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From:   Geert Uytterhoeven <geert+renesas@glider.be>
To:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Masahiro Yamada <masahiroy@kernel.org>,
        Michal Marek <michal.lkml@markovi.net>,
        Jonathan Corbet <corbet@lwn.net>, Arnd Bergmann <arnd@arndb.de>
Cc:     linux-kbuild@vger.kernel.org, linux-doc@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        Geert Uytterhoeven <geert+renesas@glider.be>
Subject: [PATCH 2/2] Documentation/kbuild: Document platform dependency practises
Date:   Tue,  8 Dec 2020 16:28:57 +0100
Message-Id: <20201208152857.2162093-3-geert+renesas@glider.be>
In-Reply-To: <20201208152857.2162093-1-geert+renesas@glider.be>
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Document best practises for using architecture and platform dependencies.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
---
 Documentation/kbuild/kconfig-language.rst | 24 +++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/Documentation/kbuild/kconfig-language.rst b/Documentation/kbuild/kconfig-language.rst
index 2b746332d8aa6bce..87e9bbe14a21ce83 100644
--- a/Documentation/kbuild/kconfig-language.rst
+++ b/Documentation/kbuild/kconfig-language.rst
@@ -564,6 +564,30 @@ common system, and detect bugs that way.
 Note that compile-tested code should avoid crashing when run on a system where
 the dependency is not met.
 
+Architecture and platform dependencies
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Due to the presence of stubs, most drivers can now be compiled on most
+architectures.  However, this does not mean it makes sense to have all drivers
+available everywhere, as the actual hardware may only exist on specific
+architectures and platforms.  This is especially true for on-SoC IP cores,
+which may be limited to a specific vendor or SoC family.
+
+To prevent asking the user about drivers that cannot be used on the system(s)
+the user is compiling a kernel for, and if it makes sense, config symbols
+controlling the compilation of a driver should contain proper dependencies,
+limiting the visibility of the symbol to (a superset of) the platform(s) the
+driver can be used on.  The dependency can be an architecture (e.g. ARM) or
+platform (e.g. ARCH_OMAP4) dependency.  This makes life simpler not only for
+distro config owners, but also for every single developer or user who
+configures a kernel.
+
+Such a dependency can be relaxed by combining it with the compile-testing rule
+above, leading to:
+
+  config FOO
+	bool "Support for foo hardware"
+	depends on ARCH_FOO_VENDOR || COMPILE_TEST
+
 Kconfig recursive dependency limitations
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
-- 
2.25.1