ptp: document behavior of PTP_STRICT_FLAGS

Commit 6138e687c7b6 ("ptp: Introduce strict checking of external time stamp
options.") added the PTP_STRICT_FLAGS to the set of flags supported for the
external timestamp request ioctl.

It is only supported by PTP_EXTTS_REQUEST2, as it was introduced the
introduction of the new ioctls. Further, the kernel has always set this
flag for PTP_EXTTS_REQUEST2 regardless of whether or not the user requested
the behavior.

This effectively means that the flag is not useful for userspace. If the
user issues a PTP_EXTTS_REQUEST ioctl, the flag is ignored due to not being
supported on the old ioctl. If the user issues a PTP_EXTTS_REQUEST2 ioctl,
the flag will be set by the kernel regardless of whether the user set the
flag in their structure.

Add a comment documenting this behavior in the uAPI header file.

Signed-off-by: Jacob Keller <[e-mail]>
Reviewed-by: Vadim Fedorenko <[e-mail]>
Acked-by: Richard Cochran <[e-mail]>
Reviewed-by: Kory Maincent <[e-mail]>
Tested-by: James Clark <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

include/uapi/linux/ptp_clock.h

···
 
 /*
  * flag fields valid for the new PTP_EXTTS_REQUEST2 ioctl.
+ *
+ * Note: PTP_STRICT_FLAGS is always enabled by the kernel for
+ * PTP_EXTTS_REQUEST2 regardless of whether it is set by userspace.
  */
 #define PTP_EXTTS_VALID_FLAGS	(PTP_ENABLE_FEATURE |	\
 				 PTP_RISING_EDGE |	\