README.cross

   1 Dracut supports running against a sysroot directory that is different
   2 from the actual root (/) directory of the running system. It is most
   3 useful for creating/bootstrapping a new system that may or may not be
   4 using the same CPU architecture as the host system, i.e. building a
   5 whole Linux system with a cross-compiler framework like Yocto.
   6
   7 The user-visible frontend change is the introduction of a new option
   8 called "-r" or "--sysroot". It expects a directory that contains the
   9 complete Linux system that has all the files (kernel drivers, firmware,
  10 executables, libraries and others) necessary to construct the initramfs.
  11
  12 E.g: dracut --sysroot /path/to/sysroot initramfs.img kernelversion
  13
  14 To support this, a new global variable was introduced inside dracut.
  15 This variable is called "dracutsysrootdir" and all the files installed
  16 into the initramfs image is searched relative to the sysroot directory.
  17 This variable can also be set externally to dracut without using option
  18 -r/--sysroot.
  19
  20 There are other details that are necessary to tweak to be able to
  21 run on cross-compiled (a.k.a. foreign) binaries.
  22
  23 dracut uses these crucial utilities during its operation:
  24
  25 ldd
  26 ===
  27 It lists dynamic library dependencies for executables or libraries
  28
  29 ldconfig
  30 ========
  31 It creates /etc/ld.so.cache, i.e. the cached information about libraries
  32 known to the system.
  33
  34 These utilities the way they exist on the host system only work on
  35 the host system.
  36
  37 To support cross-compiled binaries, a different ldd variant is needed that
  38 works on those binaries. One such ldd script is found at
  39 https://gist.github.com/jerome-pouiller/c403786c1394f53f44a3b61214489e6f
  40
  41 ldconfig in GLIBC as is does support a sysroot with its -r option.
  42
  43 Extra environment variables needed to run dracut on the sysroot are
  44 documented in the dracut(8) man page.
  45
  46 For the Plymouth boot splash to be added to the initramfs image,
  47 this gitlab PR is needed for Plymouth:
  48 https://gitlab.freedesktop.org/plymouth/plymouth/merge_requests/72
  49
  50 Extensions to be used with Yocto
  51 ================================
  52
  53 examples/yocto/classes/dracut.bbclass:
  54   Bitbake class for running dracut during the "do_rootfs" stage of
  55   creating a distribution image and from the postinstall script of
  56   a package. A kernel recipe needs this line to activate it:
  57
  58   inherit dracut
  59
  60 examples/yocto/postinst-intercepts/execute_dracut
  61   The "intercept" script used during "do_rootfs" mentioned above.
  62
  63 examples/yocto/recipes/cross-compiler-ldd_git.bb
  64   Package recipe for the cross-compiler-ldd script from the above link.
  65
  66 All the above three need to be added to a Yocto layer to their
  67 proper locations.
  68
  69 They were tested on Yocto 2.7 with MACHINE="intel-core2-32" and
  70 MACHINE="genericx86-64".