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(+)
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2.54.0
