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Subject[net-next, v3, 5/7] docs: networking: timestamping: update for DSA switches
Date
Update timestamping doc for DSA switches to describe current
implementation accurately. On TX, the skb cloning is no longer
in DSA generic code.

Signed-off-by: Yangbo Lu <yangbo.lu@nxp.com>
Acked-by: Richard Cochran <richardcochran@gmail.com>
---
Changes for v2:
- Split from tx timestamp optimization big patch.
Changes for v3:
- None.
---
Documentation/networking/timestamping.rst | 63 ++++++++++++++---------
1 file changed, 39 insertions(+), 24 deletions(-)

diff --git a/Documentation/networking/timestamping.rst b/Documentation/networking/timestamping.rst
index f682e88fa87e..7db3985359bc 100644
--- a/Documentation/networking/timestamping.rst
+++ b/Documentation/networking/timestamping.rst
@@ -630,30 +630,45 @@ hardware timestamping on it. This is because the SO_TIMESTAMPING API does not
allow the delivery of multiple hardware timestamps for the same packet, so
anybody else except for the DSA switch port must be prevented from doing so.

-In code, DSA provides for most of the infrastructure for timestamping already,
-in generic code: a BPF classifier (``ptp_classify_raw``) is used to identify
-PTP event messages (any other packets, including PTP general messages, are not
-timestamped), and provides two hooks to drivers:
-
-- ``.port_txtstamp()``: The driver is passed a clone of the timestampable skb
- to be transmitted, before actually transmitting it. Typically, a switch will
- have a PTP TX timestamp register (or sometimes a FIFO) where the timestamp
- becomes available. There may be an IRQ that is raised upon this timestamp's
- availability, or the driver might have to poll after invoking
- ``dev_queue_xmit()`` towards the host interface. Either way, in the
- ``.port_txtstamp()`` method, the driver only needs to save the clone for
- later use (when the timestamp becomes available). Each skb is annotated with
- a pointer to its clone, in ``DSA_SKB_CB(skb)->clone``, to ease the driver's
- job of keeping track of which clone belongs to which skb.
-
-- ``.port_rxtstamp()``: The original (and only) timestampable skb is provided
- to the driver, for it to annotate it with a timestamp, if that is immediately
- available, or defer to later. On reception, timestamps might either be
- available in-band (through metadata in the DSA header, or attached in other
- ways to the packet), or out-of-band (through another RX timestamping FIFO).
- Deferral on RX is typically necessary when retrieving the timestamp needs a
- sleepable context. In that case, it is the responsibility of the DSA driver
- to call ``netif_rx_ni()`` on the freshly timestamped skb.
+In the generic layer, DSA provides the following infrastructure for PTP
+timestamping:
+
+- ``.port_txtstamp()``: a hook called prior to the transmission of
+ packets with a hardware TX timestamping request from user space.
+ This is required for two-step timestamping, since the hardware
+ timestamp becomes available after the actual MAC transmission, so the
+ driver must be prepared to correlate the timestamp with the original
+ packet so that it can re-enqueue the packet back into the socket's
+ error queue. To save the packet for when the timestamp becomes
+ available, the driver can call ``skb_clone_sk`` , save the clone pointer
+ in skb->cb and enqueue a tx skb queue. Typically, a switch will have a
+ PTP TX timestamp register (or sometimes a FIFO) where the timestamp
+ becomes available. In case of a FIFO, the hardware might store
+ key-value pairs of PTP sequence ID/message type/domain number and the
+ actual timestamp. To perform the correlation correctly between the
+ packets in a queue waiting for timestamping and the actual timestamps,
+ drivers can use a BPF classifier (``ptp_classify_raw``) to identify
+ the PTP transport type, and ``ptp_parse_header`` to interpret the PTP
+ header fields. There may be an IRQ that is raised upon this
+ timestamp's availability, or the driver might have to poll after
+ invoking ``dev_queue_xmit()`` towards the host interface.
+ One-step TX timestamping do not require packet cloning, since there is
+ no follow-up message required by the PTP protocol (because the
+ TX timestamp is embedded into the packet by the MAC), and therefore
+ user space does not expect the packet annotated with the TX timestamp
+ to be re-enqueued into its socket's error queue.
+
+- ``.port_rxtstamp()``: On RX, the BPF classifier is run by DSA to
+ identify PTP event messages (any other packets, including PTP general
+ messages, are not timestamped). The original (and only) timestampable
+ skb is provided to the driver, for it to annotate it with a timestamp,
+ if that is immediately available, or defer to later. On reception,
+ timestamps might either be available in-band (through metadata in the
+ DSA header, or attached in other ways to the packet), or out-of-band
+ (through another RX timestamping FIFO). Deferral on RX is typically
+ necessary when retrieving the timestamp needs a sleepable context. In
+ that case, it is the responsibility of the DSA driver to call
+ ``netif_rx_ni()`` on the freshly timestamped skb.

3.2.2 Ethernet PHYs
^^^^^^^^^^^^^^^^^^^
--
2.25.1
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