[RFC PATCH v5 2/5] doc: describe ethdev timesync clock and Rx timestamp API

[Rajesh Kumar <rajesh3.kumar@intel.com>]

Add a programming guide page for the ethdev IEEE 1588 / PTP timesync
API, covering PHC clock management and receive timestamp extraction.

This describes existing, previously undocumented API and does not
introduce any new functionality.

Signed-off-by: Rajesh Kumar <rajesh3.kumar@intel.com>
---
 doc/guides/prog_guide/ethdev/index.rst    |  1 +
 doc/guides/prog_guide/ethdev/timesync.rst | 72 +++++++++++++++++++++++
 2 files changed, 73 insertions(+)
 create mode 100644 doc/guides/prog_guide/ethdev/timesync.rst

diff --git a/doc/guides/prog_guide/ethdev/index.rst b/doc/guides/prog_guide/ethdev/index.rst
index 392ced0a2e..bc21f28091 100644
--- a/doc/guides/prog_guide/ethdev/index.rst
+++ b/doc/guides/prog_guide/ethdev/index.rst
@@ -13,3 +13,4 @@ Ethernet Device Library
     traffic_metering_and_policing
     traffic_management
     qos_framework
+    timesync
diff --git a/doc/guides/prog_guide/ethdev/timesync.rst b/doc/guides/prog_guide/ethdev/timesync.rst
new file mode 100644
index 0000000000..59d660eba8
--- /dev/null
+++ b/doc/guides/prog_guide/ethdev/timesync.rst
@@ -0,0 +1,72 @@
+..  SPDX-License-Identifier: BSD-3-Clause
+    Copyright(c) 2026 Intel Corporation.
+
+IEEE 1588 / PTP Timesync API
+============================
+
+Overview
+--------
+
+The DPDK IEEE 1588 / Precision Time Protocol (PTP) Timesync API provides
+a standardized framework for managing PTP Hardware Clocks (PHCs) and retrieving
+precise hardware transmit (Tx) and receive (Rx) timestamps.
+
+The Timesync framework encompasses three core capabilities:
+
+1. **Clock Control & Adjustment**: Enabling/disabling hardware timestamping, reading/setting clock time, and adjusting phase/frequency.
+2. **Receive Timestamping**: Hardware capture of incoming PTP packet arrival timestamps.
+3. **Transmit Timestamping**: Hardware capture of outbound PTP packet departure timestamps.
+
+
+Clock Management & Control
+--------------------------
+
+To initialize and discipline a port's PTP Hardware Clock (PHC), the API provides:
+
+Enable / Disable
+   ``rte_eth_timesync_enable(port_id)`` enables hardware timestamping on the specified port.
+   ``rte_eth_timesync_disable(port_id)`` disables timesync offloads.
+   The application must ensure no Tx timestamp operations are in flight before disabling timesync, unless the PMD explicitly supports concurrent disable and Tx timestamp operations.
+   This does not make the PMD hardware lifecycle safe by itself.
+
+Clock Time Read / Write
+   ``rte_eth_timesync_read_time(port_id, &ts)`` reads the current PHC wall-clock time as a ``struct timespec``.
+   ``rte_eth_timesync_write_time(port_id, &ts)`` sets the PHC wall-clock time.
+
+Clock Adjustments
+   ``rte_eth_timesync_adjust_time(port_id, delta_ns)`` adjusts the clock phase by a delta offset in nanoseconds.
+   ``rte_eth_timesync_adjust_freq(port_id, scaled_ppm)`` adjusts the clock frequency in scaled parts-per-million (1 ppm = 1 << 16).
+
+
+Receive (Rx) Timestamping
+-------------------------
+
+When receive timestamping is enabled, the hardware identifies incoming PTP packets and latches their arrival time.
+
+Rx Timestamp Extraction Workflow
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+1. On packet reception via ``rte_eth_rx_burst()``, the PMD checks if the received mbuf represents a PTP packet.
+2. The PMD sets the ``RTE_MBUF_F_RX_IEEE1588_PTP`` flag in ``mbuf->ol_flags``.
+3. Depending on the PMD and hardware capability, the Rx timestamp is either:
+
+   Extracted via API
+      Application calls ``rte_eth_timesync_read_rx_timestamp(port_id, &ts, flags)``.
+
+   Inlined in Mbuf
+      Stored in a registered mbuf dynamic field (e.g. ``rte_mbuf_dyn_rx_timestamp_register()``).
+
+
+PMD Implementation Requirements
+-------------------------------
+
+To support full timesync capabilities, a Poll Mode Driver (PMD) implements the following driver contract:
+
+1. Clock Operations
+   (``timesync_enable``, ``timesync_disable``, ``timesync_read_time``, ``timesync_write_time``, ``timesync_adjust_time``, ``timesync_adjust_freq``)
+
+   Controls hardware timestamp generation and disciplines the hardware clock registers.
+
+2. Rx Timestamping (``timesync_read_rx_timestamp``)
+
+   Configures Rx filters to latch incoming PTP arrival times and flags received mbufs with ``RTE_MBUF_F_RX_IEEE1588_PTP``.
-- 
2.55.0