linux-kernel - Re: [PATCH 09/19] powerpc: boot: Build devicetrees when CONFIG

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Message-ID: <Z323v2w74_lB2Ilo@probook>
Date: Tue,  7 Jan 2025 23:24:47 +0000
From: J. Neuschäfer <j.ne@...teo.net>
To: Christophe Leroy <christophe.leroy@...roup.eu>
Cc: j.ne@...teo.net, Michael Ellerman <mpe@...erman.id.au>,
	Nicholas Piggin <npiggin@...il.com>,
	Naveen N Rao <naveen@...nel.org>,
	Madhavan Srinivasan <maddy@...ux.ibm.com>,
	Rob Herring <robh@...nel.org>,
	Krzysztof Kozlowski <krzk+dt@...nel.org>,
	Conor Dooley <conor+dt@...nel.org>,
	Linus Walleij <linus.walleij@...aro.org>,
	Bartosz Golaszewski <brgl@...ev.pl>, Frank Li <Frank.Li@....com>,
	linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
	devicetree@...r.kernel.org, linux-gpio@...r.kernel.org
Subject: Re: [PATCH 09/19] powerpc: boot: Build devicetrees when
 CONFIG_MPC831x=y

On Mon, Jan 06, 2025 at 02:55:22PM +0100, Christophe Leroy wrote:
> Le 02/01/2025 à 19:31, J. Neuschäfer via B4 Relay a écrit :
> > [Vous ne recevez pas souvent de courriers de devnull+j.ne.posteo.net@...nel.org. Découvrez pourquoi ceci est important à https://aka.ms/LearnAboutSenderIdentification ]
> > 
> > From: "J. Neuschäfer" <j.ne@...teo.net>
> > 
> > In order to produce useful FIT images, the kernel needs to know which
> > devicetrees to build. To that end, follow the same approach as other
> > architectures, and enable devicetrees per platform.
> 
> Why do you need that ?

The FIT infrastructure uses the dtbs-y variable and the dtbs-list file
built from it in order to obtain a list of DTBs to include.

Having roughly the right DTBs included by setting a simple config
option to =y is pretty convenient.

> Why not just use CONFIG_EXTRA_TARGETS for that ?

To be honest, I didn't know about CONFIG_EXTRA_TARGETS until now.
But I also don't quite understand it:

  ()  Additional default image types

I don't need an additional image type. I want additional device trees,
if anything.

One thing I specifically wanted to avoid by using FIT is having to add
more per-board bootwrapper logic to arch/powerpc/boot/Makefile.

> > 
> > Signed-off-by: J. Neuschäfer <j.ne@...teo.net>
> > ---
> > 
> > I've only enabled MPC831x devicetrees, because that's the hardware I have.
> > ---
> >   arch/powerpc/boot/dts/Makefile | 5 +++++
> >   1 file changed, 5 insertions(+)
> > 
> > diff --git a/arch/powerpc/boot/dts/Makefile b/arch/powerpc/boot/dts/Makefile
> > index 0cd0d8558b475cfe342f36f4b78240ef88dd2e37..6aee895d5baaa2c978d4b1c82a6d198d9e166ea2 100644
> > --- a/arch/powerpc/boot/dts/Makefile
> > +++ b/arch/powerpc/boot/dts/Makefile
> > @@ -3,3 +3,8 @@
> >   subdir-y += fsl
> > 
> >   dtb-$(CONFIG_OF_ALL_DTBS) := $(patsubst $(src)/%.dts,%.dtb, $(wildcard $(src)/*.dts))
> > +
> > +dtb-$(CONFIG_MPC831x) += \
> > +       kmeter1.dtb \
> > +       mpc8313erdb.dtb \
> > +       mpc8315erdb.dtb
> 
> How will it know which one of the three to put in the FIT image ?
> Or do you want all three in the FIT image ?

It includes all of them, which is an acceptable outcome.

> In that case how do you select which one to use at boot ?

The bootloader (most likely U-Boot) is expected to know the compatible
string of the board and select the right devicetree based on it.


Best regards,
J. Neuschäfer