ptp: Clarify ptp_clock_info .adjphase expects an internal servo to be used

.adjphase expects a PHC to use an internal servo algorithm to correct the
provided phase offset target in the callback. Implementation of the
internal servo algorithm are defined by the individual devices.

Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: Rahul Rameshbabu <rrameshbabu@nvidia.com>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/driver-api/ptp.rst

@@ -73,6 +73,22 @@ Writing clock drivers
    class driver, since the lock may also be needed by the clock
    driver's interrupt service routine.
 
+PTP hardware clock requirements for '.adjphase'
+-----------------------------------------------
+
+   The 'struct ptp_clock_info' interface has a '.adjphase' function.
+   This function has a set of requirements from the PHC in order to be
+   implemented.
+
+     * The PHC implements a servo algorithm internally that is used to
+       correct the offset passed in the '.adjphase' call.
+     * When other PTP adjustment functions are called, the PHC servo
+       algorithm is disabled.
+
+   **NOTE:** '.adjphase' is not a simple time adjustment functionality
+   that 'jumps' the PHC clock time based on the provided offset. It
+   should correct the offset provided using an internal algorithm.
+
 Supported hardware
 ==================
 

include/linux/ptp_clock_kernel.h

@@ -77,8 +77,10 @@ struct ptp_system_timestamp {
  *            nominal frequency in parts per million, but with a
  *            16 bit binary fractional field.
  *
- * @adjphase:  Adjusts the phase offset of the hardware clock.
- *             parameter delta: Desired change in nanoseconds.
+ * @adjphase:  Indicates that the PHC should use an internal servo
+ *             algorithm to correct the provided phase offset.
+ *             parameter delta: PHC servo phase adjustment target
+ *                              in nanoseconds.
  *
  * @adjtime:  Shifts the time of the hardware clock.
  *            parameter delta: Desired change in nanoseconds.