net: dsa: realtek: fix Kconfig to assure consistent driver linkage

The kernel test robot reported a build failure:

or1k-linux-ld: drivers/net/dsa/realtek/realtek-smi.o:(.rodata+0x16c): undefined reference to `rtl8366rb_variant'

... with the following build configuration:

CONFIG_NET_DSA_REALTEK=y
CONFIG_NET_DSA_REALTEK_SMI=y
CONFIG_NET_DSA_REALTEK_RTL8365MB=y
CONFIG_NET_DSA_REALTEK_RTL8366RB=m

The problem here is that the realtek-smi interface driver gets built-in,
while the rtl8366rb switch subdriver gets built as a module, hence the
symbol rtl8366rb_variant is not reachable when defining the OF device
table in the interface driver.

The Kconfig dependencies don't help in this scenario because they just
say that the subdriver(s) depend on at least one interface driver. In
fact, the subdrivers don't depend on the interface drivers at all, and
can even be built even in their absence. Somewhat strangely, the
interface drivers can also be built in the absence of any subdriver,
BUT, if a subdriver IS enabled, then it must be reachable according to
the linkage of the interface driver: effectively what the IS_REACHABLE()
macro achieves. If it is not reachable, the above kind of linker error
will be observed.

Rather than papering over the above build error by simply using
IS_REACHABLE(), we can do a little better and admit that it is actually
the interface drivers that have a dependency on the subdrivers. So this
patch does exactly that. Specifically, we ensure that:

1. The interface drivers' Kconfig symbols must have a value no greater
   than the value of any subdriver Kconfig symbols.

2. The subdrivers should by default enable both interface drivers, since
   most users probably want at least one of them; those interface
   drivers can be explicitly disabled however.

What this doesn't do is prevent a user from building only a subdriver,
without any interface driver. To that end, add an additional line of
help in the menu to guide users in the right direction.

Link: https://lore.kernel.org/all/202204110757.XIafvVnj-lkp@intel.com/Reported-by: kernel test robot <lkp@intel.com>
Fixes: aac94001 ("net: dsa: realtek: add new mdio interface for drivers")
Signed-off-by: Alvin Šipraga <alsi@bang-olufsen.dk>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/dsa/realtek/Kconfig

@@ -9,34 +9,46 @@ menuconfig NET_DSA_REALTEK
 	help
 	  Select to enable support for Realtek Ethernet switch chips.
 
+	  Note that at least one interface driver must be enabled for the
+	  subdrivers to be loaded. Moreover, an interface driver cannot achieve
+	  anything without at least one subdriver enabled.
+
+if NET_DSA_REALTEK
+
 config NET_DSA_REALTEK_MDIO
-	tristate "Realtek MDIO connected switch driver"
-	depends on NET_DSA_REALTEK
+	tristate "Realtek MDIO interface driver"
 	depends on OF
+	depends on NET_DSA_REALTEK_RTL8365MB || NET_DSA_REALTEK_RTL8366RB
+	depends on NET_DSA_REALTEK_RTL8365MB || !NET_DSA_REALTEK_RTL8365MB
+	depends on NET_DSA_REALTEK_RTL8366RB || !NET_DSA_REALTEK_RTL8366RB
 	help
 	  Select to enable support for registering switches configured
 	  through MDIO.
 
 config NET_DSA_REALTEK_SMI
-	tristate "Realtek SMI connected switch driver"
-	depends on NET_DSA_REALTEK
+	tristate "Realtek SMI interface driver"
 	depends on OF
+	depends on NET_DSA_REALTEK_RTL8365MB || NET_DSA_REALTEK_RTL8366RB
+	depends on NET_DSA_REALTEK_RTL8365MB || !NET_DSA_REALTEK_RTL8365MB
+	depends on NET_DSA_REALTEK_RTL8366RB || !NET_DSA_REALTEK_RTL8366RB
 	help
 	  Select to enable support for registering switches connected
 	  through SMI.
 
 config NET_DSA_REALTEK_RTL8365MB
 	tristate "Realtek RTL8365MB switch subdriver"
-	depends on NET_DSA_REALTEK
-	depends on NET_DSA_REALTEK_SMI || NET_DSA_REALTEK_MDIO
+	imply NET_DSA_REALTEK_SMI
+	imply NET_DSA_REALTEK_MDIO
 	select NET_DSA_TAG_RTL8_4
 	help
 	  Select to enable support for Realtek RTL8365MB-VC and RTL8367S.
 
 config NET_DSA_REALTEK_RTL8366RB
 	tristate "Realtek RTL8366RB switch subdriver"
-	depends on NET_DSA_REALTEK
-	depends on NET_DSA_REALTEK_SMI || NET_DSA_REALTEK_MDIO
+	imply NET_DSA_REALTEK_SMI
+	imply NET_DSA_REALTEK_MDIO
 	select NET_DSA_TAG_RTL4_A
 	help
-	  Select to enable support for Realtek RTL8366RB
+	  Select to enable support for Realtek RTL8366RB.
+
+endif