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From: Masahiro Yamada <masahiroy@kernel.org>
Date: Tue, 11 Jun 2024 23:45:14 +0900
Message-ID: <CAK7LNAQ7XAGgzhBXQWPFVgqJwdBcO3mF5pmQ3mSsmdrZ0EBL9Q@mail.gmail.com>
Subject: Re: [PATCH] scripts/make_fit: Support decomposing DTBs
To: Chen-Yu Tsai <wenst@chromium.org>
Cc: Simon Glass <sjg@chromium.org>, Nathan Chancellor <nathan@kernel.org>, 
	Nicolas Schier <nicolas@fjasle.eu>, linux-arm-kernel@lists.infradead.org, 
	linux-kernel@vger.kernel.org, linux-kbuild@vger.kernel.org, 
	Elliot Berman <quic_eberman@quicinc.com>, Devicetree List <devicetree@vger.kernel.org>, 
	Rob Herring <robh@kernel.org>, Krzysztof Kozlowski <krzk+dt@kernel.org>, Conor Dooley <conor+dt@kernel.org>
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On Tue, Jun 11, 2024 at 5:52=E2=80=AFPM Chen-Yu Tsai <wenst@chromium.org> w=
rote:
>
> On Mon, Jun 10, 2024 at 11:16=E2=80=AFPM Simon Glass <sjg@chromium.org> w=
rote:
> >
> > Hi Chen-Yu,
> >
> > On Wed, 5 Jun 2024 at 03:48, Chen-Yu Tsai <wenst@chromium.org> wrote:
> > >
> > > The kernel tree builds some "composite" DTBs, where the final DTB is =
the
> > > result of applying one or more DTB overlays on top of a base DTB with
> > > fdtoverlay.
> > >
> > > The FIT image specification already supports configurations having on=
e
> > > base DTB and overlays applied on top. It is then up to the bootloader=
 to
> > > apply said overlays and either use or pass on the final result. This
> > > allows the FIT image builder to reuse the same FDT images for multipl=
e
> > > configurations, if such cases exist.
> > >
> > > The decomposition function depends on the kernel build system, readin=
g
> > > back the .cmd files for the to-be-packaged DTB files to check for the
> > > fdtoverlay command being called. This will not work outside the kerne=
l
> > > tree. The function is off by default to keep compatibility with possi=
ble
> > > existing users.
> > >
> > > To facilitate the decomposition and keep the code clean, the model an=
d
> > > compatitble string extraction have been moved out of the output_dtb
> > > function. The FDT image description is replaced with the base file na=
me
> > > of the included image.
> > >
> > > Signed-off-by: Chen-Yu Tsai <wenst@chromium.org>
> > > ---
> > > This is a feature I alluded to in my replies to Simon's original
> > > submission of the make_fit.py script [1].
> > >
> > > This is again made a runtime argument as not all firmware out there
> > > that boot FIT images support applying overlays. Like my previous
> > > submission for disabling compression for included FDT images, the
> > > bootloader found in RK3399 and MT8173 Chromebooks do not support
> > > applying overlays. Another case of this is U-boot shipped by developm=
ent
> > > board vendors in binary form (without upstream) in an image or in
> > > SPI flash on the board that were built with OF_LIBFDT_OVERLAY=3Dn.
> > > These would fail to boot FIT images with DT overlays. One such
> > > example is my Hummingboard Pulse. In these cases the firmware is
> > > either not upgradable or very hard to upgrade.
> > >
> > > I believe there is value in supporting these cases. A common script
> > > shipped with the kernel source that can be shared by distros means
> > > the distro people don't have to reimplement this in their downstream
> > > repos or meta-packages. For ChromeOS this means reducing the amount
> > > of package code we have in shell script.
> > >
> > > [1] https://lore.kernel.org/linux-kbuild/20231207142723.GA3187877@goo=
gle.com/
> > > [2]
> > >
> > >  scripts/Makefile.lib |  1 +
> > >  scripts/make_fit.py  | 70 ++++++++++++++++++++++++++++++------------=
--
> > >  2 files changed, 49 insertions(+), 22 deletions(-)
> >
> > This is a clever way to discover the included files. Does it need to
> > rely on the Linux build information, or could this information somehow
> > be in the .dtb files? I had expected some sort of overlay scheme in
>
> (+CC DT folks and mailing list)
>
> I suppose we could make the `fdtoverlay` program embed this data during
> the kernel build. That would keep the information together, while also
> having one source of truth (the kernel Makefiles). Whether it belongs
> in the DTB or not is a separate matter.


Some time ago, I asked a similar question
with a similar motivation.

https://lore.kernel.org/devicetree-compiler/CAK7LNARV8Bo-tBXMdOu55Wg9uZRXvN=
iRdkDJ4LH8PwVMnMp4cA@mail.gmail.com/


>
> > the source, but perhaps people have given up on that?
>
> I wouldn't say given up, since we haven't agreed on anything either.
> Elliot had some concerns when I brought this up earlier [1] though.
>
> ChenYu
>
> [1] https://lore.kernel.org/linux-mediatek/20240314113908471-0700.eberman=
@hu-eberman-lv.qualcomm.com/
>

--
Best Regards
Masahiro Yamada