[PATCH net-next] documentation: networking: Add NAPI config

[Joe Damato <jdamato@fastly.com>]

Document the existence of persistent per-NAPI configuration space and
the API that drivers can opt into.

Update stale documentation which suggested that NAPI IDs cannot be
queried from userspace.

Signed-off-by: Joe Damato <jdamato@fastly.com>
---
 Documentation/networking/napi.rst | 32 ++++++++++++++++++++++++++++++-
 1 file changed, 31 insertions(+), 1 deletion(-)

diff --git a/Documentation/networking/napi.rst b/Documentation/networking/napi.rst
index f970a2be271a..de146f63f09b 100644
--- a/Documentation/networking/napi.rst
+++ b/Documentation/networking/napi.rst
@@ -171,12 +171,42 @@ a channel as an IRQ/NAPI which services queues of a given type. For example,
 a configuration of 1 ``rx``, 1 ``tx`` and 1 ``combined`` channel is expected
 to utilize 3 interrupts, 2 Rx and 2 Tx queues.
 
+Persistent NAPI config
+----------------------
+
+Drivers can opt-in to using a persistent NAPI configuration space by calling
+netif_napi_add_config. This API maps a NAPI instance to a configuration
+structure using a driver defined index value, like a queue number. If the
+driver were to destroy and recreate NAPI instances (if a user requested a queue
+count change, for example), the new NAPI instances will inherit the configuration
+settings of the NAPI configuration structure they are mapped to.
+
+Using this API allows for persistent NAPI IDs (among other settings), which can
+be beneficial to userspace programs using ``SO_INCOMING_NAPI_ID``. See the
+sections below for other NAPI configuration settings.
+
 User API
 ========
 
 User interactions with NAPI depend on NAPI instance ID. The instance IDs
 are only visible to the user thru the ``SO_INCOMING_NAPI_ID`` socket option.
-It's not currently possible to query IDs used by a given device.
+
+Users can query NAPI IDs for a device or device queue using netlink. This can
+be done programmatically in a user application or by using a script included in
+the kernel source tree: ``tools/net/ynl/pyynl/cli.py``.
+
+For example, using the script to dump all of the queues for a device (which
+will reveal each queue's NAPI ID):
+
+.. code-block:: bash
+
+   $ kernel-source/tools/net/ynl/pyynl/cli.py \
+             --spec Documentation/netlink/specs/netdev.yaml \
+             --dump queue-get \
+             --json='{"ifindex": 2}'
+
+See ``Documentation/netlink/specs/netdev.yaml`` for more details on
+available operations and attributes.
 
 Software IRQ coalescing
 -----------------------

base-commit: 7bca2b2d5fcc685b81eb32fe564689eca6a59a99
-- 
2.43.0