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

[Tom Rini <trini@konsulko.com>]

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On Thu, Oct 14, 2021 at 09:17:52AM -0600, Simon Glass wrote:
> Hi Tom,
> 
> On Thu, 14 Oct 2021 at 08:56, Tom Rini <trini@konsulko.com> wrote:
> >
> > On Wed, Oct 13, 2021 at 12:06:02PM -0600, Simon Glass wrote:
> > > Hi François,
> > >
> > > On Wed, 13 Oct 2021 at 11:35, François Ozog <francois.ozog@linaro.org> wrote:
> > > >
> > > > Hi Simon
> > > >
> > > > Le mer. 13 oct. 2021 à 16:49, Simon Glass <sjg@chromium.org> a écrit :
> > > >>
> > > >> Hi Tom, Bin,François,
> > > >>
> > > >> On Tue, 12 Oct 2021 at 19:34, Tom Rini <trini@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@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
> > > clearly.
> > >
> > > 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 have a long long reply to your first reply here saved, but, maybe
> > here's the biggest sticking point.  To be clear, you agree that U-Boot
> > needs to support being passed a device tree to use, at run time, yes?
> 
> Yes. The OF_BOARD feature provides this.
> 
> >
> > And in that case, would not be using the "fake" tree we built in?
> 
> Not at runtime.

OK.

> > So is the sticking point here that we really have two classes of
> > devices, one class where we will never ever be given the device tree at
> > run time (think BeagleBone Black) and one where we will always be given
> > one at run time (think Raspberry Pi) ?
> 
> I'm not sure it will be that black and white. I suspect there will be
> (many) boards which can boot happily with the U-Boot devicetree but
> can also accept one at runtime, if provided. For example, you may want
> to boot with or without TF-A or some other, earlier stage.

I'm not sure I see the value in making this a gray area.  There's very
much a class of "never" boards.  There's also the class of "can" today.
Maybe as part of a developer iterative flow it would be nice to not have
to re-flash the prior stage to change a DT, and just do it in U-Boot
until things are happy, but I'm not sure what the use case is for
overriding the previous stage.

Especially since the pushback on this series I think has all been "why
are we copying in a tree to build with?  We don't want to use it at run
time!".  And then softer push back like "Well, U-Boot says we have to
include the device tree file here, but we won't use it...".

> I believe we have got unstuck because OF_BOARD (perhaps inadvertently)
> provided a way to entirely omit a devicetree from U-Boot, thus making
> things like binman and U-Boot /config impossible, for example. So I
> want to claw that back, so there is always some sort of devicetree in
> U-Boot, as we have for rpi_3, etc.

I really want to see what the binary case looks like since we could then
kill off rpi_{3,3_b,4}_defconfig and I would need to see if we could
then also do a rpi_arm32_defconfig too.

I want to see less device trees in U-Boot sources, if they can come
functionally correct from the hardware/our caller.

And I'm not seeing how we make use of "U-Boot /config" if we also don't
use the device tree from build time at run time, ignoring the device
tree provided to us at run time by the caller.

-- 
Tom

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