[PATCH 00/16] fdt: Make OF_BOARD a boolean option

Simon Glass sjg at chromium.org
Wed Oct 13 20:06:02 CEST 2021

Hi François,

On Wed, 13 Oct 2021 at 11:35, François Ozog <francois.ozog at linaro.org> wrote:
> Hi Simon
> Le mer. 13 oct. 2021 à 16:49, Simon Glass <sjg at chromium.org> a écrit :
>> Hi Tom, Bin,François,
>> On Tue, 12 Oct 2021 at 19:34, Tom Rini <trini at konsulko.com> wrote:
>> >
>> > On Wed, Oct 13, 2021 at 09:29:14AM +0800, Bin Meng wrote:
>> > > Hi Simon,
>> > >
>> > > On Wed, Oct 13, 2021 at 9:01 AM Simon Glass <sjg at chromium.org> wrote:
>> > > >
>> > > > 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 at [1].
>> > > >
>> > > > 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.
>> > >
>> > > Adding device trees that are never used sounds like a hack to me.
>> > >
>> > > For QEMU, device tree is dynamically generated on the fly based on
>> > > command line parameters, and the device tree you put in this series
>> > > has various hardcoded <phandle> values which normally do not show up
>> > > in hand-written dts files.
>> > >
>> > > I am not sure I understand the whole point of this.
>> >
>> > I am also confused and do not like the idea of adding device trees for
>> > platforms that are capable of and can / do have a device tree to give us
>> > at run time.
>> (I'll just reply to this one email, since the same points applies to
>> all replies I think)
>> I have been thinking about this and discussing it with people for a
>> few months now. I've been signalling a change like this for over a
>> month now, on U-Boot contributor calls and in discussions with Linaro
>> people. I sent a patch (below) to try to explain things. I hope it is
>> not a surprise!
>> The issue here is that we need a devicetree in-tree in U-Boot, to
>> avoid the mess that has been created by OF_PRIOR_STAGE, OF_BOARD,
>> BINMAN_STANDALONE_FDT and to a lesser extent, OF_HOSTFILE. Between
>> Ilias' series and this one we can get ourselves on a stronger footing.
>> There is just OF_SEPARATE, with OF_EMBED for debugging/ELF use.
>> For more context:
>> http://patchwork.ozlabs.org/project/uboot/patch/20210919215111.3830278-3-sjg@chromium.org/
>> BTW I did suggest to QEMU ARM that they support a way of adding the
>> u-boot.dtsi but there was not much interest there (in fact the
>> maintainer would prefer there was no special support even for booting
>> Linux directly!)
> i understand their point of view and agree with it.
>> But in any case it doesn't really help U-Boot. I
>> think the path forward might be to run QEMU twice, once to get its
>> generated tree and once to give the 'merged' tree with the U-Boot
>> properties in it, if people want to use U-Boot features.
>> I do strongly believe that OF_BOARD must be a run-time option, not a
>> build-time one. It creates all sorts of problems and obscurity which
>> have taken months to unpick. See the above patch for the rationale.
>> To add to that rationale, OF_BOARD needs to be an option available to
>> any board. At some point in the future it may become a common way
>> things are done, e.g. TF-A calling U-Boot and providing a devicetree
>> to it. It doesn't make any sense to have people decide whether or not
>> to set OF_BOARD at build time, thus affecting how the image is put
>> together. We'll end up with different U-Boot build targets like
>> capricorn, capricorn_of_board and the like. It should be obvious where
>> that will lead. Instead, OF_BOARD needs to become a commonly used
>> option, perhaps enabled by most/all boards, so that this sort of build
>> explosion is not needed.
> If you mean that when boards are by construction providing a DTB to U-Boot then I agree very much. But I don’t understand how the patch set  supports it as it puts dts files for those boards to be built.
>> U-Boot needs to be flexible enough to
>> function correctly in whatever runtime environment in which it finds
>> itself.
>> Also as binman is pressed into service more and more to build the
>> complex firmware images that are becoming fashionable, it needs a
>> definition (in the devicetree) that describes how to create the image.
>> We can't support that unless we are building a devicetree, nor can the
>> running program access the image layout without that information.
>> François's point about 'don't use this with any kernel' is
>> germane...but of course I am not suggesting doing that, since OF_BOARD
>> is, still, enabled. We already use OF_BOARD for various boards that
>> include an in-tree devicetree - Raspberry Pi 1, 2 and 3, for example
>> (as I said in the cover letter "Most boards do provide one, but some
>> don't."). So this series is just completing the picture by enforcing
>> that *some sort* of devicetree is always present.
> That seems inconsistent with the OF_BOARD becomes the default.

I think the key point that will get you closer to where I am on this
issue, is that OF_BOARD needs to be a run-time option. At present it
has build-time effects and this is quite wrong. If you go through all
the material I have written on this I think I have motivated that very

Another big issue is that I believe we need ONE devicetree for U-Boot,
not two that get merged by U-Boot. Again I have gone through that in a
lot of detail.

>> I can't quite pinpoint the patch where U-Boot started allowing the
>> devicetree to be omitted, but if people are interested I could try a
>> little harder. It was certainly not my intention (putting on my
>> device-tree-maintainer hat) to go down that path and it slipped in
>> somehow in all the confusion. I'm not sure anyone could tell you that
>> rpi_3 has an in-tree devicetree but rpi_4 does not...
>> Anyway this series is very modest. It just adds the requirement that
>> all in-tree boards have some sort of sample devicetree and preferably
>> some documentation as to where it might come from at runtime.
> That’s a very good goal. But adding files in dts make them not samples but templates to be used and replace board provided DTB.
> If you push all your DTS files in documentation, you do what you say: adding sample files.
>> That
>> should not be a tough call IMO. Assuming we can get the validation in
>> place (mentioned by Rob Herring recently) it will be quite natural to
>> sync them between (presumably) Linux and U-Boot.
>> I am also quite happy to discuss what should actually be in these
>> devicetree files and whether some of them should be essentially empty.
>> As you probably noticed, some of them are empty since I literally
>> could not figure out where they come from! But there needs to be at
>> least some skeleton for U-Boot to progress, since devicetree is
>> critical to its feature set.
> absolutely. And thank you for your efforts to make that center stage. This is also Linaro Edge group mist challenging  task on the next 6 moths. Knowing that we have lived in a floating situation for over 10 years, I just hope we get consensus across projects and distro providers about the right end goal and migration strategy.

Thank you.

>> It is high time we tidied all this up. I predict it will be much
>> harder, and much more confusing, in 6 months.

Just to set a road map here in case you can help unblock anything,
here are the things I am aware of, excluding the things I have

- this series
- the devicetree docs patch
- devicetree bindings upstream for U-Boot (first patch under discussion)
- bloblist as a means of passing devicetree, ACPI, tiny config info as
C structs to U-Boot (needs to be written)
- VPL so we can handle verification (patches pending)
- bootflow / VBE v2 series (coming next week)


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