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Subject: Re: [RFC 0/2] kbuild: Add support to build overlays (%.dtbo)
To:     Rob Herring <robh+dt@kernel.org>,
        Viresh Kumar <viresh.kumar@linaro.org>,
        Arnd Bergmann <arnd@arndb.de>, Olof Johansson <olof@lixom.net>
Cc:     Pantelis Antoniou <pantelis.antoniou@konsulko.com>,
        Masahiro Yamada <masahiroy@kernel.org>,
        Michal Marek <michal.lkml@markovi.net>,
        devicetree@vger.kernel.org,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Bill Mills <bill.mills@linaro.org>, tero.kristo@gmail.com
References: <cover.1609844956.git.viresh.kumar@linaro.org>
 <CAL_JsqJMr3vfz2B29vzvFALCt_5-J__eJv2TZHJ0sR9nM=xXaw@mail.gmail.com>
From:   Frank Rowand <frowand.list@gmail.com>
Message-ID: <1cbefc63-4185-def5-2afa-d929a44e4e1f@gmail.com>
Date:   Mon, 11 Jan 2021 18:25:03 -0600
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On 1/5/21 9:21 AM, Rob Herring wrote:
> On Tue, Jan 5, 2021 at 4:24 AM Viresh Kumar <viresh.kumar@linaro.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 ?
> 
> 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

(Thinking ahead....) I'm not sure how to fit connector nodes and the
corresponding plugin overlays into this model.  A single plugin .dtbo
will need to be relocated onto one or more connector nodes.  fdtoverlay
and the run time overlay apply code do not know how to do this yet.

-Frank

> 
> (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.
> 
> 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).
> 
>> Also fdtoverlay is an external entity right now, and is not part of the
>> kernel. Do we need to make it part of the kernel ? Or keep using the
>> external entity ?
> 
> Part of the kernel. We just need to add it to the dtc sync script and
> makefile I think.
> 
> Rob
>