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>

1 files changed, 28 insertions, 20 deletions

diff --git a/include/linux/ptp_clock_kernel.h b/include/linux/ptp_clock_kernel.h
index 71fac9237725..2e5565067355 100644
--- a/include/linux/ptp_clock_kernel.h
+++ b/include/linux/ptp_clock_kernel.h
@@ -215,7 +215,7 @@ static inline long scaled_ppm_to_ppb(long ppm)
 	return (long)ppb;
 }
 
-#if IS_REACHABLE(CONFIG_PTP_1588_CLOCK)
+#if IS_ENABLED(CONFIG_PTP_1588_CLOCK)
 
 /**
  * ptp_clock_register() - register a PTP hardware clock driver
@@ -307,6 +307,33 @@ int ptp_schedule_worker(struct ptp_clock *ptp, unsigned long delay);
  */
 void ptp_cancel_worker_sync(struct ptp_clock *ptp);
 
+#else
+static inline struct ptp_clock *ptp_clock_register(struct ptp_clock_info *info,
+						   struct device *parent)
+{ return NULL; }
+static inline int ptp_clock_unregister(struct ptp_clock *ptp)
+{ return 0; }
+static inline void ptp_clock_event(struct ptp_clock *ptp,
+				   struct ptp_clock_event *event)
+{ }
+static inline int ptp_clock_index(struct ptp_clock *ptp)
+{ return -1; }
+static inline int ptp_find_pin(struct ptp_clock *ptp,
+			       enum ptp_pin_function func, unsigned int chan)
+{ return -1; }
+static inline int ptp_schedule_worker(struct ptp_clock *ptp,
+				      unsigned long delay)
+{ return -EOPNOTSUPP; }
+static inline void ptp_cancel_worker_sync(struct ptp_clock *ptp)
+{ }
+#endif
+
+#if IS_BUILTIN(CONFIG_PTP_1588_CLOCK)
+/*
+ * These are called by the network core, and don't work if PTP is in
+ * a loadable module.
+ */
+
 /**
  * ptp_get_vclocks_index() - get all vclocks index on pclock, and
  *                           caller is responsible to free memory
@@ -327,26 +354,7 @@ int ptp_get_vclocks_index(int pclock_index, int **vclock_index);
  */
 void ptp_convert_timestamp(struct skb_shared_hwtstamps *hwtstamps,
 			   int vclock_index);
-
 #else
-static inline struct ptp_clock *ptp_clock_register(struct ptp_clock_info *info,
-						   struct device *parent)
-{ return NULL; }
-static inline int ptp_clock_unregister(struct ptp_clock *ptp)
-{ return 0; }
-static inline void ptp_clock_event(struct ptp_clock *ptp,
-				   struct ptp_clock_event *event)
-{ }
-static inline int ptp_clock_index(struct ptp_clock *ptp)
-{ return -1; }
-static inline int ptp_find_pin(struct ptp_clock *ptp,
-			       enum ptp_pin_function func, unsigned int chan)
-{ return -1; }
-static inline int ptp_schedule_worker(struct ptp_clock *ptp,
-				      unsigned long delay)
-{ return -EOPNOTSUPP; }
-static inline void ptp_cancel_worker_sync(struct ptp_clock *ptp)
-{ }
 static inline int ptp_get_vclocks_index(int pclock_index, int **vclock_index)
 { return 0; }
 static inline void ptp_convert_timestamp(struct skb_shared_hwtstamps *hwtstamps,