ELinOS comes with range of selected hardware boards supported out of box. To extend possibilities for the customers, ELinOS allows the use of external Linux kernels instead of the integrated ones.

At present, big part of vendors provide their BSPs as overlay for Yocto Linux distribution – which usually also includes a custom Linux kernel branch. However some issues must still be considered – among others,  the vendor BSPs are often not synchronized/merged with stable kernel releases. This task is left up to the customer to handle. Additionally, the more complex hardware devices often require several user-space components, some of them even proprietary.

Integrating the Linux kernel tree in ELinOS project only requires a few steps. The advantage resulting from this are the fast turn-around times and the graphical configuration editors that ELinOS provides also in this configuration.

Initial BSP Preparation

The only inherent limitation in using external BSPs is that each ELinOS version requires a minimal kernel version – in case of ELinOS 7.0 it is v4.9. The ELinOS glibc is cross-compiled against this version thus using an older kernel version is not supported.

We will show on the example of the i.MX 8 QuadMax Multisensory Enablement Kit board how to setup the Yocto Linux kernel in ELinOS. After consulting the kernel recipe in https://source.codeaurora.org/external/imx/meta-imx/tree/meta-bsp/recipes-kernel/linux?h=gatesgarth-5.10.9-1.0.0, we will take the kernel tree https://source.codeaurora.org/external/imx/linux-imx and tag lf-5.10.y-1.0.0, and integrate that into the so-called system project.

System Configuration

We start by cloning the kernel GIT repository:

The next step, creating an ELinOS system project, helps with integrating the kernel with user space binaries. For this, CODEO will be used, which is the SYSGO Eclipse-based project configuration/build tool. Create a new system project and in the “Target Properties” dialogue, fill in the respective entries:

When using the Generic Platform BSP ELinOS cannot automatically enable the board specific drivers and architecture options. Thus an initial kernel config file or default config should be provided. The suitable initial Linux kernel config can usually be found in the Yocto recipe, or alternatively in the vendor BSP documentation.

Now that the project is prepared, some additional configuration may be done. ELinOS Feature Configurator allows controlling various aspects of the system – both regarding kernel and user-space features. When using the Generic Platform BSP board-specific drivers are not available. For this purpose you can specify an initial vendor specific configuration. This configuration should contain the required drivers for the board.

A quite common issue with vendor kernels is that not all valid kernel configurations have been tested. In particular, quite often the board specific kernel configuration for the drivers cannot be modified. For example disabling the Power Management interface often results in a non-compilable kernel. Thus make sure to enable required options such as SMP support and Power Management support in the ELinOS Feature Configurator, which consequently also adjust the kernel configuration:

To manually adjust the kernel options, the built-in ELinOS Kernel Configurator can be used. Alternatively also the standard kernel configurator tool can be used by calling for example ‘elinos-buildkernel xconfig‘ from within the project directory. In this case, we will enable the missing IMX SCU Protocol driver along with its Mailbox dependency, and disable the nouveau driver (because it’s huge and unused on the i.MX8 board):

Board Startup

Finally, once the project has been built, the images are present in the project’s directory:

One of them is the kernel image combined with initramfs (root filesystem), the other one is the device tree that the u-boot may modify slightly to match the board configuration. Note that the base zImage generated is 23 MiB including 5MiB for the BusyBox based root filesystem. The kernel size can be reduced significantly by removing various unused features. For this purpose you can have a look at the unused media and ethernet drivers.

To finally boot the built system using the pre-installed u-boot, for example a TFTP server may be used. The bootloader downloads the binaries and on Aarch64, the booti command is usually used to put it all together:

More information also at www.sysgo.com/elinos