ptp: Avoid deadlocks in the programmable pin code.

The PTP Hardware Clock (PHC) subsystem offers an API for configuring
programmable pins.  User space sets or gets the settings using ioctls,
and drivers verify dialed settings via a callback.  Drivers may also
query pin settings by calling the ptp_find_pin() method.

Although the core subsystem protects concurrent access to the pin
settings, the implementation places illogical restrictions on how
drivers may call ptp_find_pin().  When enabling an auxiliary function
via the .enable(on=1) callback, drivers may invoke the pin finding
method, but when disabling with .enable(on=0) drivers are not
permitted to do so.  With the exception of the mv88e6xxx, all of the
PHC drivers do respect this restriction, but still the locking pattern
is both confusing and unnecessary.

This patch changes the locking implementation to allow PHC drivers to
freely call ptp_find_pin() from their .enable() and .verify()
callbacks.

V2 ChangeLog:
- fixed spelling in the kernel doc
- add Vladimir's tested by tag

Signed-off-by: Richard Cochran <richardcochran@gmail.com>
Reported-by: Yangbo Lu <yangbo.lu@nxp.com>
Tested-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 19 insertions, 0 deletions

diff --git a/include/linux/ptp_clock_kernel.h b/include/linux/ptp_clock_kernel.h
index c64a1ef87240..121a7eda4593 100644
--- a/include/linux/ptp_clock_kernel.h
+++ b/include/linux/ptp_clock_kernel.h
@@ -223,6 +223,12 @@ extern s32 scaled_ppm_to_ppb(long ppm);
 /**
  * ptp_find_pin() - obtain the pin index of a given auxiliary function
  *
+ * The caller must hold ptp_clock::pincfg_mux.  Drivers do not have
+ * access to that mutex as ptp_clock is an opaque type.  However, the
+ * core code acquires the mutex before invoking the driver's
+ * ptp_clock_info::enable() callback, and so drivers may call this
+ * function from that context.
+ *
  * @ptp:    The clock obtained from ptp_clock_register().
  * @func:   One of the ptp_pin_function enumerated values.
  * @chan:   The particular functional channel to find.
@@ -234,6 +240,19 @@ int ptp_find_pin(struct ptp_clock *ptp,
 		 enum ptp_pin_function func, unsigned int chan);
 
 /**
+ * ptp_find_pin_unlocked() - wrapper for ptp_find_pin()
+ *
+ * This function acquires the ptp_clock::pincfg_mux mutex before
+ * invoking ptp_find_pin().  Instead of using this function, drivers
+ * should most likely call ptp_find_pin() directly from their
+ * ptp_clock_info::enable() method.
+ *
+ */
+
+int ptp_find_pin_unlocked(struct ptp_clock *ptp,
+			  enum ptp_pin_function func, unsigned int chan);
+
+/**
  * ptp_schedule_worker() - schedule ptp auxiliary work
  *
  * @ptp:    The clock obtained from ptp_clock_register().