net: ena: Add PHC documentation

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

Signed-off-by: Amit Bernstein <[e-mail]>
Signed-off-by: David Arinzon <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

Documentation/networking/device_drivers/ethernet/amazon/ena.rst

···
 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.
+
+**PHC activation**
+
+The feature is turned off by default, in order to turn the feature on, the ENA driver
+can be loaded in the following way:
+
+- devlink:
+
+.. code-block:: shell
+
+  sudo devlink dev param set pci/<domain:bus:slot.function> name enable_phc value true cmode driverinit
+  sudo devlink dev reload pci/<domain:bus:slot.function>
+  # for example:
+  sudo devlink dev param set pci/0000:00:06.0 name enable_phc value true cmode driverinit
+  sudo devlink dev reload pci/0000:00:06.0
+
+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_ts statistic.
+
+**PHC statistics**
+
+PHC can be monitored using debugfs (if mounted):
+
+.. code-block:: shell
+
+  sudo cat /sys/kernel/debug/<domain:bus:slot.function>/phc_stats
+
+  # for example:
+  sudo cat /sys/kernel/debug/0000:00:06.0/phc_stats
+
+PHC errors must remain below 1% of all PHC requests to maintain the desired level of accuracy and reliability
+
+=================   ======================================================
+**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_dv**      | Number of failed get time attempts due to device errors (entering into block state).
+**phc_err_ts**      | Number of failed get time attempts due to timestamp errors (entering into block state),
+                    | This occurs if driver exceeded the request limit or device received an invalid timestamp.
+=================   ======================================================
+
+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
 ==========