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

[Tom Rini]

On Thu, Dec 02, 2021 at 07:03:30PM -0700, Simon Glass wrote:
> Hi Andre,
> 
> On Thu, 2 Dec 2021 at 18:59, Andre Przywara <andre.przywara at arm.com> wrote:
> >
> > On Thu, 2 Dec 2021 13:34:07 -0500
> > Tom Rini <trini at konsulko.com> wrote:
> >
> > Hi,
> >
> > > On Thu, Dec 02, 2021 at 11:17:38AM -0700, Simon Glass 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:
> > >
> > > And the rest of us wouldn't keep trying to argue otherwise if we didn't
> > > see problems with it, still.
> > >
> > > > - U-Boot should not have its own nodes/properties
> > >
> > > The caveat there is that aren't documented upstream bindings.  I think
> > > at this point the lack of screaming and otherwise "wait, no no no
> > > don't!" that your current patch has gotten means it's time for a pull
> > > request, and for that to go in, and so this line of argument would be
> > > simply removed.
> > >
> > > > - U-Boot should not have DTs in-tree
> > >
> > > ... for the cases where the DTs are not used at run time, yes.
> > >
> > > > - U-Boot should have DTs only when essential
> > >
> > > I don't understand this point.  Can you please elaborate?
> > >
> > > > - U-Boot should have DTs in-tree for all boards
> > >
> > > This is the line you're pushing and almost every other reviewer
> > > disagrees with.
> > >
> > > > What's the downside here anyway?
> > >
> > > - A lack of clarity.  We have dts files, you modify those dts files,
> > >   they aren't used.  What's the point?  Oh, you forgot to tweak
> > >   something else.  Wait, now nothing works.  Oh, it's a mismatch between
> > >   what this dts was at one point, and what it needs to be now to
> > >   actually work.
> > > - We're adding more ongoing sync-up work.  While I loudly applaud the
> > >   custodians that are keeping their dts files in sync very regularly,
> > >   and I sympathize with the custodians that want to do it more, but are
> > >   unable to find the time, I do not want to add more of this work.  Even
> > >   more so when it's unclear who would be doing it.  Or what the use is.
> > >
> > > There's probably more if I think about it harder, but those are the
> > > first to spring to mind.
> > >
> > > > > > 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?
> > >
> > > Well, in the QEMU instance, they're only as correct as the parameters
> > > passed to qemu-system-foo when you did -dumpdtb to start with.  Lets
> > > take TPM as that now should show up in the device tree, or not,
> > > depending on if we have the backend side of it?  Or all of the examples
> > > of how to arbitrarily configure a system as Heinrich noted.
> > >
> > > Or the Pi examples where we need to use the device tree passed to us
> > > because config.txt is the official way to modify things in the device
> > > tree on that platform.
> >
> > I feel like a lot of the confusion stems from the very different
> > roles that U-Boot plays on various platforms:
> > - In the traditional way U-Boot is the first and only piece of code
> >   that runs between reset and the kernel. Having the DT as part of the
> >   U-Boot image, and thus in the U-Boot source tree, makes sense, given
> >   that we talk about a particular board only.
> > - Many platforms run other pieces of software (TF-A, SCP firmware)
> >   alongside or before U-Boot, but still U-Boot is the main attraction,
> >   and is doing firmware and setup duties. Most cheap ARM SoCs (sunxi,
> >   RK) fall under this category. Depending on the particular firmware
> >   setup, having the DTs in the tree (as a copy of the canonical Linux
> >   source) again makes sense, and the DTB should probably be part of the
> >   built U-Boot image as well, unless there is some better place.
> >
> > But there is a quite different category of boards, where U-Boot is a
> > mere *loader*, and UEFI provider. The heavy lifting of platform setup
> > (clocks, DRAM, power) is either done by prior firmware code, or by a
> > separate management processor. The RPis, ARM Juno boards, Highbank and
> > many other (typically advanced) platforms fall under this category.
> > U-Boot should be happy about the lesser burden, and just consume
> > whatever DTB it finds in memory.
> > Virtual/dynamic platforms like QEMU or the ARM FVP models also fall
> > into this category: for a virtual platform hardware setup is mostly not
> > needed (DRAM, clock gating), or the emulator takes care of this already.
> >
> > For all those platforms the DTB naturally lives with the other firmware
> > bits already, or is even amended by them, and U-Boot should not try to
> > duplicate this, especially when the hardware is somewhat dynamic.
> 
> Yes that is all understood and have been bashed to death in various
> threads. Still, it is not unreasonable, I think, for U-Boot to have a
> way to use an in-tree devicetree for development and testing purposes.
> It also reasonable, I think, to require some in-tree documentation
> about how to get U-Boot running on the board.

OK, but what at least I'm saying (and I think others are too), is that
it's also not unreasonable to say that on some platforms that
development and testing purpose might require the developer to enable
things, rather than be the out of the box case (and may be more or less
hard to do, depending on the platform).

-- 
Tom
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