[RFC PATCH 0/5] Allow for removal of DT nodes and properties

[Rob Herring]

On Thu, Sep 14, 2023 at 5:42 PM Simon Glass <sjg at chromium.org> wrote:
>
> Hi Rob,
>
> On Wed, 13 Sept 2023 at 16:39, Rob Herring <robh at kernel.org> wrote:
> >
> > On Fri, Sep 8, 2023 at 1:06 PM Mark Kettenis <mark.kettenis at xs4all.nl> wrote:
> > >
> > > > From: Jassi Brar <jassisinghbrar at gmail.com>
> > > > Date: Fri, 8 Sep 2023 09:37:12 -0500
> > > >
> > > > Hi Simon,
> > > >
> > > > On Thu, Sep 7, 2023 at 3:08 PM Simon Glass <sjg at chromium.org> wrote:
> > > > > On Wed, 6 Sept 2023 at 23:20, Ilias Apalodimas
> > > > > >
> > > > > > > > I beg to differ. Devicetree is more than just hardware and always has
> > > > > > > > been. See, for example the /chosen and /options nodes.
> > > > > > >
> > > > > > > There are exceptions...
> > > > > > >
> > > > > >
> > > > > > We've been this over and over again and frankly it gets a bit annoying.
> > > > > > It's called *DEVICE* tree for a reason.  As Rob pointed out there are
> > > > > > exceptions, but those made a lot of sense.  Having arbitrary internal ABI
> > > > > > stuff of various projects in the schema just defeats the definition of a
> > > > > > spec.
> > > > >
> > > > > Our efforts should not just be about internal ABI, but working to
> > > > > provide a consistent configuration system for all firmware elements.
> > > > >
> > > > Sure, programmatically we can pass any data/info via DT, however it is
> > > > only meant to map hardware features onto data structures.
> > > >
> > > > devicetree.org  landing page
> > > >     "The devicetree is a data structure for describing hardware."
> > > >
> > > > devicetree-specification-v0.3.pdf  Chapter-2 Line-1
> > > >    "DTSpec specifies a construct called a devicetree to describe
> > > > system hardware."
> > >
> > > But it doesn't say that it describes *only* hardware.  And it does
> > > describe more than just hardware.  There is /chosen to specify
> > > firmware configuration and there are several examples of device tree
> > > bindings that describe non-discoverable firmware interfaces in the
> > > Linux tree.  And it has been that way since the days of IEEE 1275.
> > > There are also bindings describing things like the firmware partition
> > > layout.
> >
> > Yes. The exact title for 1275[1] is: IEEE Standard for Boot
> > (Initialization Configuration)
> > Firmware: Core Requirements and Practices
> >
> > I see "configuration" in there. However, in the OF case, it's
> > generally how firmware configured the hardware and what it discovered.
> > That's a little different than telling the OS how to configure the
> > hardware which is what we do with FDT.
>
> For the /options node it says "The properties of this node are the
> system’s configuration variables."
>
> Then there is section 7.4.4 which has a large list of options which
> don't seem to be so narrowly defined.
>
> In any case, this is not quite the point, which IMO is that we need DT
> to support firmware use cases, whether or not a 29-year-old spec
> thought of it or not. In fact it is amazing to me how forward-looking
> Open Firmware was.
>
> > Then there's stuff that's how
> > to configure Linux which we try to reject.
>
> Fair enough: Linux has user-space for that.

Yep, the question I usually ask is who needs to configure something
and what determines the config. Changing things with sysfs is much
easier than changing the DT provided by firmware.

> >
> > Once we get into configuration of the OS/client (including u-boot),
> > making that an ABI is a lot harder and if we use DT for that, I don't
> > think it should be an ABI.
> >
> > > > If we want to digress from the spec, we need the majority of
> > > > developers to switch sides :)  which is unlikely to happen and rightly
> > > > so, imho.
> > >
> > > It isn't even clear what the spec is.  There is the document you
> > > reference above, there are the yaml files at
> > > https://github.com/devicetree-org/dt-schema and then there are
> > > additional yaml files in the Linux tree.  As far as I know it isn't
> > > written down anywhere that those are the only places where device tree
> > > bindings can live.
> >
> > There's not any restriction.
> >
> > My intention with dtschema schemas is to only have "spec level"
> > schemas. (Stuff we'd add to DTSpec (but don't because no one wants to
> > write specs).) Hardware specific stuff lives somewhere else. That
> > happens to be the Linux tree because that is where all the h/w support
> > is (nothing else is close (by far)). Last I checked, we've got ~3700
> > schemas and ~1500 unconverted bindings.
>
> That scope is quite a bit narrower than I understood it to be. It
> seems to leave a significant gap between the Linux repo and yours.
>
> >
> > > Anyway, let's face it, there is some consensus among developers that
> > > what Simon has done in U-Boot is pushing the use of devicetree beyond
> > > the point where a significant fraction of developers thinks it makes
> > > sense.  And I think I agree with that.  But you can't beat him with
> > > the spec to make your point.
> > >
> > > Now the devicetree is cleverly constructed such that it is possible to
> > > define additional bindings without the risk of conflicting with
> > > bindings developed by other parties.  In particular if U-Boot is
> > > augmenting a device tree with properties that are prefixed with
> > > "u-boot," this isn't going to hurt an operating system that uses such
> > > an augmented device tree.
> >
> > Until someone has some great idea to start using them. If the OS
> > doesn't need something, then why pass it in the first place? What
> > purpose does it serve? It's an invitation for someone somewhere to
> > start using them.
> >
> > The downside of keeping the nodes is it creates an ABI where there
> > doesn't need to be one necessarily.
>
> I'd love to get away from the idea that DT is just for the OS. We are
> trying to use it for firmware-to-firmware communication, too. The
> programs on each side of that interface may be different projects. For
> that, we do need to have a binding, otherwise we end up with series
> like this one.

I don't think DT is just for the OS, but DT for the OS is an ABI and
other cases may not be. Or they just don't need to follow all the same
rules. Zephyr is doing their own thing for example.

I don't think /options/u-boot is an ABI. AIUI, u-boot populates the
node and consumes it. No ABI there (or limited to SPL to u-boot
proper). I suppose a prior firmware stage could populate it, but that
doesn't scale as then the prior stage has to know details of the next
stage.

> > > The real problem is that some folks developed a certification program
> > > that allegedly requires schema verification and now propose adding
> > > code to U-Boot that doesn't really solve any problem.  My proposed
> > > solution would be to change said certification program to allow
> > > firmware to augment the device tree with properties and nodes with
> > > compatibles that are in the namespace controlled by the firmware.
> >
> > I don't think we should decide what to do here based on said
> > certification program. It can adapt to what's decided. Probably, the
> > /option nodes will be stripped out or ignored for certification.
> >
> > I accepted u-boot options node schema into dtschema, but now have
> > second thoughts on that. Now I'm getting more u-boot specific
> > (perhaps, not clear) stuff and widevine stuff internal to a TEE.
>
> Where should these bindings go such that ARM / Linaro are not trying
> to remove them? I would be OK with moving them out somewhere else, but
> how are people supposed to deal with such fragmentation? My
> understanding was that dt-schema was an attempt to set up a neutral
> area where bindings could be accepted that were not just for
> Linux...did that change?

No change, just not communicated I guess. And again, bindings are not
"just for Linux". They are hosted there because that is where *all*
the hardware support is (by far). But we'll probably never get past
the "Linux binding" perception no matter what we do or how many times
I say it.

To rephrase things a bit, I'm happy to host anything that's
multi-project, not a large number of bindings, and not a
device/hardware specific binding. The device specific bindings live in
the kernel tree primarily. For any new binding (foos/#foo-cell type
ones), I want to see multiple users. Really for these, probably best
to start with them in Linux repo (or elsewhere) and then promote them
to dtschema. That gives a little flexibility in changing/abandoning
them.

Removing nodes and/or properties and where things live are mostly
independent discussions. SystemReady can adapt to whatever is decided
for the former. In general, IMO, when passing DT from stage N to N+1,
the N+1 stage should remove things which only apply to N+2 stage. For
example, kexec removes /chosen and recreates it for the next kernel.

Rob