incompatible device trees between u-boot and linux

[Rob Herring]

On Wed, Aug 25, 2021 at 10:12 AM Vladimir Oltean <olteanv at gmail.com> wrote:
>
> On Wed, Aug 25, 2021 at 10:26:10AM -0400, Tom Rini wrote:
> > On Wed, Aug 25, 2021 at 05:18:16PM +0300, Vladimir Oltean wrote:
> > > On Wed, Aug 25, 2021 at 10:00:45AM -0400, Tom Rini wrote:
> > > > On Wed, Aug 25, 2021 at 03:58:10PM +0200, Michael Walle wrote:
> > > >
> > > > > Hi,
> > > > >
> > > > > I noticed that there is a fallback to the u-boot device tree for linux
> > > > > (esp. EFI boot) if no other device tree was found, see [1]. It seems this
> > > > > is working fine for imx devices, for example, where you can just boot a
> > > > > stock installer iso via EFI. It will just work and it is quite a nice
> > > > > feature as a fallback.
> > > > >
> > > > > Now for the layerscape architecture, the ls1028a in my case, things are
> > > > > more difficult because the bindings differ between u-boot and linux - one
> > > > > which comes to mind is DSA and ethernet.
> > > > >
> > > > > Which begs the general question, is it encouraged to have both bindings
> > > > > diverge? To me it seems, that most bindings in u-boot are ad-hoc and there
> > > > > is no real review or alignment but just added as needed, which is ok if
> > > > > they are local to u-boot. But since they are nowadays passed to linux
> > > > > (by default!) I'm not so sure anymore.
> > > > >
> > > > > OTOH The whole structure around a .dts{,i} and -u-boot.dtsi looks like
> > > > > they should (could?) be shared between linux and u-boot.
> > > > >
> > > > > -michael
> > > > >
> > > > > [1]
> > > > > https://elixir.bootlin.com/u-boot/v2021.10-rc2/source/common/board_r.c#L471
> > > >
> > > > The U-Boot device tree is supposed to be able to be passed on to Linux
> > > > and Just Work.  The bindings are not supposed to be different between
> > > > the two (except for when we take the binding while it's being hashed out
> > > > upstream BUT THEN RESYNCED).
> > >
> > > You might need to spell that out a bit clearer.
> > >
> > > You are saying that both U-Boot and Linux are allowed to have their own
> > > custom properties (like 'u-boot,dm-spl' for U-Boot, and 'managed = "in-band-status"'
> > > for Linux), as long as the device tree files themselves are in sync, and
> > > the subset of the device tree blob understood by Linux (i.e. the U-Boot
> > > blob sans the U-Boot specifics) is compatible with the Linux DT blob?
> >
> > I don't know what about the Linux example makes it Linux specific.  But
> > yes, 'u-boot,dm-spl' is clearly in our namespace and should be ignored
> > by Linux.  The whole reason we have the -u-boot.dtsi automatic drop-in
> > logic (as much as it can be used is that device trees are device trees
>         ^
>         I don't think this parenthesis ever closes...
>
> > and describe the hardware and developers don't need to write a device
> > tree for Linux and a device tree for U-Boot and a device tree for
> > FreeBSD and ...  So yes, you're supposed to use the device tree for a
>                 ^
>                 so I never get the answer to "the whole reason is...".
>
> > platform and it works here and there and every where.
>
> The fact that only Linux uses it makes it Linux specific.
>
> > > To expand even further on that, it means we should put 'managed = "in-band-status"'
> > > in U-Boot, which is a Linux phylink device tree property, even if U-Boot
> > > does not use phylink?
> >
> > We should be able to drop in the device trees from Linux and use them.
> > Custodians should be re-syncing them periodically.  Some are, even.
>
> Are you ready to take up device tree bindings for PTP timers, PCIe root
> complex event collectors, cascaded interrupt controllers, things which
> U-Boot will never ever need to support?
>
> At least in Linux there is a policy to not add device tree nodes that do
> not have drivers.

That is not the policy. The policy is DT nodes must have binding
(schema) documentation and the binding should be complete as possible
(not what some driver currently uses). However, for complex bindings,
it might be difficult to write the binding in absence of a driver.

> Is the same policy not true for U-Boot? At least your
> ./scripts/checkpatch.pl does have the same "check for DT compatible
> documentation" section as Linux. You might consider removing it if you
> want people to not strip the DTs they submit to U-Boot.

That hacky checkpatch.pl check is going away. While somewhat
effective, it will take 'the' or any other word in a binding doc as a
valid compatible. With the schemas now, we have an exact list of what
compatibles are documented and that is checked on dts files. Though
anything with a free form text binding will be reported as
undocumented, but I view that as a todo list.

> And why do we even maintain the device tree bindings in Linux at all?
> It seems rather counter-productive for both ends to do that, if it is
> expected that the kernel works with DT blobs provided by third parties too,
> and if all third parties need to resync with it (there are other boot
> loaders too beyond U-Boot, and other kernels beyond Linux). Somehow it
> doesn't feel right for the reference to be the Linux kernel. Maybe this
> is something that needs to be brought up with higher-level Linux maintainers.

This has been discussed to death, but the short answer is convenience.
Where does the largest collection of h/w support and knowledge live?
The Linux kernel. By far. u-boot is probably second, but there's lots
of h/w on platforms u-boot doesn't care about. We could move portions
or all of DT stuff out, but then who is going to review it and check
whether bindings are accepted before a driver or dts uses them? I'm
sure some kernel or u-boot subsystem maintainers might sign up
initially, but are they really going to stay around? In the kernel,
they have to pay attention (to varying levels).

Rob