[RFC 0/1] ethdev: per-packet Tx timestamp slot management

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

The current DPDK ethdev time synchronization framework is architected
around a single, shared hardware latch. The existing API,
`rte_eth_timesync_read_tx_timestamp()`, assumes a serialization model
where only one TX timestamp is outstanding at any given time.

This model creates severe limitations for modern high-throughput network
interface cards (NICs). When multiple packets requiring precise
transmit timestamps are sent concurrently, the shared latch becomes a
race-condition bottleneck. It makes timestamp retrieval unreliable and
drops accuracy. Furthermore, Poll Mode Drivers (PMDs) backed by
hardware that supports independent, per-packet timestamping slots have
no way to expose this capability to the user.

To solve this, this RFC introduces a formal slot-based lifecycle API
for per-packet Tx timestamp management. The API decouples timestamp
tracking from the global latch model, enabling true asynchronous,
parallel hardware timestamping.

Key Components of the Proposal:
================================
1. **Slot Lifecycle Management APIs**:
   - `rte_eth_timesync_tx_timestamp_slot_alloc()`: Allocates and locks
     a unique hardware slot prior to frame transmission.
   - `rte_eth_timesync_tx_timestamp_stamp_mbuf()`: Embeds the allocated
     slot handle into an mbuf dynamic field, allowing the PMD to
     program the specific hardware descriptor during the Tx burst.
   - `rte_eth_timesync_read_tx_timestamp_slot()`: Asynchronously polls
     a specific slot for its captured value, safely returning `-EAGAIN`
     if the hardware has not yet written back the timestamp.
   - `rte_eth_timesync_tx_timestamp_slot_release()`: Recycles the
     hardware slot back to the PMD resource pool after successful
     retrieval or an application timeout.

2. **Dual-Domain Timing Mechanics**:
   - Introduces `struct rte_eth_timesync_dual_domain_timestamp` to
     simultaneously capture both the adjusted PTP Hardware Clock (PHC)
     domain (`adjusted_ns`) and the raw, free-running cycles domain
     (`cycles_ns`).
   - A dedicated `valid_mask` tracks the validity of each time domain
     independently, giving applications granular telemetry options.

Scope of this RFC & Open Questions for the Community:
======================================================
This RFC establishes the structural definition of the APIs and driver
interfaces. Before proceeding to a full `v1` patch with active PMD
implementations, we would highly appreciate the community's feedback
on the following design choices:

- **Mbuf Dynamic Field Layout**: Is the encapsulation of the slot
  handle into an mbuf dynamic field using standard dynamic registration
  the preferred mechanism for passing steering context to the Tx
  descriptor path?
- **Dual-Domain Structure**: Does tracking both the adjusted PHC and
  the cycle counter within `rte_eth_timesync_dual_domain_timestamp`
  cover the needs of other hardware vendors supporting concurrent
  domains?
- **Error States**: Is `-EAGAIN` an acceptable return code for
  non-blocking polling of incomplete timestamps, or should we consider
  an explicit bitmask status?

Please review the proposed design. We welcome your feedback, design
suggestions, and critique.

Rajesh Kumar (1):
  ethdev: add per-packet Tx timestamp slot APIs

 lib/ethdev/ethdev_driver.h |  19 ++++++
 lib/ethdev/rte_ethdev.c    | 121 +++++++++++++++++++++++++++++++++++++
 lib/ethdev/rte_ethdev.h    | 108 +++++++++++++++++++++++++++++++++
 3 files changed, 248 insertions(+)

-- 
2.54.0