Adapt bootlargs file to support btrfs snapshotter

Signed-off-by: David Cassany <dcassany@suse.com>

.obs/specfile/elemental.spec

@@ -142,6 +142,8 @@ rm -rf %{buildroot}/usr/libexec/.placeholder
 %defattr(-,root,root,-)
 %doc README.md
 %license LICENSE
+%dir %{_sysconfdir}/elemental/config.d
+%config %{_sysconfdir}/elemental/config.d/snapshotter.yaml 
 %dir %{_sysconfdir}/cos
 %config %{_sysconfdir}/cos/bootargs.cfg
 %dir %{_sysconfdir}/dracut.conf.d

framework/files/etc/cos/bootargs.cfg

@@ -1,7 +1,27 @@
-set kernel=/boot/vmlinuz
+# bootargs.cfg inherits from grub.cfg several context variables:
+#   'img' => defines the image path to boot from. Active img is statically defined, does not require a value
+#   'state_label' => label of the state partition filesystem
+#   'oem_label' => label of the oem partition filesystem
+#   'recovery_label' => label of the recovery partition filesystem
+#   'snapshotter' => snapshotter type, assumes loopdevice type if undefined
+#
+# In addition bootargs.cfg is responsible of setting the following variables:
+#   'kernelcmd' => essential kernel command line parameters (all elemental specific and non elemental specific)
+#   'kernel' => kernel binary path within the target image
+#   'initramfs' => initramfs binary path within the target image
+
+if [ -n "${img}" ]; then
+  set img_arg="elemental.image=${img}"
+fi
+
 if [ "${mode}" == "recovery" ]; then
-  set kernelcmd="console=tty1 root=LABEL=$recovery_label elemental.image=${img} rd.neednet=0 elemental.oemlabel=$oem_label"
+  set kernelcmd="console=tty1 root=LABEL=${recovery_label} ${img_arg} elemental.mode=${mode} rd.neednet=0 elemental.oemlabel=${oem_label}"
 else
-  set kernelcmd="console=tty1 root=LABEL=$state_label elemental.image=${img} panic=5 rd.neednet=0 elemental.oemlabel=$oem_label fsck.mode=force fsck.repair=yes"
+  if [ "${snapshotter}" == "btrfs" ]; then
+    set snap_arg="elemental.snapshotter=btrfs"
+  fi
+  set kernelcmd="console=tty1 root=LABEL=${state_label} ${img_arg} ${snap_arg} elemental.mode=${mode} panic=5 rd.neednet=0 elemental.oemlabel=${oem_label} fsck.mode=force fsck.repair=yes"
 fi
+
 set initramfs=/boot/initrd
+set kernel=/boot/vmlinuz

framework/files/etc/elemental/config.d/snapshotter.yaml

@@ -0,0 +1,3 @@
+snapshotter:
+  type: btrfs
+  max-snaps: 4