From: Arnd Bergmann <arnd@arndb.de>

This problem frequently comes up in randconfig testing, with
drivers failing to link because of a dependency on an optional
feature.

The Kconfig language for this is very confusing, so try to
document it in "Kconfig hints" section.

Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
v2: fix typos pointed out by Nicolas Schier
---
 Documentation/kbuild/kconfig-language.rst | 26 +++++++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/Documentation/kbuild/kconfig-language.rst b/Documentation/kbuild/kconfig-language.rst
index 858ed5d80defe..0135905c0aa35 100644
--- a/Documentation/kbuild/kconfig-language.rst
+++ b/Documentation/kbuild/kconfig-language.rst
@@ -573,6 +573,32 @@ above, leading to:
 	bool "Support for foo hardware"
 	depends on ARCH_FOO_VENDOR || COMPILE_TEST
 
+Optional dependencies
+~~~~~~~~~~~~~~~~~~~~~
+
+Some drivers are able to optionally use a feature from another module
+or build cleanly with that module disabled, but cause a link failure
+when trying to use that loadable module from a built-in driver.
+
+The most common way to express this optional dependency in Kconfig logic
+uses the slightly counterintuitive::
+
+  config FOO
+	tristate "Support for foo hardware"
+	depends on BAR || !BAR
+
+This means that there is either a dependency on BAR that disallows
+the combination of FOO=y with BAR=m, or BAR is completely disabled.
+For a more formalized approach if there are multiple drivers that have
+the same dependency, a helper symbol can be used, like::
+
+  config FOO
+	tristate "Support for foo hardware"
+	depends on BAR_OPTIONAL
+
+  config BAR_OPTIONAL
+	def_tristate BAR || !BAR
+
 Kconfig recursive dependency limitations
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
-- 
2.39.2

On Mon, Sep 18, 2023 at 4:20 AM Arnd Bergmann <arnd@kernel.org> wrote:
>
> From: Arnd Bergmann <arnd@arndb.de>
>
> This problem frequently comes up in randconfig testing, with
> drivers failing to link because of a dependency on an optional
> feature.
>
> The Kconfig language for this is very confusing, so try to
> document it in "Kconfig hints" section.
>
> Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
> Reviewed-by: Sakari Ailus <sakari.ailus@linux.intel.com>
> Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> ---
> v2: fix typos pointed out by Nicolas Schier


Applied to linux-kbuild. Thanks.



-- 
Best Regards
Masahiro Yamada

On Mon 25 Sep 2023 16:06:41 GMT, Masahiro Yamada wrote:
> On Mon, Sep 18, 2023 at 4:20 AM Arnd Bergmann <arnd@kernel.org> 
> wrote:
> >
> > From: Arnd Bergmann <arnd@arndb.de>
> >
> > This problem frequently comes up in randconfig testing, with
> > drivers failing to link because of a dependency on an optional
> > feature.
> >
> > The Kconfig language for this is very confusing, so try to
> > document it in "Kconfig hints" section.
> >
> > Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
> > Reviewed-by: Sakari Ailus <sakari.ailus@linux.intel.com>
> > Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>
> > Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> > ---
> > v2: fix typos pointed out by Nicolas Schier
> 
> 
> Applied to linux-kbuild. Thanks.

Hi Masahiro,

this patch seems to got lost, or did you intentionally removed it 
again?

Kind regards,
Nicolas

On Thu, Feb 15, 2024 at 9:39 PM Nicolas Schier <nicolas@fjasle.eu> wrote:
>
> On Mon 25 Sep 2023 16:06:41 GMT, Masahiro Yamada wrote:
> > On Mon, Sep 18, 2023 at 4:20 AM Arnd Bergmann <arnd@kernel.org>
> > wrote:
> > >
> > > From: Arnd Bergmann <arnd@arndb.de>
> > >
> > > This problem frequently comes up in randconfig testing, with
> > > drivers failing to link because of a dependency on an optional
> > > feature.
> > >
> > > The Kconfig language for this is very confusing, so try to
> > > document it in "Kconfig hints" section.
> > >
> > > Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
> > > Reviewed-by: Sakari Ailus <sakari.ailus@linux.intel.com>
> > > Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>
> > > Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> > > ---
> > > v2: fix typos pointed out by Nicolas Schier
> >
> >
> > Applied to linux-kbuild. Thanks.
>
> Hi Masahiro,
>
> this patch seems to got lost, or did you intentionally removed it
> again?
>
> Kind regards,
> Nicolas



Oh?

I can see it in the mainline.




commit 28d49e171676afb7df7f47798391364af9abed7f
Author:     Arnd Bergmann <arnd@arndb.de>
AuthorDate: Sun Sep 17 21:19:59 2023 +0200
Commit:     Masahiro Yamada <masahiroy@kernel.org>
CommitDate: Mon Sep 25 16:01:05 2023 +0900

    Documentation: kbuild: explain handling optional dependencies





-- 
Best Regards
Masahiro Yamada

On Thu 15 Feb 2024 22:40:46 GMT, Masahiro Yamada wrote:
> On Thu, Feb 15, 2024 at 9:39 PM Nicolas Schier <nicolas@fjasle.eu> 
> wrote:
> >
> > On Mon 25 Sep 2023 16:06:41 GMT, Masahiro Yamada wrote:
> > > On Mon, Sep 18, 2023 at 4:20 AM Arnd Bergmann <arnd@kernel.org>
> > > wrote:
> > > >
> > > > From: Arnd Bergmann <arnd@arndb.de>
> > > >
> > > > This problem frequently comes up in randconfig testing, with
> > > > drivers failing to link because of a dependency on an optional
> > > > feature.
> > > >
> > > > The Kconfig language for this is very confusing, so try to
> > > > document it in "Kconfig hints" section.
> > > >
> > > > Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
> > > > Reviewed-by: Sakari Ailus <sakari.ailus@linux.intel.com>
> > > > Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>
> > > > Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> > > > ---
> > > > v2: fix typos pointed out by Nicolas Schier
> > >
> > >
> > > Applied to linux-kbuild. Thanks.
> >
> > Hi Masahiro,
> >
> > this patch seems to got lost, or did you intentionally removed it
> > again?
> >
> > Kind regards,
> > Nicolas
> 
> 
> 
> Oh?
> 
> I can see it in the mainline.

ups, I'm sorry for the noise.  You are right, I had an heavily 
out-dated tree at hand and wasn't aware of it ...

Kind regards,
Nicolas