Re: [Buildroot] [PATCH 00/23] Support building the Linux kernel out of tree with LINUX_OVERRIDE_SRCDIR

From: Thomas Petazzoni via buildroot <buildroot@buildroot.org>
To: Florian Fainelli <florian.fainelli@broadcom.com>
Cc: buildroot@buildroot.org,
	"Eric Le Bihan" <eric.le.bihan.dev@free.fr>,
	"Gary Bisson" <bisson.gary@gmail.com>,
	"Jérôme Pouiller" <jezz@sysmic.org>,
	"Bernd Kuhls" <bernd@kuhls.net>,
	"Thomas Perale" <thomas.perale@mind.be>,
	"Maxime Chevallier" <maxime.chevallier@bootlin.com>,
	"Refik Tuzakli" <tuzakli.refik@gmail.com>,
	"Francois Perrad" <francois.perrad.86@gmail.com>,
	"Arnout Vandecappelle" <arnout@rnout.be>,
	"Petr Vorel" <petr.vorel@gmail.com>,
	"Spenser Gilliland" <spenser@gillilanding.com>,
	"Giulio Benetti" <giulio.benetti@benettiengineering.com>,
	"Pascal de Bruijn" <p.debruijn@unilogic.nl>,
	"Luca Ceresoli" <luca.ceresoli@bootlin.com>,
	"Romain Naour" <romain.naour@gmail.com>,
	"Mahyar Koshkouei" <mahyar.koshkouei@gmail.com>
Subject: Re: [Buildroot] [PATCH 00/23] Support building the Linux kernel out of tree with LINUX_OVERRIDE_SRCDIR
Date: Sat, 24 Oct 2026 22:44:32 +0200	[thread overview]
Message-ID: <at0YSHfI0gcUfMtX@windsurf> (raw)
In-Reply-To: <20261023214807.2151753-1-florian.fainelli@broadcom.com>

Hello Florian,

Wow, blast from the past here. Tough (but interesting!) topic.

On Fri, Oct 23, 2026 at 02:47:44PM -0700, Florian Fainelli via buildroot wrote:

> Patch 9, the kernel:
>   - add <pkg>_SUPPORTS_OUT_OF_TREE. When it is YES and <pkg>_OVERRIDE_SRCDIR
>     is set, the rsync step is skipped, the stamp files go in the build
>     directory and kconfig is called with -C $(SRCDIR) O=$(BUILDDIR);
>   - opt linux in, in the same patch. Kbuild requires a clean source tree
>     for O=: a tree with a stale .config, include/config or
>     arch/*/include/generated is rejected by kbuild itself.

So it means out of tree build is only used when <pkg>_OVERRIDE_SRCDIR
is used, and for a "normal" build, you still use in-tree build? (Note:
I didn't check the patches yet).

If we were to merge something like this, we would need to switch to
out-of-tree build completely, not just for the specific
<pkg>_OVERRIDE_SRCDIR situation.

Thomas
-- 
Thomas Petazzoni, co-owner and CEO, Bootlin
Embedded Linux and Kernel engineering and training
https://bootlin.com
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