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 <jacob.e.keller@intel.com>
Reviewed-by: Vadim Fedorenko <vadim.fedorenko@linux.dev>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Reviewed-by: Kory Maincent <kory.maincent@bootlin.com>
Tested-by: James Clark <jjc@jclark.com>
Link: https://patch.msgid.link/20250918-jk-fix-bcm-phy-supported-flags-v1-3-747b60407c9c@intel.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/include/uapi/linux/ptp_clock.h b/include/uapi/linux/ptp_clock.h
index 18eefa6d93d62fc34edae8ae904db8cfad978c83..2c3346e91dbe599a53d30d2395812d04a0729ba2 100644 (file)
--- a/include/uapi/linux/ptp_clock.h
+++ b/include/uapi/linux/ptp_clock.h
@@ -37,6 +37,9 @@
 
 /*
  * 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 |      \