Writing Bitbake Recipes

In order to package your application and include it in the root filesystem image, you must write a BitBake recipe for it.

When starting from scratch, it is easiest to learn by example from existing recipes. 

Example HelloWorld recipe using autotools

For software that uses autotools (./configure; make; make install), writing recipes can be very simple:

SRC_URI specifies the location to download the source from. It can take the form of any standard URL using http://, ftp://, etc. It can also fetch from SCM systems, such as git in the example above.

PR is the package revision variable. Any time a recipe is updated that should require the package to be rebuilt, this variable should be incremented.

inherit autotools brings in support for the package to be built using autotools, and thus no other instructions on how to compile and install the software are needed unless something needs to be customized.

S is the source directory variable. This specifies where the source code will exist after it is fetched from SRC_URI and unpacked. The default value is ${WORKDIR}/${PN}-${PV}, where PN is the package name and PV is the package version. Both PN and PV are set by default using the filename of the recipe, where the filename has the format PN_PV.bb.

Example HelloWorld recipe using a single source file

This example shows a simple case of building a helloworld.c file directly using the default compiler (gcc). Since it isn’t using autotools or make, we have to tell BitBake how to build it explicitly.

In this case, SRC_URI specifies a file that must exist locally with the recipe. Since there is no code to download and unpack, we set S to WORKDIR since that is where helloworld.c will be copied to before it is built.

WORKDIR is located at ${OETREE}/<build directory>/tmp/work/armv8a-poky-linux/<package name and version> for most packages. If the package is machine-specific (rather than generic for the armv8a architecture), it may be located in the smarcimx8mm4g-poky-linux subdirectory depending on your hardware (this applies to kernel packages, images, etc).

do_compile defines how to compile the source. In this case, we just call gcc directly. If it isn’t defined, do_compile runs make in the source directory by default.

do_install defines how to install the application. This example runs install to create a bin directory where the application will be copied to and then copies the application there with permissions set to 755.

D is the destination directory where the application is installed to before it is packaged.

${bindir} is the directory where most binary applications are installed, typically /usr/bin.

For a more in-depth explanation of BitBake recipes, syntax, and variables, see the Recipe Chapter of the OpenEmbedded User Manual.

Setup eMMC Manually

Setting up eMMC usually is the last step at development stage after the development work is done at your SD card or NFS environments. From software point of view, eMMC is nothing but a non-removable SD card on board. For SMARC-iMX8MM, the SD card is always emulated as /dev/mmcblk1 and on-module eMMC is always emulated as /dev/mmcblk0. Setting up eMMC now is nothing but changing the device descriptor. 

This section gives a step-by-step procedure to setup eMMC flash. Users can write a shell script your own at production to simplify the steps.

First, we need to backup the final firmware from your SD card or NFS.

Prepare for eMMC binaries from SD card (or NFS):

Insert SD card into your Linux PC. For these instructions, we are assuming: DISK=/dev/mmcblk0, "lsblk" is very useful for determining the device id.

For these instruction, we are assuming: DISK=/dev/mmcblk0, "lsblk" is very useful for determining the device id.

$ export DISK=/dev/mmcblk0

Mount Partitions:

On some systems, these partitions may be auto-mounted...

Copy Image to rootfs partition:

$ sudo cp -v Image /media/rootfs/home/root

Copy uEnv.txt to rootfs partition:

Copy and paste the following contents to /media/rootfs/home/root ($ sudo vim /media/rootfs/home/root/uEnv.txt)

Copy device tree blob to rootfs partition:

Copy boot file to rootfs partition:

Copy real rootfs to rootfs partition:

$ pushd /media/rootfs

$ sudo tar cvfz ~/smarcimx8mm-emmc-rootfs.tar.gz .

$ sudo mv ~/smarcimx8mm-emmc-rootfs.tar.gz /media/rootfs/home/root

$ popd

Remove SD card:

Copy Binaries to eMMC from SD card:

Insert this SD card into your SMARC-iMX8MM device.

Now it will be almost the same as you did when setup your SD card, but the eMMC device descriptor is /dev/mmcblk0 now. Booting up the device.

$ export DISK=/dev/mmcblk0

Erase eMMC:

Create Partition Layout:

Format Partitions:

$ sudo mkfs.vfat -F 16 ${DISK}p1 -n boot

$ sudo mkfs.ext4 ${DISK}p2 -L rootfs

Mount Partitions:

Install binaries for partition 1

Copy uEnv.txt/Image/*.dtb to the boot partition

Install Kernel Device Tree Binary

$ sudo mkdir -p /media/boot/dtbs

$ sudo cp -v imx8mm-smarc.dtb /media/boot/dtbs/

Install Root File System

$ sudo tar -zxvf smarcimx8mm-emmc-rootfs.tar.gz -C /media/rootfs