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Diffstat (limited to 'documentation/dev-manual/dev-manual-common-tasks.xml')
| -rw-r--r-- | documentation/dev-manual/dev-manual-common-tasks.xml | 73 |
1 files changed, 73 insertions, 0 deletions
diff --git a/documentation/dev-manual/dev-manual-common-tasks.xml b/documentation/dev-manual/dev-manual-common-tasks.xml index 603ca1ff8e..f3f2a4b026 100644 --- a/documentation/dev-manual/dev-manual-common-tasks.xml +++ b/documentation/dev-manual/dev-manual-common-tasks.xml @@ -5475,6 +5475,79 @@ </section> </section> + <section id='building-an-initramfs-image'> + <title>Building an Initial RAM Filesystem (initramfs) Image</title> + + <para> + initramfs is the successor of Initial RAM Disk (initrd). + It is a "copy in and out" (cpio) archive of the initial file system + that gets loaded into memory during the Linux startup process. + Because Linux uses the contents of the archive during + initialization, the initramfs needs to contain all of the device + drivers and tools needed to mount the final root filesystem. + </para> + + <para> + To build an initramfs image and bundle it into the kernel, set the + following variables: + <literallayout class='monospaced'> + INITRAMFS_IMAGE_BUNDLE = "1" + INITRAMFS_IMAGE = "<replaceable>image_recipe_name</replaceable>" + </literallayout> + Setting the + <ulink url='&YOCTO_DOCS_REF_URL;#var-INITRAMFS_IMAGE_BUNDLE'><filename>INITRAMFS_IMAGE_BUNDLE</filename></ulink> + flag causes the initramfs created by the recipe + and defined by + <ulink url='&YOCTO_DOCS_REF_URL;#var-INITRAMFS_IMAGE'><filename>INITRAMFS_IMAGE</filename></ulink> + to be unpacked into the <filename>${B}/usr/</filename> directory. + The unpacked initramfs is then passed to the kernel's + <filename>Makefile</filename> using the + <ulink url='&YOCTO_DOCS_REF_URL;#var-CONFIG_INITRAMFS_SOURCE'><filename>CONFIG_INITRAMFS_SOURCE</filename></ulink> + variable, allowing initramfs to be built in to the kernel + normally. + <note> + The preferred method is to use the + <filename>INITRAMFS_IMAGE</filename> variable rather than the + <filename>INITRAMFS_TASK</filename> variable. + Setting <filename>INITRAMFS_TASK</filename> is supported for + backward compatibility. + However, use of this variable has circular dependency + problems. + See the + <ulink url='&YOCTO_DOCS_REF_URL;#var-INITRAMFS_IMAGE_BUNDLE'><filename>INITRAMFS_IMAGE_BUNDLE</filename></ulink> + variable for additional information on these dependency + problems. + </note> + </para> + + <para> + The recipe that <filename>INITRAMFS_IMAGE</filename> + points to must produce a <filename>.cpio.gz</filename>, + <filename>.cpio.tar</filename>, <filename>.cpio.lz4</filename>, + <filename>.cpio.lzma</filename>, or + <filename>.cpio.xz</filename> file. + You can ensure you produce one of these <filename>.cpio.*</filename> + files by setting the + <ulink url='&YOCTO_DOCS_REF_URL;#var-IMAGE_FSTYPES'><filename>IMAGE_FSTYPES</filename></ulink> + variable in your configuration file to one or more of the above + file types. + <note> + If you add items to the initramfs image by way of its recipe, + you should use + <ulink url='&YOCTO_DOCS_REF_URL;#var-PACKAGE_INSTALL'><filename>PACKAGE_INSTALL</filename></ulink> + rather than + <ulink url='&YOCTO_DOCS_REF_URL;#var-IMAGE_INSTALL'><filename>IMAGE_INSTALL</filename></ulink>. + <filename>PACKAGE_INSTALL</filename> 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 + <ulink url='&YOCTO_DOCS_REF_URL;#ref-classes-image'><filename>image</filename></ulink> + or + <ulink url='&YOCTO_DOCS_REF_URL;#ref-classes-core-image'><filename>core-image</filename></ulink> + classes. + </note> + </para> + </section> + <section id='configuring-the-kernel'> <title>Configuring the Kernel</title> |