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

From: Florian Fainelli via buildroot <buildroot@buildroot.org>
To: Thomas Petazzoni <thomas.petazzoni@bootlin.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: Mon, 26 Oct 2026 14:49:53 -0700	[thread overview]
Message-ID: <fceb1e19-e9d3-4342-b1c3-d10e175dc47d@broadcom.com> (raw)
In-Reply-To: <at0YSHfI0gcUfMtX@windsurf>

On 10/24/26 13:44, Thomas Petazzoni wrote:
> Hello Florian,
> 
> Wow, blast from the past here. Tough (but interesting!) topic.

Indeed, this had been bugging me for a long time, so here we go again.

> 
> 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).

Correct. Out-of-tree is only used when <pkg>_OVERRIDE_SRCDIR is set and 
the package opts in with <pkg>_SUPPORTS_OUT_OF_TREE = YES (only linux 
for now).

For a normal build the tarball is extracted into the build directory as 
before, and _SRCDIR, _BUILDDIR and _DIR are all the same directory, so 
there is no change for any package. The goal is to avoid the rsync of a 
developer's source tree, which does not exist for a downloaded tarball. 
Other packages could opt in later with the same variable if there is 
interest.

> 
> 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.

I wonder if this makes sense however, not arguing about it, but trying 
to think whether it brings anything?
-- 
Florian
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