boot loader: support root=UUID

As mentioned when introducing the VM images
(https://bugzilla.yoctoproject.org/show_bug.cgi?id=7374), the
resulting images only work when the image is mounted as a disk that
results in the hard-coded path (/dev/sda in the current
default). Using the file system UUID to find the rootfs is more
flexible.

To enable this for boot-direct.bbclass and thus image-vm.bbclass (aka
FSTYPEs vdi/vmdk/qcow2), set SYSLINUX_ROOT =
"root=UUID=<<uuid-of-rootfs>>". The rootfs image must use an ext file
system.

The special string will get replaced in the APPEND line with the
actual UUID when the boot loader (grub-efi, syslinux or gummiboot)
writes the boot loader configuration files. At that time, the rootfs
image has already been created and its UUID can be extracted using
"tune2fs -l", which also should be available because the
e2fsprogs-native tools were needed to create the image in the first
place.

(From OE-Core rev: 1e29d77d0d33ee216b43022439876863f0db39bb)

Signed-off-by: Patrick Ohly <patrick.ohly@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

1 files changed, 24 insertions, 0 deletions

diff --git a/meta/classes/fs-uuid.bbclass b/meta/classes/fs-uuid.bbclass
new file mode 100644
index 0000000000..bd2613cf10
--- /dev/null
+++ b/meta/classes/fs-uuid.bbclass
@@ -0,0 +1,24 @@
+# Extract UUID from ${ROOTFS}, which must have been built
+# by the time that this function gets called. Only works
+# on ext file systems and depends on tune2fs.
+def get_rootfs_uuid(d):
+    import subprocess
+    rootfs = d.getVar('ROOTFS', True)
+    output = subprocess.check_output(['tune2fs', '-l', rootfs])
+    for line in output.split('\n'):
+        if line.startswith('Filesystem UUID:'):
+            uuid = line.split()[-1]
+            bb.note('UUID of %s: %s' % (rootfs, uuid))
+            return uuid
+    bb.fatal('Could not determine filesystem UUID of %s' % rootfs)
+
+# Replace the special <<uuid-of-rootfs>> inside a string (like the
+# root= APPEND string in a syslinux.cfg or gummiboot entry) with the
+# actual UUID of the rootfs. Does nothing if the special string
+# is not used.
+def replace_rootfs_uuid(d, string):
+    UUID_PLACEHOLDER = '<<uuid-of-rootfs>>'
+    if UUID_PLACEHOLDER in string:
+        uuid = get_rootfs_uuid(d)
+        string = string.replace(UUID_PLACEHOLDER, uuid)
+    return string