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

["François Ozog" <francois.ozog@linaro.org>]

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Le jeu. 14 oct. 2021 à 18:24, Andre Przywara <andre.przywara@arm.com> a
écrit :

> On Thu, 14 Oct 2021 09:17:52 -0600
> Simon Glass <sjg@chromium.org> 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.
> >
> > >
> > > 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 don't understand this: as Tom mentioned this is a platform decision:
> either it provides a DT somewhere (flash, EEPROM, prior firmware stage),
> or it doesn't. Sure, you can always hack your own DT in, somehow, for
> development or experimentation purposes, but that is a separate issue.
>
> Most of those platforms actually utilise some dynamic DTs, btw, where
> software amends the DT on the fly:
> - Highbank has a stub DT in SPI flash, and the management controller
> firmware detects the size and some extra DRAM (DIMMs!) parameters at boot
> time, and writes the /memory node accordingly.
> - RPi3/4 have DT overlay files on the SD card, and depending on what a
> user wrote in config.txt, they get applied to the DT (or not).
> - Even for the Allwinner H616 we amend the OF_SEPARATE provided DT copy in
> DRAM in TF-A, to carve out the DRAM region TF-A occupies.
> - QEMU is the obvious example, where the whole machine is highly dynamic,
> and there is no such thing as a fixed DT (how many cores?, how much
> memory?, how many virtio devices?, flash?, SCSI?)
>
> > 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.
>
> I have the feeling this is because we abuse DT for this. Conceptually the
> DT is not a configuration file, but a hardware description.

+42!!!

> I see that it
> is also a nice and flexible, hierarchical key/value storage, for which we
> have code in anyway, so it makes hardcoding data in the code easier to
> avoid.
> But as we see now, this has problems as well.
>
> So shall we separate those use cases? And attach just a tree with /binman
> and /config (in DTB format), but treat this separately from the hardware
> description? (Admittedly I have still to wrap my head around why we need
> /binman at U-Boot runtime in the first place.)

+1

>
>
> Cheers,
> Andre
>
> > 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.
> >
> > Regards,
> > Simon
>
> --
François-Frédéric Ozog | *Director Business Development*
T: +33.67221.6485
francois.ozog@linaro.org | Skype: ffozog

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