approach are documented in another patch in this
> > > > > > > > series: "doc: Add documentation about devicetree usage"
> > > > > > > >
> > > > > > > > In practice, OF_BOARD is not really distinct from OF_SEPARATE. Any board
> > > > > > > > can obtain its devicetree at runtime, even it is has a devicetree built
> > > > > > > > in U-Boot. This is because U-Boot may be a second-stage bootloader and its
> > > > > > > > caller may have a better idea about the hardware available in the machine.
> > > > > > > > This is the case with a few QEMU boards, for example.
> > > > > > > >
> > > > > > > > So it makes no sense to have OF_BOARD as a 'choice'. It should be an
> > > > > > > > option, available with either OF_SEPARATE or OF_EMBED.
> > > > > > > >
> > > > > > > > This series makes this change, adding various missing devicetree files
> > > > > > > > (and placeholders) to make the build work.
> > > > > > > >
> > > > > > > > Note: If board maintainers are able to add their own patch to add the
> > > > > > > > files, some patches in this series can be dropped.
> > > > > > > >
> > > > > > > > It also provides a few qemu clean-ups discovered along the way. The
> > > > > > > > qemu-riscv64_spl problem is fixed.
> > > > > > > >
> > > > > > > > [1]
> > > > > > > > https://patchwork.ozlabs.org/project/uboot/patch/20210919215111.3830278-3-sjg@chromium.org/
> > > > > > > >
> > > > > > > > Changes in v6:
> > > > > > > > - Fix description of OF_BOARD so it refers just to the current state
> > > > > > > > - Explain that the 'two devicetrees' refers to two *control* devicetrees
> > > > > > > > - Expand the commit message based on comments
> > > > > > > > - Expand the commit message based on comments  
> > > > > > >
> > > > > > > You haven’t addressed any concerns expressed on the mailing list.so I am
> > > > > > > not in favor of this new version either.
> > > > > > > If you make a version without « fake DTs » as you name them, there are good
> > > > > > > advances in the documentation and other areas that would be better in
> > > > > > > mainline….
> > > > > > > If I am the only one thinking this way and the patch can be accepted, I
> > > > > > > would love there is a warning in capital letters at the top of the DTS fake
> > > > > > > files that explains the intent of this fake DT, the possible outcomes of
> > > > > > > not using the one provided by the platform and the right way of dealing
> > > > > > > with DTs for the platform.  
> > > > > >
> > > > > > This is the part that I too am still unhappy about.  I do not want
> > > > > > reference or fake or whatever device trees in the U-Boot source tree.
> > > > > > We should be able to _remove_ the ones we have, that are not required,
> > > > > > with doc/board/...rst explaining how to get / view one.  Not adding
> > > > > > more.  
> > > > >
> > > > > I understand you don't like it and that others don't as well. I wish
> > > > > it had not come to this.
> > > > >
> > > > > However we are only talking about 10 boards, three of which don't even
> > > > > have a devicetree anywhere I can find.
> > > > >
> > > > > I think on balance this is a substantial clean-up. I am happy to add
> > > > > whatever caveats and documentation people want to clarify what is
> > > > > going on here. I'm happy to look at future options where the
> > > > > devicetree is hosted elsewhere, so long as it is trivial to build it
> > > > > within U-Boot for development purposes.
> > > > >
> > > > > I'll also note that the bootstd series shows the devicetree source:
> > > > >
> > > > > Core:  246 devices, 88 uclasses, devicetree: board
> > > > >
> > > > > But for now, I still feel this is the best path forward.  
> > > >
> > > > I'm not sure how to proceed here.  The reviews are rather strongly
> > > > against the "include a device tree that won't be used".  The use case of
> > > > "but for development someone might need to modify the device tree" is
> > > > handled by platforms documenting where / how to get the real one.  We
> > > > should even update the Kconfig help to note that if you enable this your
> > > > board docs MUST explain where the device tree can be seen (or have some
> > > > legal reason you think it's OK to not...).  
> > >
> > > Right, we can do lots of things as we have discussed. I am very
> > > willing to work on these and make sure it is hard to do the thing. But
> > > this series is long enough already.  
> >
> > Yes, I think the rest of us had hoped you would come around to all of
> > our reasoning by this point, is why this is taking so long.
> >  
> 
> Look, if I thought this was all wrong I would not be doing it. We have
> a range of opinions:
> 
> - U-Boot should not have its own nodes/properties
> - U-Boot should not have DTs in-tree
> - U-Boot should have DTs only when essential
> - U-Boot should have DTs in-tree for all boards
> 
> What's the downside here anyway?
> 
> > > It is more than just development. A devicetree is needed for binman to
> > > work, even if it is empty. The documentation idea doesn't really work,
> > > as I think I have proven by the difficulty in getting this series
> > > together. An automated mechanism that runs in CI might be acceptable,
> > > but that is in the future. For now, I believe it just HAS to be
> > > in-tree.  
> >
> > I still don't see any reason why we need these incorrect and not
> > functionally used device trees in-tree when a dummy invalid tree is
> > enough to make things link.  We're dealing with real "we must have X.bin
> > in the output for things to function" issues on other platforms with
> > binman right now.  Using a dummy dts should be fine.  
> 
> Incorrect in what way?
> 
> How do I get a real one for development? How do I turn off OF_BOARD
> and use the in-tree one?
> 
> The documentation approach is not good enough.
> 
> >  
> > > > And yes, we're "only" talking about 10 platforms, which include things
> > > > like the "everyone" has one Pi family, the extraordinarily flexible (and
> > > > so easy for the reference device tree to be very wrong) QEMU families
> > > > and then platforms that are including a dts in-tree now because they
> > > > were told that was required.  
> > >
> > > Actually it is only rpi4 that doesn't have an in-tree DT. There is
> > > actually no reason for it not to, e.g. Linux has it. Why not U-Boot?
> > > The argument is the same.  
> >
> > Because we don't need it!  It serves no purpose!  It exists in Linux
> > as that's the primary device tree source repository.  We could _copy_ it
> > in, if it was useful.  But then we need to re-sync it every so often,
> > and for less clear reasons than all of the platforms that we need to
> > sync with the kernel for, AND we use the tree.  
> 
> So some people don't need it and it serves no purpose for them. But
> why do they care? It is not hurting anyone. This is all overblown.
> 
> >
> > There's even an argument to be made that it IS in Linux because when you
> > build that dtb, it's what the firmware then ships and uses and provides
> > to everyone at run time, possibly along with whatever other
> > modifications the binary firmware did (see the assorted threads,
> > including one this week about the problems we have because we don't just
> > always use the dtb provided to us at run time).
> >  
> > > Most QEMU boards have an in-tree devicetree. It is only ARM (now
> > > copied by RISC-V) which doesn't.  
> >
> > Yes, these are more examples of "someone said we need to copy it in, so
> > we copy it in".  
> 
> No that's not correct. With x86, ppc, integrator, ast2500 and many
> others we *need* the DT and *it is not* created by QEMU.

Maybe because those are fixed platforms, and/or have ways to autodetect
most of their hardware? I feel like U-Boot just has a DT here because
it uses it as some kind of configuration file, short of hardcoding the
hardware information.
I think this is one reason that x86 PCs don't use DTs: there is only
*one* fixed base platform (1981 IBM PC, with all its oddities), plus
probeable hardware (PCI) for most of the devices.

I personally absolutely love that QEMU creates the DT based on its
command line parameters: that's the way it should be, and the only way
it makes sense to me for this dynamic platform. Having some random QEMU
DT in the U-Boot tree sounds wrong, IMHO.

Cheers,
Andre.

> 
> >  
> > > > How about adjusting the make logic so that if a tree isn't found, we use
> > > > a dummy minimal valid dts file?  
> > >
> > > This is what I have done for the boards where I could not figure out
> > > how to get any sort of DT, yes. But I don't think that should be the
> > > default.  
> >
> > The more I think about this, the more I think dummy minimal valid dts
> > should be the fall-back default.  This then solves the "I'm a developer,
> > I need to modify the dts files" case because you then just provide the
> > dts instead where it should go, and it's used.  
> 
> How does it solve it? I don't even know how to get it in many cases.
> If it is a dummy then I cannot actually use it for development, right?
> 
> Regards,
> Simon