device: introduce generic function to inherit MTU from parent

Introduce a generic function to set a MTU based on parent's one. Also
define a device-specific @mtu_parent_delta value that specifies the
difference from parent MTU that should be set by default. For VLAN it
is zero but other interface types (for example MACsec) require a
positive value due to encapsulation overhead.

(cherry picked from commit 5cf57f452233b1211fcff7c8cf689d9d5cb48092)
(cherry picked from commit 73597864bb0a229ce33d9072a3f73f96443f1fe7)

4 files changed, 36 insertions, 21 deletions

diff --git a/src/devices/nm-device-private.h b/src/devices/nm-device-private.h
index dd98e92f73..667ca6fe0f 100644
--- a/src/devices/nm-device-private.h
+++ b/src/devices/nm-device-private.h
@@ -177,6 +177,9 @@ guint32 nm_device_get_configured_mtu_from_connection (NMDevice *device,
 
 guint32 nm_device_get_configured_mtu_for_wired (NMDevice *self, NMDeviceMtuSource *out_source);
 
+guint32 nm_device_get_configured_mtu_wired_parent (NMDevice *self,
+                                                   NMDeviceMtuSource *out_source);
+
 void nm_device_commit_mtu (NMDevice *self);
 
 /*****************************************************************************/
diff --git a/src/devices/nm-device-vlan.c b/src/devices/nm-device-vlan.c
index ace6a24bdb..e0f2bd324e 100644
--- a/src/devices/nm-device-vlan.c
+++ b/src/devices/nm-device-vlan.c
@@ -523,26 +523,6 @@ act_stage1_prepare (NMDevice *device, NMDeviceStateReason *out_failure_reason)
 	return ret;
 }
 
-static guint32
-get_configured_mtu (NMDevice *self, NMDeviceMtuSource *out_source)
-{
-	guint32 mtu = 0;
-	int ifindex;
-
-	mtu = nm_device_get_configured_mtu_for_wired (self, out_source);
-	if (*out_source != NM_DEVICE_MTU_SOURCE_NONE)
-		return mtu;
-
-	/* Inherit the MTU from parent device, if any */
-	ifindex = nm_device_parent_get_ifindex (self);
-	if (ifindex > 0) {
-		mtu = nm_platform_link_get_mtu (nm_device_get_platform (NM_DEVICE (self)), ifindex);
-		*out_source = NM_DEVICE_MTU_SOURCE_PARENT;
-	}
-
-	return mtu;
-}
-
 /*****************************************************************************/
 
 static void
@@ -598,13 +578,14 @@ nm_device_vlan_class_init (NMDeviceVlanClass *klass)
 	device_class->connection_type_supported = NM_SETTING_VLAN_SETTING_NAME;
 	device_class->connection_type_check_compatible = NM_SETTING_VLAN_SETTING_NAME;
 	device_class->link_types = NM_DEVICE_DEFINE_LINK_TYPES (NM_LINK_TYPE_VLAN);
+	device_class->mtu_parent_delta = 0; /* VLANs can have the same MTU of parent */
 
 	device_class->create_and_realize = create_and_realize;
 	device_class->link_changed = link_changed;
 	device_class->unrealize_notify = unrealize_notify;
 	device_class->get_generic_capabilities = get_generic_capabilities;
 	device_class->act_stage1_prepare = act_stage1_prepare;
-	device_class->get_configured_mtu = get_configured_mtu;
+	device_class->get_configured_mtu = nm_device_get_configured_mtu_wired_parent;
 	device_class->is_available = is_available;
 	device_class->parent_changed_notify = parent_changed_notify;
 
diff --git a/src/devices/nm-device.c b/src/devices/nm-device.c
index 570c6dc5c7..f6c709d317 100644
--- a/src/devices/nm-device.c
+++ b/src/devices/nm-device.c
@@ -9075,6 +9075,33 @@ nm_device_get_configured_mtu_for_wired (NMDevice *self, NMDeviceMtuSource *out_s
 	                                                     out_source);
 }
 
+guint32
+nm_device_get_configured_mtu_wired_parent (NMDevice *self,
+                                           NMDeviceMtuSource *out_source)
+{
+	guint32 mtu = 0;
+	int ifindex;
+
+	mtu = nm_device_get_configured_mtu_for_wired (self, out_source);
+	if (*out_source != NM_DEVICE_MTU_SOURCE_NONE) {
+		nm_assert (mtu > 0);
+		return mtu;
+	}
+
+	/* Inherit the MTU from parent device, if any */
+	ifindex = nm_device_parent_get_ifindex (self);
+	if (ifindex > 0) {
+		mtu = nm_platform_link_get_mtu (nm_device_get_platform (self), ifindex);
+		if (mtu >= NM_DEVICE_GET_CLASS (self)->mtu_parent_delta) {
+			mtu -= NM_DEVICE_GET_CLASS (self)->mtu_parent_delta;
+			*out_source = NM_DEVICE_MTU_SOURCE_PARENT;
+		} else
+			mtu = 0;
+	}
+
+	return mtu;
+}
+
 /*****************************************************************************/
 
 static void
diff --git a/src/devices/nm-device.h b/src/devices/nm-device.h
index d11c33ab46..fee3b7e563 100644
--- a/src/devices/nm-device.h
+++ b/src/devices/nm-device.h
@@ -240,6 +240,10 @@ typedef struct _NMDeviceClass {
 
 	const NMLinkType *link_types;
 
+	/* if the device MTU is set based on parent's one, this specifies
+	 * a delta in the MTU allowed value due the encapsulation overhead */
+	guint16 mtu_parent_delta;
+
 	/* Whether the device type is a master-type. This depends purely on the
 	 * type (NMDeviceClass), not the actual device instance. */
 	bool is_master:1;