We recommend you create your own layer and make changes to the existing BSP there. This will make it easier to update the BSP. Instructions and tips on creating your own layer are available here: http://www.yoctoproject.org/docs/2.2/dev-manual/dev-manual.html#creating-your-own-layer.
Appending Recipes
To modify an existing recipe in your own layer, use a bbappend file. The following is an example of modifying the u-boot-phytec_2017.01 recipe, u-boot-phytec_2017.01.bb, located at $YOCTO_DIR/sources/meta-phytec/meta-phytec-ti/recipes-bsp/u-boot/u-boot-phytec_2017.01.bb.
Create a recipes-bsp/u-boot/ directory in your own meta-layer to place the bbappend file in. Make sure that the new file matches the .bb file name exactly. Alternatively, you may use % after the underscore in place of the specific version for portability with future versions of the recipe.
mkdir $YOCTO_DIR/sources/<YOUR_META_LAYER>/recipes-bsp/u-boot/ vim $YOCTO_DIR/sources/<YOUR_META_LAYER>/recipes-bsp/u-boot/u-boot-phytec_%.bbappend
For information on how to write a recipe, see chapter 5.3.4 of the Yocto Development Manual: http://www.yoctoproject.org/docs/2.2/dev-manual/dev-manual.html#understanding-recipe-syntax
Adding Packages to the build
There are various ways to add a package to the BSP. For example, packages and package groups can be added to image recipes. See the Yocto Development manual for how to customize an image: http://www.yoctoproject.org/docs/2.2/dev-manual/dev-manual.html#usingpoky-extend-customimage-imagefeatures.
The following instructions demonstrate how to add a package to the local build of the BSP. First, search for the corresponding recipe and which layer the recipe is in. This link is a useful tool for doing so: http://layers.openembedded.org/layerindex/branch/morty/layers/.
If the package is in the meta-openembedded layer, the recipe is already available in your build tree. Add the following line to $YOCTO_DIR/build/conf/local.conf:
IMAGE_INSTALL_append = " <package>"
The leading whitespace between the " and the package name is necessary for the append command.
If you need to add a layer to the BSP, clone or extract it to the $YOCTO_DIR/sources/ directory. Then, modify $YOCTO_DIR/build/conf/bblayers.conf to include this new layer in BBLAYERS:
BBLAYERS += "${BSPDIR}/sources/<new_layer>"
Configuring the Kernel
The kernel configuration menu allows the user to adjust drivers and support included in a Linux Kernel build. Run the following command from the build directory:
cd $YOCTO_DIR/build bitbake linux-phytec-ti -c menuconfig
Then rebuild the kernel:
bitbake linux-phytec-ti -f -c compile && bitbake linux-phytec-ti
To rebuild the root filesystem:
bitbake arago-core-tisdk-image
Customizing the Device Tree
The device tree is a data structure for describing hardware, and is a way of separating machine specific information from the kernel. For information on the device tree concept, devicetree.org is a good source: http://devicetree.org/Device_Tree_Usage.
Device trees for PHYTEC products consist of a board 'dts' file, a master SOM 'dtsi' file, a carrier board 'dtsi' file, SOM variant 'dtsi' files, and expansion board 'dtsi' files.
Board DTS file: Includes all 'dtsi' files that are to be built for this board target, and does not individually handle any nodes.
SOM DTSI file: Includes the processor 'dtsi' and contains and enables nodes for all buses/devices located on the SOM (e.g. eMMC flash).
Carrier Board DTSI file: Peripherals whose signals are routed through the SOM but whose hardware is located on the carrier board are defined and enabled in the carrier board 'dtsi' (e.g. SD card).
SOM Variant DTSI files: The SOM variant 'dtsi' files represent the differences between a SOM variant and the fully-featured SOM, or different memory configurations (e.g. disables QSPI NOR if not populated).
Expansion Board DTSI files: Expansion boards are additional boards that add support for additional features and are designed to plug into a specific interface/connector. These files include everything except the hardware pinmux to support the new board's features (e.g. wifi).
The kernel source directory has very good documentation and examples on what bindings are supported for specific peripherals: Documentation/devicetree/bindings/.