ethernet: fix PTP_1588_CLOCK dependencies

The 'imply' keyword does not do what most people think it does, it only
politely asks Kconfig to turn on another symbol, but does not prevent
it from being disabled manually or built as a loadable module when the
user is built-in. In the ICE driver, the latter now causes a link failure:

aarch64-linux-ld: drivers/net/ethernet/intel/ice/ice_main.o: in function `ice_eth_ioctl':
ice_main.c:(.text+0x13b0): undefined reference to `ice_ptp_get_ts_config'
ice_main.c:(.text+0x13b0): relocation truncated to fit: R_AARCH64_CALL26 against undefined symbol `ice_ptp_get_ts_config'
aarch64-linux-ld: ice_main.c:(.text+0x13bc): undefined reference to `ice_ptp_set_ts_config'
ice_main.c:(.text+0x13bc): relocation truncated to fit: R_AARCH64_CALL26 against undefined symbol `ice_ptp_set_ts_config'
aarch64-linux-ld: drivers/net/ethernet/intel/ice/ice_main.o: in function `ice_prepare_for_reset':
ice_main.c:(.text+0x31fc): undefined reference to `ice_ptp_release'
ice_main.c:(.text+0x31fc): relocation truncated to fit: R_AARCH64_CALL26 against undefined symbol `ice_ptp_release'
aarch64-linux-ld: drivers/net/ethernet/intel/ice/ice_main.o: in function `ice_rebuild':

This is a recurring problem in many drivers, and we have discussed
it several times befores, without reaching a consensus. I'm providing
a link to the previous email thread for reference, which discusses
some related problems.

To solve the dependency issue better than the 'imply' keyword, introduce a
separate Kconfig symbol "CONFIG_PTP_1588_CLOCK_OPTIONAL" that any driver
can depend on if it is able to use PTP support when available, but works
fine without it. Whenever CONFIG_PTP_1588_CLOCK=m, those drivers are
then prevented from being built-in, the same way as with a 'depends on
PTP_1588_CLOCK || !PTP_1588_CLOCK' dependency that does the same trick,
but that can be rather confusing when you first see it.

Since this should cover the dependencies correctly, the IS_REACHABLE()
hack in the header is no longer needed now, and can be turned back
into a normal IS_ENABLED() check. Any driver that gets the dependency
wrong will now cause a link time failure rather than being unable to use
PTP support when that is in a loadable module.

However, the two recently added ptp_get_vclocks_index() and
ptp_convert_timestamp() interfaces are only called from builtin code with
ethtool and socket timestamps, so keep the current behavior by stubbing
those out completely when PTP is in a loadable module. This should be
addressed properly in a follow-up.

As Richard suggested, we may want to actually turn PTP support into a
'bool' option later on, preventing it from being a loadable module
altogether, which would be one way to solve the problem with the ethtool
interface.

Fixes: 06c16d89d2cb ("ice: register 1588 PTP clock device object for E810 devices")
Link: https://lore.kernel.org/netdev/20210804121318.337276-1-arnd@kernel.org/
Link: https://lore.kernel.org/netdev/CAK8P3a06enZOf=XyZ+zcAwBczv41UuCTz+=0FMf2gBz1_cOnZQ@mail.gmail.com/
Link: https://lore.kernel.org/netdev/CAK8P3a3=eOxE-K25754+fB_-i_0BZzf9a9RfPTX3ppSwu9WZXw@mail.gmail.com/
Link: https://lore.kernel.org/netdev/20210726084540.3282344-1-arnd@kernel.org/
Acked-by: Shannon Nelson <snelson@pensando.io>
Acked-by: Jacob Keller <jacob.e.keller@intel.com>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20210812183509.1362782-1-arnd@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

2 files changed, 16 insertions, 1 deletions

diff --git a/drivers/ptp/Kconfig b/drivers/ptp/Kconfig
index 8b08745e1ca1..e82b4a678acb 100644
--- a/drivers/ptp/Kconfig
+++ b/drivers/ptp/Kconfig
@@ -8,6 +8,7 @@ menu "PTP clock support"
 config PTP_1588_CLOCK
 	tristate "PTP clock support"
 	depends on NET && POSIX_TIMERS
+	default ETHERNET
 	select PPS
 	select NET_PTP_CLASSIFY
 	help
@@ -26,6 +27,18 @@ config PTP_1588_CLOCK
 	  To compile this driver as a module, choose M here: the module
 	  will be called ptp.
 
+config PTP_1588_CLOCK_OPTIONAL
+	tristate
+	default y if PTP_1588_CLOCK=n
+	default PTP_1588_CLOCK
+	help
+	  Drivers that can optionally use the PTP_1588_CLOCK framework
+	  should depend on this symbol to prevent them from being built
+	  into vmlinux while the PTP support itself is in a loadable
+	  module.
+	  If PTP support is disabled, this dependency will still be
+	  met, and drivers refer to dummy helpers.
+
 config PTP_1588_CLOCK_DTE
 	tristate "Broadcom DTE as PTP clock"
 	depends on PTP_1588_CLOCK
@@ -91,7 +104,7 @@ config PTP_1588_CLOCK_PCH
 	tristate "Intel PCH EG20T as PTP clock"
 	depends on X86_32 || COMPILE_TEST
 	depends on HAS_IOMEM && NET
-	imply PTP_1588_CLOCK
+	depends on PTP_1588_CLOCK
 	help
 	  This driver adds support for using the PCH EG20T as a PTP
 	  clock. The hardware supports time stamping of PTP packets
diff --git a/drivers/ptp/ptp_vclock.c b/drivers/ptp/ptp_vclock.c
index e0f87c57749a..baee0379482b 100644
--- a/drivers/ptp/ptp_vclock.c
+++ b/drivers/ptp/ptp_vclock.c
@@ -149,6 +149,7 @@ void ptp_vclock_unregister(struct ptp_vclock *vclock)
 	kfree(vclock);
 }
 
+#if IS_BUILTIN(CONFIG_PTP_1588_CLOCK)
 int ptp_get_vclocks_index(int pclock_index, int **vclock_index)
 {
 	char name[PTP_CLOCK_NAME_LEN] = "";
@@ -217,3 +218,4 @@ void ptp_convert_timestamp(struct skb_shared_hwtstamps *hwtstamps,
 	hwtstamps->hwtstamp = ns_to_ktime(ns);
 }
 EXPORT_SYMBOL(ptp_convert_timestamp);
+#endif