dsa: Allow core and drivers to be built as modules

Change the kconfig types to tristate and adjust the condition for
declaring net_device::dsa_ptr to allow for this.

Adjust the makefile so that if NET_DSA_MV88E6123_61_65=y and
NET_DSA_MV88E6131=m or vice versa then both drivers are built-in.  We
could leave these options as bool and make NET_DSA_MV88E6XXX a
user-selected option, but that would break existing configurations.
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/netdevice.h

@@ -1079,7 +1079,7 @@ struct net_device {
 #if defined(CONFIG_VLAN_8021Q) || defined(CONFIG_VLAN_8021Q_MODULE)
 	struct vlan_group __rcu	*vlgrp;		/* VLAN group */
 #endif
-#ifdef CONFIG_NET_DSA
+#if IS_ENABLED(CONFIG_NET_DSA)
 	struct dsa_switch_tree	*dsa_ptr;	/* dsa specific data */
 #endif
 	void 			*atalk_ptr;	/* AppleTalk link 	*/

net/dsa/Kconfig

 menuconfig NET_DSA
-	bool "Distributed Switch Architecture support"
+	tristate "Distributed Switch Architecture support"
 	default n
 	depends on EXPERIMENTAL && NETDEVICES && !S390
 	select PHYLIB
@@ -26,11 +26,11 @@ config NET_DSA_TAG_TRAILER
 
 # switch drivers
 config NET_DSA_MV88E6XXX
-	bool
+	tristate
 	default n
 
 config NET_DSA_MV88E6060
-	bool "Marvell 88E6060 ethernet switch chip support"
+	tristate "Marvell 88E6060 ethernet switch chip support"
 	select NET_DSA_TAG_TRAILER
 	---help---
 	  This enables support for the Marvell 88E6060 ethernet switch
@@ -41,7 +41,7 @@ config NET_DSA_MV88E6XXX_NEED_PPU
 	default n
 
 config NET_DSA_MV88E6131
-	bool "Marvell 88E6085/6095/6095F/6131 ethernet switch chip support"
+	tristate "Marvell 88E6085/6095/6095F/6131 ethernet switch chip support"
 	select NET_DSA_MV88E6XXX
 	select NET_DSA_MV88E6XXX_NEED_PPU
 	select NET_DSA_TAG_DSA
@@ -50,7 +50,7 @@ config NET_DSA_MV88E6131
 	  ethernet switch chips.
 
 config NET_DSA_MV88E6123_61_65
-	bool "Marvell 88E6123/6161/6165 ethernet switch chip support"
+	tristate "Marvell 88E6123/6161/6165 ethernet switch chip support"
 	select NET_DSA_MV88E6XXX
 	select NET_DSA_TAG_EDSA
 	---help---

net/dsa/Makefile

@@ -11,5 +11,9 @@ dsa_core-$(CONFIG_NET_DSA_TAG_TRAILER) += tag_trailer.o
 obj-$(CONFIG_NET_DSA_MV88E6060) += mv88e6060.o
 obj-$(CONFIG_NET_DSA_MV88E6XXX) += mv88e6xxx_drv.o
 mv88e6xxx_drv-y += mv88e6xxx.o
-mv88e6xxx_drv-$(CONFIG_NET_DSA_MV88E6123_61_65) += mv88e6123_61_65.o
-mv88e6xxx_drv-$(CONFIG_NET_DSA_MV88E6131) += mv88e6131.o
+ifdef CONFIG_NET_DSA_MV88E6123_61_65
+mv88e6xxx_drv-y += mv88e6123_61_65.o
+endif
+ifdef CONFIG_NET_DSA_MV88E6131
+mv88e6xxx_drv-y += mv88e6131.o
+endif