kernel-dev: document KCONFIG_MODE

(From yocto-docs rev: 12aa6f9c6af68ea03fbb056677213b00d693cf5f)

Signed-off-by: Daniel Wagenknecht <dwagenknecht@emlix.com>
Reviewed-by: Michael Opdenacker <michael.opdenacker@bootlin.com>
Reviewed-by: Quentin Schulz <foss@0leil.net>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

2 files changed, 47 insertions, 0 deletions

diff --git a/documentation/kernel-dev/common.rst b/documentation/kernel-dev/common.rst
index 3f35d8412f..5935d6f560 100644
--- a/documentation/kernel-dev/common.rst
+++ b/documentation/kernel-dev/common.rst
@@ -650,6 +650,15 @@ extends the :term:`FILESPATH`
 variable (search directories) to include the ``${PN}`` directory you
 created to hold the configuration changes.
 
+You can also use a regular ``defconfig`` file, as generated by the
+:ref:`ref-tasks-savedefconfig`
+task instead of a complete ``.config`` file. This only specifies the
+non-default configuration values.  You need to additionally set
+:term:`KCONFIG_MODE`
+in the linux-yocto ``.bbappend`` file in your layer::
+
+   KCONFIG_MODE = "alldefconfig"
+
 .. note::
 
    The build system applies the configurations from the ``defconfig``
diff --git a/documentation/ref-manual/variables.rst b/documentation/ref-manual/variables.rst
index c339d45e18..957362bb6d 100644
--- a/documentation/ref-manual/variables.rst
+++ b/documentation/ref-manual/variables.rst
@@ -3745,6 +3745,44 @@ system and gives an overview of their function and contents.
       ":ref:`kernel-dev/common:using an "in-tree" \`\`defconfig\`\` file`"
       section in the Yocto Project Linux Kernel Development Manual.
 
+   :term:`KCONFIG_MODE`
+      When used with the :ref:`kernel-yocto <ref-classes-kernel-yocto>`
+      class, specifies the kernel configuration values to use for options
+      not specified in the provided ``defconfig`` file. Valid options are::
+
+         KCONFIG_MODE = "alldefconfig"
+         KCONFIG_MODE = "allnoconfig"
+
+      In ``alldefconfig`` mode the options not explicitly specified will be
+      assigned their Kconfig default value. In ``allnoconfig`` mode the
+      options not explicitly specified will be disabled in the kernel
+      config.
+
+      In case ``KCONFIG_MODE`` is not set the behaviour will depend on where
+      the ``defconfig`` file is coming from. An "in-tree" ``defconfig`` file
+      will be handled in ``alldefconfig`` mode, a ``defconfig`` file placed
+      in ``${WORKDIR}`` through a meta-layer will be handled in
+      ``allnoconfig`` mode.
+
+      An "in-tree" ``defconfig`` file can be selected via the
+      :term:`KBUILD_DEFCONFIG` variable. ``KCONFIG_MODE`` does not need to
+      be explicitly set.
+
+      A ``defconfig`` file compatible with ``allnoconfig`` mode can be
+      generated by copying the ``.config`` file from a working Linux kernel
+      build, renaming it to ``defconfig`` and placing it into the Linux
+      kernel ``${WORKDIR}`` through your meta-layer. ``KCONFIG_MODE`` does
+      not need to be explicitly set.
+
+      A ``defconfig`` file compatible with ``alldefconfig`` mode can be
+      generated using the
+      :ref:`ref-tasks-savedefconfig`
+      task and placed into the Linux kernel ``${WORKDIR}`` through your
+      meta-layer. Explicitely set ``KCONFIG_MODE``::
+
+         KCONFIG_MODE = "alldefconfig"
+
+
    :term:`KERNEL_ALT_IMAGETYPE`
       Specifies an alternate kernel image type for creation in addition to
       the kernel image type specified using the