[PATCH v4 net-next 3/3] net: ena: Add PHC documentation

[David Arinzon <darinzon@amazon.com>]

Provide the relevant information and guidelines
about the feature support in the ENA driver.

Signed-off-by: Amit Bernstein <amitbern@amazon.com>
Signed-off-by: David Arinzon <darinzon@amazon.com>
---
 .../device_drivers/ethernet/amazon/ena.rst    | 78 +++++++++++++++++++
 1 file changed, 78 insertions(+)

diff --git a/Documentation/networking/device_drivers/ethernet/amazon/ena.rst b/Documentation/networking/device_drivers/ethernet/amazon/ena.rst
index 4561e8ab..12b13da0 100644
--- a/Documentation/networking/device_drivers/ethernet/amazon/ena.rst
+++ b/Documentation/networking/device_drivers/ethernet/amazon/ena.rst
@@ -56,6 +56,7 @@ ena_netdev.[ch]     Main Linux kernel driver.
 ena_ethtool.c       ethtool callbacks.
 ena_xdp.[ch]        XDP files
 ena_pci_id_tbl.h    Supported device IDs.
+ena_phc.[ch]        PTP hardware clock infrastructure (see `PHC`_ for more info)
 =================   ======================================================
 
 Management Interface:
@@ -221,6 +222,83 @@ descriptor it was received on would be recycled. When a packet smaller
 than RX copybreak bytes is received, it is copied into a new memory
 buffer and the RX descriptor is returned to HW.
 
+.. _`PHC`:
+
+PTP Hardware Clock (PHC)
+========================
+.. _`ptp-userspace-api`: https://docs.kernel.org/driver-api/ptp.html#ptp-hardware-clock-user-space-api
+.. _`testptp`: https://elixir.bootlin.com/linux/latest/source/tools/testing/selftests/ptp/testptp.c
+
+ENA Linux driver supports PTP hardware clock providing timestamp reference to achieve nanosecond resolution.
+
+**PHC support**
+
+PHC depends on the PTP module, which needs to be either loaded as a module or compiled into the kernel.
+
+Verify if the PTP module is present:
+
+.. code-block:: shell
+
+  grep -w '^CONFIG_PTP_1588_CLOCK=[ym]' /boot/config-`uname -r`
+
+- If no output is provided, the ENA driver cannot be loaded with PHC support.
+
+- ``CONFIG_PTP_1588_CLOCK=y``: the PTP module is already compiled and loaded inside the kernel binary file.
+
+- ``CONFIG_PTP_1588_CLOCK=m``: the PTP module needs to be loaded prior to loading the ENA driver:
+
+Load PTP module:
+
+.. code-block:: shell
+
+  sudo modprobe ptp
+
+All available PTP clock sources can be tracked here:
+
+.. code-block:: shell
+
+  ls /sys/class/ptp
+
+PHC support and capabilities can be verified using ethtool:
+
+.. code-block:: shell
+
+  ethtool -T <interface>
+
+**PHC timestamp**
+
+To retrieve PHC timestamp, use `ptp-userspace-api`_, usage example using `testptp`_:
+
+.. code-block:: shell
+
+  testptp -d /dev/ptp$(ethtool -T <interface> | awk '/PTP Hardware Clock:/ {print $NF}') -k 1
+
+PHC get time requests should be within reasonable bounds,
+avoid excessive utilization to ensure optimal performance and efficiency.
+The ENA device restricts the frequency of PHC get time requests to a maximum
+of 125 requests per second. If this limit is surpassed, the get time request
+will fail, leading to an increment in the phc_err statistic.
+
+**PHC statistics**
+
+PHC can be monitored using :code:`ethtool -S` counters:
+
+=================   ======================================================
+**phc_cnt**         Number of successful retrieved timestamps (below expire timeout).
+**phc_exp**         Number of expired retrieved timestamps (above expire timeout).
+**phc_skp**         Number of skipped get time attempts (during block period).
+**phc_err**         Number of failed get time attempts (entering into block state).
+=================   ======================================================
+
+PHC timeouts:
+
+=================   ======================================================
+**expire**          Max time for a valid timestamp retrieval, passing this threshold will fail
+                    the get time request and block new requests until block timeout.
+**block**           Blocking period starts once get time request expires or fails, all get time
+                    requests during block period will be skipped.
+=================   ======================================================
+
 Statistics
 ==========
 
-- 
2.40.1