3.10.3. Building an Initial RAM Filesystem (initramfs) Image

An initial RAM filesystem (initramfs) image provides a temporary root filesystem used for early system initialization (e.g. loading of modules needed to locate and mount the "real" root filesystem).

Note

The initramfs image is the successor of initial RAM disk (initrd). It is a "copy in and out" (cpio) archive of the initial filesystem that gets loaded into memory during the Linux startup process. Because Linux uses the contents of the archive during initialization, the initramfs image needs to contain all of the device drivers and tools needed to mount the final root filesystem.

Follow these steps to create an initramfs image:

  1. Create the initramfs Image Recipe: You can reference the core-image-minimal-initramfs.bb recipe found in the meta/recipes-core directory of the Source Directory as an example from which to work.

  2. Decide if You Need to Bundle the initramfs Image Into the Kernel Image: If you want the initramfs image that is built to be bundled in with the kernel image, set the INITRAMFS_IMAGE_BUNDLE variable to "1" in your local.conf configuration file and set the INITRAMFS_IMAGE variable in the recipe that builds the kernel image.

    Tip

    It is recommended that you do bundle the initramfs image with the kernel image to avoid circular dependencies between the kernel recipe and the initramfs recipe should the initramfs image include kernel modules.

    Setting the INITRAMFS_IMAGE_BUNDLE flag causes the initramfs image to be unpacked into the ${B}/usr/ directory. The unpacked initramfs image is then passed to the kernel's Makefile using the CONFIG_INITRAMFS_SOURCE variable, allowing the initramfs image to be built into the kernel normally.

    Note

    If you choose to not bundle the initramfs image with the kernel image, you are essentially using an Initial RAM Disk (initrd). Creating an initrd is handled primarily through the INITRD_IMAGE, INITRD_LIVE, and INITRD_IMAGE_LIVE variables. For more information, see the image-live.bbclass file.

  3. Optionally Add Items to the initramfs Image Through the initramfs Image Recipe: If you add items to the initramfs image by way of its recipe, you should use PACKAGE_INSTALL rather than IMAGE_INSTALL. PACKAGE_INSTALL gives more direct control of what is added to the image as compared to the defaults you might not necessarily want that are set by the image or core-image classes.

  4. Build the Kernel Image and the initramfs Image: Build your kernel image using BitBake. Because the initramfs image recipe is a dependency of the kernel image, the initramfs image is built as well and bundled with the kernel image if you used the INITRAMFS_IMAGE_BUNDLE variable described earlier.