linux-kernel - Re: [PATCH] kbuild: install-extmod-build: do not exclude scripts/dtc/libfdt/

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Message-ID: <CAL_Jsq+t4JtiZRpqVOC8D=3u3jBq2Z+CtkiCz2tt=x0JP3tj7Q@mail.gmail.com>
Date: Tue, 3 Feb 2026 20:56:56 -0600
From: Rob Herring <robh@...nel.org>
To: Nathan Chancellor <nathan@...nel.org>
Cc: Icenowy Zheng <zhengxingda@...as.ac.cn>, Nicolas Schier <nsc@...nel.org>, 
	Masahiro Yamada <masahiroy@...nel.org>, Abel Vesa <abelvesa@...nel.org>, 
	Mingcong Bai <jeffbai@...c.io>, WangYuli <wangyuli@...ontech.com>, 
	Inochi Amaoto <inochiama@...il.com>, James Le Cuirot <chewi@...too.org>, linux-kbuild@...r.kernel.org, 
	linux-kernel@...r.kernel.org, Icenowy Zheng <uwu@...nowy.me>, Rong Zhang <i@...g.moe>, 
	Saravana Kannan <saravanak@...nel.org>, devicetree@...r.kernel.org
Subject: Re: [PATCH] kbuild: install-extmod-build: do not exclude scripts/dtc/libfdt/

On Tue, Feb 3, 2026 at 8:16 PM Nathan Chancellor <nathan@...nel.org> wrote:
>
> + Rob, Saravana, and devicetree@ since this concerns files they own.
>
> On Sun, Feb 01, 2026 at 09:02:59PM +0800, Icenowy Zheng wrote:
> > There exists a header file in include/linux/ called libfdt.h that is
> > just a wrapper for libfdt header file in scripts/dtc/libfdt/. This makes
> > the headers inside libfdt copy at scripts/dtc/libfdt/ part of the kernel
> > headers for building external modules.
> >
> > Do not exclude them, otherwise modules that include <linux/libfdt.h>
> > will fail to build externally.
> >
> > Fixes: aaed5c7739be ("kbuild: slim down package for building external modules")
> > Signed-off-by: Icenowy Zheng <zhengxingda@...as.ac.cn>
>
> This does indeed bring back scripts/dtc/libfdt back into the headers
> package that I examined. However, how does including libfdt.h in an
> external module actually work, even with this change? libfdt appears to
> be built into vmlinux IIUC and I do not see any EXPORT_SYMBOLs in the
> list, so how can you actually use any of the functions from libfdt
> within the module? Would you just build and link the pieces that your
> module needs using the other source files?

Right. Modules should not use libfdt functions.

Rob