[PATCH 00/16] fdt: Make OF_BOARD a boolean option
Heinrich Schuchardt
heinrich.schuchardt at canonical.com
Wed Oct 27 15:23:01 CEST 2021
On 10/27/21 15:15, François Ozog wrote:
> Hi,
>
> On Wed, 27 Oct 2021 at 14:48, Tom Rini <trini at konsulko.com
> <mailto:trini at konsulko.com>> wrote:
>
> On Fri, Oct 15, 2021 at 12:03:44PM -0600, Simon Glass wrote:
> > Hi all,
> >
> > On Thu, 14 Oct 2021 at 09:28, Tom Rini <trini at konsulko.com
> <mailto:trini at konsulko.com>> wrote:
> > >
> > > 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 at konsulko.com
> <mailto:trini at 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 at linaro.org <mailto:francois.ozog at linaro.org>> wrote:
> > > > > > >
> > > > > > > Hi Simon
> > > > > > >
> > > > > > > Le mer. 13 oct. 2021 à 16:49, Simon Glass
> <sjg at chromium.org <mailto:sjg at chromium.org>> a écrit :
> > > > > > >>
> > > > > > >> Hi Tom, Bin,François,
> > > > > > >>
> > > > > > >> On Tue, 12 Oct 2021 at 19:34, Tom Rini
> <trini at konsulko.com <mailto: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 <mailto: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/
> <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...".
> >
> > See below.
> >
> > >
> > > > 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.
> >
> > Firstly I should say that I find building firmware very messy and
> > confusing these days. Lots of things to build and it's hard to find
> > the instructions. It doesn't have to be that way, but if we carry on
> > as we are, it will continue to be messy and in five years you will
> > need a Ph.D and a lucky charm to boot on any modern board. My
> > objective here is to simplify things, bringing some consistency
> to the
> > different components. Binman was one effort there. I feel that
> putting
> > at least the U-Boot house in order, in my role as devicetree
> > maintainer (and as author of devicetree support in U-Boot back in
> > 2011), is the next step.
>
> Yes, it's Not Great. I don't like my handful of build-BOARD.sh scripts
> that know where to grab other known-good binaries of varying licenses
> that are needed to assemble something that boots.
>
> > If we set things up correctly and agree on the bindings, devicetree
> > can be the unifying configuration mechanism through the whole of
> > firmware (except for very early bits) and into the OS, this will set
> > us up very well to deal with the complexity that is coming.
> >
> > Anyway, here are the mental steps that I've gone through over the
> past
> > two months:
> >
> > Step 1: At present, some people think U-Boot is not even allowed to
> > have its own nodes/properties in the DT.
>
> In my view U-Boot shall be able to leverage device tree format (source
> and binary) to store its own data.
> When you say "the" DT, I always think this is "the" DT that is passed to
> OS and in "that" DT, there should be no U-Boot entries. As stated in
> another mail thread, I also refer to a place in a FIP where that dynamic
> config DT is meant to be stored: NT_FW_CONFIG.
> But there can be U-Boot defined bindings in "a" control/dynamic config
> DT; Trusted Firmware does that.
It ends up in that we need two separate devicetrees.
One passed to U-Boot for fixups and further passed to the OS. This
devicetree may originate from a prior boot stage, from a file loaded by
U-Boot, or from a later bootstage, e.g systemd-boot's devicetree
command. This devicetree will not contain any U-Boot specific information.
A second devicetree to hold everything that U-Boot needs for its
internal purposes.
Best regards
Heinrich
>
> It is an abuse of the
> > devicetree standard, like the /chosen node but with less history. We
> > should sacrifice efficiency, expedience and expandability on the
> altar
> > of 'devicetree is a hardware description'. How do we get over that
> > one? Wel, I just think we need to accept that U-Boot uses devicetree
> > for its own purposes, as well as for booting the OS. I am not saying
>
> Yes, we need to have properties present in the device tree, and just
> like how "linux," is a valid vendor prefix for the linux kernel (but not
> used I would expect by the BSD families) we have cases that need
> "u-boot," properties.
>
> > it always has to have those properties, but with existing features
> > like verified boot, SPL as well as complex firmware images where
> > U-Boot needs to be able to find things in the image, it is essential.
> > So let's just assume that we need this everywhere, since we certainly
> > need it in at least some places.
>
> No, we can't / shouldn't assume we need this everywhere. A lot of
> places? Yes. But some features are going to be optional. A valid must
> be supported use case is something like a Pi where the hardware gives us
> a device tree, the tree is correct and some features in U-Boot aren't
> needed (SPL) nor possibly supported immediately (verified boot). We can
> go off on a tangent about how useful it would be to have HW platforms
> that are both common and can demonstrate a number of features, but
> that's its own problem to solve.
>
> > (stop reading here if you disagree, because nothing below will make
> > any sense...you can still use U-Boot v2011.06 which doesn't have
> > OF_CONTROL :-)
> >
> > Step 2: Assume U-Boot has its own nodes/properties. How do they get
> > there? Well, we have u-boot.dtsi files for that (the 2016 patch
> > "6d427c6b1fa binman: Automatically include a U-Boot .dtsi file"), we
> > have binman definitions, etc. So we need a way to overlay those
> things
> > into the DT. We already support this for in-tree DTs, so IMO this is
> > easy. Just require every board to have an in-tree DT. It helps with
> > discoverability and documentation, anyway. That is this series.
> >
> > (I think most of us are at the beginning of step 2, unsure about it
> > and worried about step 3)
> >
> > Step 3: Ah, but there are flows (i.e. boards that use a particular
> > flow only, or boards that sometimes use a flow) which need the DT to
> > come from a prior stage. How to handle that? IMO that is only
> going to
> > grow as every man and his dog get into the write-a-bootloader
> > business. We need a way to provide the U-Boot nodes/properties in a
> > form that the prior stage can consume and integrate with its build
> > system. Is TF-A the only thing being discussed here? If so, let's
> just
> > do it. We have the u-boot.dtsi and we can use binman to put the image
> > together, for example. Or we can get clever and create some sort of
> > overlay dtb.
> >
> > Step 3a. But I don't want to do that. a) If U-Boot needs this stuff
> > then it will need to build it in and use two devicetrees, one
> internal
> > and one from the prior stage....well that is not very efficient
> and it
> > is going to be confusing for people to figure out what U-Boot is
> > actually doing. But we actually already do that in a lot of cases
> > where U-Boot passes a DT to the kernel which is different to the one
> > it uses. So perhaps we have three devicetrees? OMG. b) Well then
> > U-Boot can have its own small devicetree with its bits and then
> U-Boot
> > can merge the two when it starts. Again that is not very efficient.
>
> Does not need to merge the two. hence it does not have any influence on
> efficiency.
> For properties access, trusted firmware has defined an abstract way to
> get them:
> https://trustedfirmware-a.readthedocs.io/en/latest/components/fconf/index.html
> <https://trustedfirmware-a.readthedocs.io/en/latest/components/fconf/index.html>.
>
> The properties are currently implemented as DT but TF.ORG
> <http://TF.ORG> could decide to move to CBOR.
> The API will remain so that a change in backend will not influence
> existing code.
> I think you are too focused on "THE" device tree. "THE" device tree that
> is passed to the OS
> shall be hardware description and not a hacky place to fit any piece of
> metadata.
> I would argue that /chosen shall not even be there as most if not all
> information can be passed as OS command line. And actually for the UEFI
> contract, /chosen should go empty.
>
> It
> > means that U-Boot cannot be controlled by the prior stage (e.g.
> to get
> > its public key from there or to enable/disable the console), so
> > unified firmware config is not possible. It will get very confusing,
> > particularly for debugging U-Boot. c) Some other scheme to avoid
> > accepting step 3...please stop!
>
> How the nodes should get there is how the rest of the nodes in a system
> get there. Bindings are submitted and reviewed. The authoritative
> source of the dtses in question then has them, like any other property.
>
> > Step 4: Yes, but there is QEMU, which makes the devicetree up out of
> > whole cloth. What about that? Well, we are just going to have to deal
> > with that. We can easily merge in the U-Boot nodes/properties and
> > update the U-Boot CI scripts to do this, as needed, e.g. with
> > qemu-riscv64_spl. It's only one use case, although Xen might do
> > something similar.
> >
> > To my mind, that deals with both the build-time and run-time issues.
> > We have a discoverable DT in U-Boot, which should be considered the
> > source of truth for most boards. We can sync it with Linux
> > automatically with the tooling that I hope Rob Herring will come up
> > with. We can use an empty one where there really is no default,
> > although I'd argue that is making perfect an enemy of the good.
> >
> > Step 5: If we get clever and want to remove them from the U-Boot tree
> > and pick them up from somewhere else, we can do that with sufficient
> > tooling. Perhaps we should set a timeline for that? A year? Two? Six?
>
> For SystemReady compliant boards, this has to come much faster.
> Do you think distros will keep providing DTs for ever? I bet not.
>
> These last two paragraphs condense what I think is honestly close to a
> decade of debate / discussion down to a fiat "U-Boot will have the DTS
> files". I don't want that. I don't think any of the other projects
> that want to leverage DTS files want that.
>
> > To repeat, if we set things up correctly and agree on the bindings,
> > devicetree can be the unifying configuration mechanism through the
> > whole of firmware (except for very early bits) and into the OS. I
> feel
> > this will set us up very well to deal with the complexity that is
> > coming.
>
> Sure, it could. But that doesn't mean that U-Boot is where the dts
> files live.
>
> --
> Tom
>
>
>
> --
>
> François-Frédéric Ozog | /Director Business Development/
> T: +33.67221.6485
> francois.ozog at linaro.org <mailto:francois.ozog at linaro.org> | Skype: ffozog
>
>
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