bsp-guide, kernel-dev: Updates to how kernel metadata is found

Fixes [YOCTO #10946]

There was insufficient information in the combination of the
BSP Guide and the Kernel Development Manual on just how to locate
and use kernel metadata.

 * bsp-guide - Removed the detailed append file example for the
   kernel recipe.  This is moved now to the chapter in the kernel
   manual that describes append files.

 * kernel-dev - Placed the example from the BSP Guide into the
   section that describes kernel append files.  Cleaned up some
   terminology issues throughout chapter 3.  Added information
   about how BitBake picks up kernel metadata when the metadata
   is in a hierarchical directory and not just a simple *.scc
   file.

(From yocto-docs rev: 1048acb7127e77ca9c1f524a208fe25344fcb57c)

Signed-off-by: Scott Rifenbark <srifenbark@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

1 files changed, 147 insertions, 2 deletions

diff --git a/documentation/kernel-dev/kernel-dev-common.xml b/documentation/kernel-dev/kernel-dev-common.xml
index a9aafd3c21..d49aa3ce17 100644
--- a/documentation/kernel-dev/kernel-dev-common.xml
+++ b/documentation/kernel-dev/kernel-dev-common.xml
@@ -84,11 +84,11 @@
                 You also name it accordingly based on the linux-yocto recipe
                 you are using.
                 For example, if you are modifying the
-                <filename>meta/recipes-kernel/linux/linux-yocto_3.19.bb</filename>
+                <filename>meta/recipes-kernel/linux/linux-yocto_4.4.bb</filename>
                 recipe, the append file will typically be located as follows
                 within your custom layer:
                 <literallayout class='monospaced'>
-     <replaceable>your-layer</replaceable>/recipes-kernel/linux/linux-yocto_3.19.bbappend
+     <replaceable>your-layer</replaceable>/recipes-kernel/linux/linux-yocto_4.4.bbappend
                 </literallayout>
                 The append file should initially extend the
                 <ulink url='&YOCTO_DOCS_REF_URL;#var-FILESPATH'><filename>FILESPATH</filename></ulink>
@@ -114,6 +114,151 @@
                     <ulink url='&YOCTO_DOCS_BSP_URL;'>Yocto Project Board Support Package (BSP) Developer's Guide</ulink>.
                 </note>
             </para>
+
+            <para>
+                As an example, consider the following append file
+                used by the BSPs in <filename>meta-yocto-bsp</filename>:
+                <literallayout class='monospaced'>
+     meta-yocto-bsp/recipes-kernel/linux/linux-yocto_4.4.bbappend
+                </literallayout>
+                The following listing shows the file.
+                Be aware that the actual commit ID strings in this
+                example listing might be different than the actual strings
+                in the file from the <filename>meta-yocto-bsp</filename>
+                layer upstream.
+                <literallayout class='monospaced'>
+     KBRANCH_genericx86  = "standard/base"
+     KBRANCH_genericx86-64  = "standard/base"
+
+     KMACHINE_genericx86 ?= "common-pc"
+     KMACHINE_genericx86-64 ?= "common-pc-64"
+     KBRANCH_edgerouter = "standard/edgerouter"
+     KBRANCH_beaglebone = "standard/beaglebone"
+     KBRANCH_mpc8315e-rdb = "standard/fsl-mpc8315e-rdb"
+
+     SRCREV_machine_genericx86    ?= "ad8b1d659ddd2699ebf7d50ef9de8940b157bfc2"
+     SRCREV_machine_genericx86-64 ?= "ad8b1d659ddd2699ebf7d50ef9de8940b157bfc2"
+     SRCREV_machine_edgerouter ?= "cebe1ad56aebd89e0de29412e19433fb441bf13c"
+     SRCREV_machine_beaglebone ?= "cebe1ad56aebd89e0de29412e19433fb441bf13c"
+     SRCREV_machine_mpc8315e-rdb ?= "06c0dbdcba374ca7f92a53d69292d6bb7bc9b0f3"
+
+     COMPATIBLE_MACHINE_genericx86 = "genericx86"
+     COMPATIBLE_MACHINE_genericx86-64 = "genericx86-64"
+     COMPATIBLE_MACHINE_edgerouter = "edgerouter"
+     COMPATIBLE_MACHINE_beaglebone = "beaglebone"
+     COMPATIBLE_MACHINE_mpc8315e-rdb = "mpc8315e-rdb"
+
+     LINUX_VERSION_genericx86 = "4.4.41"
+     LINUX_VERSION_genericx86-64 = "4.4.41"
+     LINUX_VERSION_edgerouter = "4.4.53"
+     LINUX_VERSION_beaglebone = "4.4.53"
+     LINUX_VERSION_mpc8315e-rdb = "4.4.53"
+                </literallayout>
+                This append file contains statements used to support
+                several BSPs that ship with the Yocto Project.
+                The file defines machines using the
+                <ulink url='&YOCTO_DOCS_REF_URL;#var-COMPATIBLE_MACHINE'><filename>COMPATIBLE_MACHINE</filename></ulink>
+                variable and uses the
+                <ulink url='&YOCTO_DOCS_REF_URL;#var-KMACHINE'><filename>KMACHINE</filename></ulink>
+                variable to ensure the machine name used by the OpenEmbedded
+                build system maps to the machine name used by the Linux Yocto
+                kernel.
+                The file also uses the optional
+                <ulink url='&YOCTO_DOCS_REF_URL;#var-KBRANCH'><filename>KBRANCH</filename></ulink>
+                variable to ensure the build process uses the
+                appropriate kernel branch.
+            </para>
+
+            <para>
+                Although this particular example does not use it, the
+                <ulink url='&YOCTO_DOCS_REF_URL;#var-KERNEL_FEATURES'><filename>KERNEL_FEATURES</filename></ulink>
+                variable could be used to enable features specific to
+                the kernel.
+                The append file points to specific commits in the
+                <ulink url='&YOCTO_DOCS_DEV_URL;#source-directory'>Source Directory</ulink>
+                Git repository and the <filename>meta</filename> Git repository
+                branches to identify the exact kernel needed to build the
+                BSP.
+            </para>
+
+            <para>
+                One thing missing in this particular BSP, which you will
+                typically need when developing a BSP, is the kernel configuration
+                file (<filename>.config</filename>) for your BSP.
+                When developing a BSP, you probably have a kernel configuration
+                file or a set of kernel configuration files that, when taken
+                together, define the kernel configuration for your BSP.
+                You can accomplish this definition by putting the configurations
+                in a file or a set of files inside a directory located at the
+                same level as your kernel's append file and having the same
+                name as the kernel's main recipe file.
+                With all these conditions met, simply reference those files in the
+                <ulink url='&YOCTO_DOCS_REF_URL;#var-SRC_URI'><filename>SRC_URI</filename></ulink>
+                statement in the append file.
+            </para>
+
+            <para>
+                For example, suppose you had some configuration options
+                in a file called <filename>network_configs.cfg</filename>.
+                You can place that file inside a directory named
+                <filename>linux-yocto</filename> and then add
+                a <filename>SRC_URI</filename> statement such as the
+                following to the append file.
+                When the OpenEmbedded build system builds the kernel, the
+                configuration options are picked up and applied.
+                <literallayout class='monospaced'>
+     SRC_URI += "file://network_configs.cfg"
+                </literallayout>
+            </para>
+
+            <para>
+                To group related configurations into multiple files, you
+                perform a similar procedure.
+                Here is an example that groups separate configurations
+                specifically for Ethernet and graphics into their own
+                files and adds the configurations by using a
+                <filename>SRC_URI</filename> statement like the following
+                in your append file:
+                <literallayout class='monospaced'>
+     SRC_URI += "file://myconfig.cfg \
+                 file://eth.cfg \
+                 file://gfx.cfg"
+                </literallayout>
+            </para>
+
+            <para>
+                Another variable you can use in your kernel recipe append
+                file is the
+                <ulink url='&YOCTO_DOCS_REF_URL;#var-FILESEXTRAPATHS'><filename>FILESEXTRAPATHS</filename></ulink>
+                variable.
+                When you use this statement, you are extending the locations
+                used by the OpenEmbedded system to look for files and
+                patches as the recipe is processed.
+            </para>
+
+            <note>
+                <para>
+                    Other methods exist to accomplish grouping and defining configuration options.
+                    For example, if you are working with a local clone of the kernel repository,
+                    you could checkout the kernel's <filename>meta</filename> branch, make your changes,
+                    and then push the changes to the local bare clone of the kernel.
+                    The result is that you directly add configuration options to the
+                    <filename>meta</filename> branch for your BSP.
+                    The configuration options will likely end up in that location anyway if the BSP gets
+                    added to the Yocto Project.
+                </para>
+
+                <para>
+                    In general, however, the Yocto Project maintainers take care of moving the
+                    <filename>SRC_URI</filename>-specified
+                    configuration options to the kernel's <filename>meta</filename> branch.
+                    Not only is it easier for BSP developers to not have to worry about putting those
+                    configurations in the branch, but having the maintainers do it allows them to apply
+                    'global' knowledge about the kinds of common configuration options multiple BSPs in
+                    the tree are typically using.
+                    This allows for promotion of common configurations into common features.
+                </para>
+            </note>
         </section>
 
         <section id='applying-patches'>