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Diffstat (limited to 'Documentation/networking/timestamping.rst')
| -rw-r--r-- | Documentation/networking/timestamping.rst | 63 |
1 files 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 ^^^^^^^^^^^^^^^^^^^ |