Re: [PATCH v4 net-next 1/7] ptp: Add ioctls for PHC timestamps with quality attributes

On 7/13/2026 7:03 PM, Arthur Kiyanovski wrote:

Introduce two new ioctls that extend existing PTP timestamp interfaces
with clock quality information:

- PTP_SYS_OFFSET_EXTENDED_ATTRS: Extends PTP_SYS_OFFSET_EXTENDED
- PTP_SYS_OFFSET_PRECISE_ATTRS: Extends PTP_SYS_OFFSET_PRECISE

These ioctls provide quality attributes alongside timestamps:

1. error_bound: Maximum deviation from true time (nanoseconds), based
   on device's internal clock state
2. clock_status: Synchronization state (unknown, initializing,
   synchronized, free-running, unreliable)
3. timescale: Time reference (TAI, UTC, etc.)
4. counter_value: Raw system counter (e.g. TSC ticks) captured by the
   timekeeping core alongside each system timestamp
5. counter_id: Identifies the counter source (e.g. TSC, ARM arch counter)

This supports three use cases:

1. Managed PHC devices (e.g., ENA, vmclock) that maintain their own
   synchronization and can report quality metrics directly to userspace
   without requiring ptp4l

2. Applications that need complete time quality information in a single
   call, regardless of how the PHC is synchronized

3. VMMs that need raw system counter values paired
   with PTP timestamps for feed-forward clock calibration, avoiding the
   feedback loop inherent in NTP-style synchronization

I'm also wondering if this can expose device-known error bounds on
timestamps even for devices which are operated as synchronized by ptp4l..

Timescale definitions use a Continuity/Discipline framework to describe
timeline properties and steering behavior consistently across all
entries.

This implementation is based on the original RFC and the UAPI design
discussion linked below.

Not a dig against this patch set, nor a request that you work to
implement anything else, but I am beginning to wonder if/when it would
make sense to transition from ioctl-based implementation to genetlink or
something. We did something similar for ethtool ioctls a few years ago.
I know the maintainer for PTP has some distaste for netlink and prefers
the simplicity of the ioctls.. but I think we're moving past where the
ioctls are "simple". Now that we have ynl tools, it has gotten easier to
implement properly. It makes extending the API much easier for the
future vs the array of ioctls we now carry for legacy implementations.

Do you have any thought on how ptp4l synchronizing the clock should
impact the clock status here? Is this intended purely for device/drivers
which have their own synchronization and not for ones which expose a
clock that is synchronized by userspace? Would it make sense to have a
mode that is something like "this clock has been modified by userspace"
after any call to the .adjtime or .adjfreq is made?

+
+/*
+ * Clock timescale values for struct ptp_clock_attrs.timescale.
+ *
+ * These definitions describe the mathematical properties and reference
+ * epochs of the timescale provided by the PHC.
+ *
+ * Discipline: Describes the frequency/phase steering behavior.
+ * Continuity: Describes whether the timeline is uninterrupted.
+ */
+enum ptp_clock_timescale {
+	/* Unknown or unspecified timescale */
+	PTP_TIMESCALE_UNKNOWN = 0,
+
+	/********************* Absolute Atomic Timescales *********************
+	 * These timescales are continuous, monotonic standards based on atomic
+	 * physics. They do not experience phase jumps.
+	 **********************************************************************/
+
+	/**
+	 * International Atomic Time (TAI)
+	 * Epoch: 1958-01-01 00:00:00.
+	 * Continuity: Strictly monotonic and continuous; no leap seconds.
+	 * Discipline: Primary atomic reference; no phase jumps.
+	 */
+	PTP_TIMESCALE_TAI = 1,
+
+	/**
+	 * Terrestrial Time (TT)
+	 * Epoch: 1958-01-01 00:00:00.
+	 * Continuity: Strictly monotonic and continuous; no leap seconds.
+	 * Discipline: Defined as TAI + 32.184s constant offset.
+	 */
+	PTP_TIMESCALE_TT = 2,
+
+	/**
+	 * Global Positioning System (GPS) Time
+	 * Epoch: 1980-01-06 00:00:00.
+	 * Continuity: Strictly monotonic and continuous; no leap seconds.
+	 * Discipline: Defined by the GPS constellation; fixed offset from TAI.
+	 */
+	PTP_TIMESCALE_GPS = 3,
+
+	/****************** UTC-Based Timescales (Civil Time) *****************
+	 * These timescales are derived from TAI but adjusted to align with
+	 * the Earth's rotation, primarily through leap seconds.
+	 **********************************************************************/
+
+	/**
+	 * Coordinated Universal Time (UTC) - Wall-clock (CLOCK_REALTIME)
+	 * Epoch: 1970-01-01 00:00:00 (Unix epoch).
+	 * Continuity: Discontinuous; subject to 1-second leap second
+	 *             phase jumps.
+	 * Discipline: Frequency steered; incorporates leap second corrections.
+	 *
+	 * Note: Leap-smeared UTC MUST NOT be advertised as PTP_TIMESCALE_UTC.
+	 * Smear algorithms are not standardized and the resulting timescale
+	 * is ambiguous. Implementations using smeared UTC MUST advertise
+	 * PTP_TIMESCALE_UNKNOWN or PTP_TIMESCALE_PROPRIETARY instead.
+	 */
+	PTP_TIMESCALE_UTC = 4,
+
+	/**
+	 * POSIX Time (Unix Time)
+	 * Epoch: 1970-01-01 00:00:00.
+	 * Continuity: Discontinuous; leap seconds handled by
+	 *             repeating/skipping values.
+	 * Discipline: Follows UTC frequency steering and phase jumps.
+	 */
+	PTP_TIMESCALE_POSIX = 5,
+
+	/****************** System-Relative Monotonic Clocks ******************
+	 * These timescales are relative to a system event (like boot)
+	 * and are not synchronized to an external atomic standard.
+	 **********************************************************************/
+
+	/**
+	 * Monotonic System Clock (CLOCK_MONOTONIC)
+	 * Epoch: Arbitrary (System boot time).
+	 * Continuity: Strictly monotonic; no leap seconds.
+	 * Discipline: Frequency steered to match system reference;
+	 *             does not advance during suspend.
+	 */
+	PTP_TIMESCALE_MONOTONIC = 6,
+
+	/**
+	 * Raw Monotonic System Clock (CLOCK_MONOTONIC_RAW)
+	 * Epoch: Arbitrary (System boot time).
+	 * Continuity: Strictly monotonic; no leap seconds.
+	 * Discipline: Raw hardware oscillator; no frequency steering
+	 *             or discipline.
+	 */
+	PTP_TIMESCALE_MONOTONIC_RAW = 7,
+
+	/**
+	 * Boot Time System Clock (CLOCK_BOOTTIME)
+	 * Epoch: Arbitrary (System boot time).
+	 * Continuity: Strictly monotonic and continuous; no leap seconds.
+	 * Discipline: Frequency steered to match system reference;
+	 *             advances during suspend.
+	 */
+	PTP_TIMESCALE_BOOTTIME = 8,
+
+	/********************** Vendor-Specific Timescale *********************/
+
+	/* A proprietary or vendor-specific timescale with custom rules. */
+	PTP_TIMESCALE_PROPRIETARY = 9,
+};

I appreciate the detailed explanations here as it helps to disambiguate
the modes.

I do kind of wish we had opted for bit flags here given the number of
ints being used as booleans.. :( A lot of wasted reserved space.