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Date:   Tue, 12 Jan 2021 02:26:41 +0900
From:   Masahiro Yamada <masahiroy@...nel.org>
To:     Rob Herring <robh+dt@...nel.org>
Cc:     Viresh Kumar <viresh.kumar@...aro.org>,
        Arnd Bergmann <arnd@...db.de>, Olof Johansson <olof@...om.net>,
        Pantelis Antoniou <pantelis.antoniou@...sulko.com>,
        Frank Rowand <frowand.list@...il.com>,
        Michal Marek <michal.lkml@...kovi.net>,
        DTML <devicetree@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Bill Mills <bill.mills@...aro.org>, tero.kristo@...il.com
Subject: Re: [RFC 0/2] kbuild: Add support to build overlays (%.dtbo)

On Fri, Jan 8, 2021 at 4:02 AM Rob Herring <robh+dt@...nel.org> wrote:
>
> On Wed, Jan 6, 2021 at 10:35 PM Masahiro Yamada <masahiroy@...nel.org> wrote:
> >
> > On Wed, Jan 6, 2021 at 12:21 AM Rob Herring <robh+dt@...nel.org> wrote:
> > >
> > > On Tue, Jan 5, 2021 at 4:24 AM Viresh Kumar <viresh.kumar@...aro.org> wrote:
> > > >
> > > > Hello,
> > > >
> > > > Here is an attempt to make some changes in the kernel to allow building
> > > > of device tree overlays.
> > > >
> > > > While at it, I would also like to discuss about how we should mention
> > > > the base DT blobs in the Makefiles for the overlays, so they can be
> > > > build tested to make sure the overlays apply properly.
> > > >
> > > > A simple way is to mention that with -base extension, like this:
> > > >
> > > > $(overlay-file)-base := platform-base.dtb
> > > >
> > > > Any other preference ?
> >
> >
> >
> > Viresh's patch is not enough.
> >
> > We will need to change .gitignore
> > and scripts/Makefile.dtbinst as well.
> >
> >
> > In my understanding, the build rule is completely the same
> > between .dtb and .dtbo
> > As Rob mentioned, I am not sure if we really need/want
> > a separate extension.
> >
> >
> > A counter approach is to use an extension like '.ovl.dtb'
> > It clarifies it is an overlay fragment without changing
> > anything in our build system or the upstream DTC project.
> >
> > We use chained extension in some places, for example,
> > .dt.yaml for schema yaml files.
> >
> >
> >
> > dtb-$(CONFIG_ARCH_FOO) += \
> >     foo-board.dtb \
> >     foo-overlay1.ovl.dtb \
> >     foo-overlay2.ovl.dtb
> >
> >
> > Overlay DT source file names must end with '.ovl.dts'
>
> I like that suggestion as then it's also clear looking at the source
> files which ones are overlays. Or we'd need .dtso to be consistent.
>
>
> > > I think we'll want something similar to how '-objs' works for modules:
> > >
> > > foo-board-1-dtbs := foo-board.dtb foo-overlay1.dtbo
> > > foo-board-2-dtbs := foo-board.dtb foo-overlay2.dtbo
> > > foo-board-1-2-dtbs := foo-board.dtb foo-overlay1.dtbo foo-overlay2.dtbo
> > > dtbs-y += foo-board-1.dtb foo-board-2.dtb foo-board-1-2.dtb
> > >
> > > (One difference here is we will want all the intermediate targets
> > > unlike .o files.)
> > >
> > > You wouldn't necessarily have all the above combinations, but you have
> > > to allow for them. I'm not sure how we'd handle applying any common
> > > overlays where the base and overlay are in different directories.
> >
> >
> > I guess the motivation for supporting -dtbs is to
> > add per-board -@ option only when it contains *.dtbo pattern.
>
> I hadn't thought that far, but yeah, that would be good. Really, I
> just want it to be controlled per SoC family at least.
>
> > But, as you notice, if the overlay files are located
> > under drivers/, it is difficult to add -@ per board.
>
> Generally, they shouldn't be. The exceptions are what we already have
> there which are old dt fixups and unittests.
>
> We want the stripped DT repo (devicetree-rebasing) to have all this
> and drivers/ is stripped out. (Which reminds me, the DT repo will need
> some work to support all this. It's a different build sys.)
>
> > Another scenario is, some people may want to compile
> > downstream overlay files (i.e. similar concept as external modules),
> > then we have no idea which base board should be given with the -@ flag.
> >
> >
> > I'd rather be tempted to add it globally
> >
> >
> > ifdef CONFIG_OF_OVERLAY
> > DTC_FLAGS += -@
> > endif
>
> We've already rejected doing that. Turning on '-@' can grow the dtb
> size by a significant amount which could be problematic for some
> boards.
>
>
> > > Another thing here is adding all the above is not really going to
> > > scale on arm32 where we have a single dts directory. We need to move
> > > things to per vendor/soc family directories. I have the script to do
> > > this. We just need to agree on the vendor names and get Arnd/Olof to
> > > run it. I also want that so we can enable schema checks by default
> > > once a vendor is warning free (the whole tree is going to take
> > > forever).
> >
> >
> > If this is a big churn, perhaps we could make it extreme
> > to decouple DT and Linux-arch.
>
> I would be fine with that, but I don't think we'll get agreement
> there. With that amount of change, we'll be discussing git submodule
> again.
>
> Rereading the thread on vendor directories[1], we may just move boards
> one vendor at a time. We could just make that a prerequisite for
> vendor supporting overlays.
>
> > arch/*/boot/dts/*.dts
> >  ->  dts/<vendor>/*.dts
> >
> > Documentation/devicetree/bindings
> >  -> dts/Bindings/
> >
> > include/dt-bindings/
> >  -> dts/include/dt-bindings/
> >
> >
> >
> > Then, other project can take dts/
> > to reuse for them.
>
> This is already possible with devicetree-rebasing.git. Though it is
> still by arch.


Yes, I know this project.

There are still cross-references between arm and arm64.

Associating DT with Linux-arch is not good
because it is possible to boot the 32-bit kernel (arch/arm/)
on the 64-bit boards (arch/arm64/boot/dts/).






> Rob
>
> [1] https://lore.kernel.org/linux-devicetree/20181204183649.GA5716@bogus/



-- 
Best Regards
Masahiro Yamada

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