docs: networking: document NAPI

Add basic documentation about NAPI. We can stop linking to the ancient
doc on the LF wiki.

Link: https://lore.kernel.org/all/[e-mail]/
Reviewed-by: Bagas Sanjaya <[e-mail]>
Reviewed-by: Toke Høiland-Jørgensen <[e-mail]>
Acked-by: Pavel Pisa <[e-mail]> # for ctucanfd-driver.rst
Reviewed-by: Tony Nguyen <[e-mail]>
Reviewed-by: Florian Fainelli <[e-mail]>
Reviewed-by: Stephen Hemminger <[e-mail]>
Reviewed-by: Randy Dunlap <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

Documentation/networking/device_drivers/can/ctu/ctucanfd-driver.rst

···
 enabling interrupts, handling an incoming IRQ in ISR, re-enabling the
 softirq and switching context back to softirq.
 
-More detailed documentation of NAPI may be found on the pages of Linux
-Foundation `<https://wiki.linuxfoundation.org/networking/napi>`_.
+See :ref:`Documentation/networking/napi.rst <napi>` for more information.
 
 Integrating the core to Xilinx Zynq
 -----------------------------------

Documentation/networking/device_drivers/ethernet/intel/e100.rst

···
 
 NAPI (Rx polling mode) is supported in the e100 driver.
 
-See https://wiki.linuxfoundation.org/networking/napi for more
-information on NAPI.
+See :ref:`Documentation/networking/napi.rst <napi>` for more information.
 
 Multiple Interfaces on Same Ethernet Broadcast Network
 ------------------------------------------------------

Documentation/networking/device_drivers/ethernet/intel/i40e.rst

···
 NAPI
 ----
 NAPI (Rx polling mode) is supported in the i40e driver.
-For more information on NAPI, see
-https://wiki.linuxfoundation.org/networking/napi
+
+See :ref:`Documentation/networking/napi.rst <napi>` for more information.
 
 Flow Control
 ------------

Documentation/networking/device_drivers/ethernet/intel/ice.rst

···
 
 NAPI
 ----
-This driver supports NAPI (Rx polling mode).
-For more information on NAPI, see
-https://wiki.linuxfoundation.org/networking/napi
 
+This driver supports NAPI (Rx polling mode).
+
+See :ref:`Documentation/networking/napi.rst <napi>` for more information.
 
 MACVLAN
 -------

Documentation/networking/index.rst

···
    mpls-sysctl
    mptcp-sysctl
    multiqueue
+   napi
    netconsole
    netdev-features
    netdevices

Documentation/networking/napi.rst

···
+.. SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+
+.. _napi:
+
+====
+NAPI
+====
+
+NAPI is the event handling mechanism used by the Linux networking stack.
+The name NAPI no longer stands for anything in particular [#]_.
+
+In basic operation the device notifies the host about new events
+via an interrupt.
+The host then schedules a NAPI instance to process the events.
+The device may also be polled for events via NAPI without receiving
+interrupts first (:ref:`busy polling<poll>`).
+
+NAPI processing usually happens in the software interrupt context,
+but there is an option to use :ref:`separate kernel threads<threaded>`
+for NAPI processing.
+
+All in all NAPI abstracts away from the drivers the context and configuration
+of event (packet Rx and Tx) processing.
+
+Driver API
+==========
+
+The two most important elements of NAPI are the struct napi_struct
+and the associated poll method. struct napi_struct holds the state
+of the NAPI instance while the method is the driver-specific event
+handler. The method will typically free Tx packets that have been
+transmitted and process newly received packets.
+
+.. _drv_ctrl:
+
+Control API
+-----------
+
+netif_napi_add() and netif_napi_del() add/remove a NAPI instance
+from the system. The instances are attached to the netdevice passed
+as argument (and will be deleted automatically when netdevice is
+unregistered). Instances are added in a disabled state.
+
+napi_enable() and napi_disable() manage the disabled state.
+A disabled NAPI can't be scheduled and its poll method is guaranteed
+to not be invoked. napi_disable() waits for ownership of the NAPI
+instance to be released.
+
+The control APIs are not idempotent. Control API calls are safe against
+concurrent use of datapath APIs but an incorrect sequence of control API
+calls may result in crashes, deadlocks, or race conditions. For example,
+calling napi_disable() multiple times in a row will deadlock.
+
+Datapath API
+------------
+
+napi_schedule() is the basic method of scheduling a NAPI poll.
+Drivers should call this function in their interrupt handler
+(see :ref:`drv_sched` for more info). A successful call to napi_schedule()
+will take ownership of the NAPI instance.
+
+Later, after NAPI is scheduled, the driver's poll method will be
+called to process the events/packets. The method takes a ``budget``
+argument - drivers can process completions for any number of Tx
+packets but should only process up to ``budget`` number of
+Rx packets. Rx processing is usually much more expensive.
+
+In other words, it is recommended to ignore the budget argument when
+performing TX buffer reclamation to ensure that the reclamation is not
+arbitrarily bounded; however, it is required to honor the budget argument
+for RX processing.
+
+.. warning::
+
+   The ``budget`` argument may be 0 if core tries to only process Tx completions
+   and no Rx packets.
+
+The poll method returns the amount of work done. If the driver still
+has outstanding work to do (e.g. ``budget`` was exhausted)
+the poll method should return exactly ``budget``. In that case,
+the NAPI instance will be serviced/polled again (without the
+need to be scheduled).
+
+If event processing has been completed (all outstanding packets
+processed) the poll method should call napi_complete_done()
+before returning. napi_complete_done() releases the ownership
+of the instance.
+
+.. warning::
+
+   The case of finishing all events and using exactly ``budget``
+   must be handled carefully. There is no way to report this
+   (rare) condition to the stack, so the driver must either
+   not call napi_complete_done() and wait to be called again,
+   or return ``budget - 1``.
+
+   If the ``budget`` is 0 napi_complete_done() should never be called.
+
+Call sequence
+-------------
+
+Drivers should not make assumptions about the exact sequencing
+of calls. The poll method may be called without the driver scheduling
+the instance (unless the instance is disabled). Similarly,
+it's not guaranteed that the poll method will be called, even
+if napi_schedule() succeeded (e.g. if the instance gets disabled).
+
+As mentioned in the :ref:`drv_ctrl` section - napi_disable() and subsequent
+calls to the poll method only wait for the ownership of the instance
+to be released, not for the poll method to exit. This means that
+drivers should avoid accessing any data structures after calling
+napi_complete_done().
+
+.. _drv_sched:
+
+Scheduling and IRQ masking
+--------------------------
+
+Drivers should keep the interrupts masked after scheduling
+the NAPI instance - until NAPI polling finishes any further
+interrupts are unnecessary.
+
+Drivers which have to mask the interrupts explicitly (as opposed
+to IRQ being auto-masked by the device) should use the napi_schedule_prep()
+and __napi_schedule() calls:
+
+.. code-block:: c
+
+  if (napi_schedule_prep(&v->napi)) {
+      mydrv_mask_rxtx_irq(v->idx);
+      /* schedule after masking to avoid races */
+      __napi_schedule(&v->napi);
+  }
+
+IRQ should only be unmasked after a successful call to napi_complete_done():
+
+.. code-block:: c
+
+  if (budget && napi_complete_done(&v->napi, work_done)) {
+    mydrv_unmask_rxtx_irq(v->idx);
+    return min(work_done, budget - 1);
+  }
+
+napi_schedule_irqoff() is a variant of napi_schedule() which takes advantage
+of guarantees given by being invoked in IRQ context (no need to
+mask interrupts). Note that PREEMPT_RT forces all interrupts
+to be threaded so the interrupt may need to be marked ``IRQF_NO_THREAD``
+to avoid issues on real-time kernel configurations.
+
+Instance to queue mapping
+-------------------------
+
+Modern devices have multiple NAPI instances (struct napi_struct) per
+interface. There is no strong requirement on how the instances are
+mapped to queues and interrupts. NAPI is primarily a polling/processing
+abstraction without specific user-facing semantics. That said, most networking
+devices end up using NAPI in fairly similar ways.
+
+NAPI instances most often correspond 1:1:1 to interrupts and queue pairs
+(queue pair is a set of a single Rx and single Tx queue).
+
+In less common cases a NAPI instance may be used for multiple queues
+or Rx and Tx queues can be serviced by separate NAPI instances on a single
+core. Regardless of the queue assignment, however, there is usually still
+a 1:1 mapping between NAPI instances and interrupts.
+
+It's worth noting that the ethtool API uses a "channel" terminology where
+each channel can be either ``rx``, ``tx`` or ``combined``. It's not clear
+what constitutes a channel; the recommended interpretation is to understand
+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.
+
+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.
+
+Software IRQ coalescing
+-----------------------
+
+NAPI does not perform any explicit event coalescing by default.
+In most scenarios batching happens due to IRQ coalescing which is done
+by the device. There are cases where software coalescing is helpful.
+
+NAPI can be configured to arm a repoll timer instead of unmasking
+the hardware interrupts as soon as all packets are processed.
+The ``gro_flush_timeout`` sysfs configuration of the netdevice
+is reused to control the delay of the timer, while
+``napi_defer_hard_irqs`` controls the number of consecutive empty polls
+before NAPI gives up and goes back to using hardware IRQs.
+
+.. _poll:
+
+Busy polling
+------------
+
+Busy polling allows a user process to check for incoming packets before
+the device interrupt fires. As is the case with any busy polling it trades
+off CPU cycles for lower latency (production uses of NAPI busy polling
+are not well known).
+
+Busy polling is enabled by either setting ``SO_BUSY_POLL`` on
+selected sockets or using the global ``net.core.busy_poll`` and
+``net.core.busy_read`` sysctls. An io_uring API for NAPI busy polling
+also exists.
+
+IRQ mitigation
+---------------
+
+While busy polling is supposed to be used by low latency applications,
+a similar mechanism can be used for IRQ mitigation.
+
+Very high request-per-second applications (especially routing/forwarding
+applications and especially applications using AF_XDP sockets) may not
+want to be interrupted until they finish processing a request or a batch
+of packets.
+
+Such applications can pledge to the kernel that they will perform a busy
+polling operation periodically, and the driver should keep the device IRQs
+permanently masked. This mode is enabled by using the ``SO_PREFER_BUSY_POLL``
+socket option. To avoid system misbehavior the pledge is revoked
+if ``gro_flush_timeout`` passes without any busy poll call.
+
+The NAPI budget for busy polling is lower than the default (which makes
+sense given the low latency intention of normal busy polling). This is
+not the case with IRQ mitigation, however, so the budget can be adjusted
+with the ``SO_BUSY_POLL_BUDGET`` socket option.
+
+.. _threaded:
+
+Threaded NAPI
+-------------
+
+Threaded NAPI is an operating mode that uses dedicated kernel
+threads rather than software IRQ context for NAPI processing.
+The configuration is per netdevice and will affect all
+NAPI instances of that device. Each NAPI instance will spawn a separate
+thread (called ``napi/${ifc-name}-${napi-id}``).
+
+It is recommended to pin each kernel thread to a single CPU, the same
+CPU as the CPU which services the interrupt. Note that the mapping
+between IRQs and NAPI instances may not be trivial (and is driver
+dependent). The NAPI instance IDs will be assigned in the opposite
+order than the process IDs of the kernel threads.
+
+Threaded NAPI is controlled by writing 0/1 to the ``threaded`` file in
+netdev's sysfs directory.
+
+.. rubric:: Footnotes
+
+.. [#] NAPI was originally referred to as New API in 2.4 Linux.

include/linux/netdevice.h

···
 	return false;
 }
 
-bool napi_complete_done(struct napi_struct *n, int work_done);
 /**
- *	napi_complete - NAPI processing complete
- *	@n: NAPI context
+ * napi_complete_done - NAPI processing complete
+ * @n: NAPI context
+ * @work_done: number of packets processed
  *
- * Mark NAPI processing as complete.
- * Consider using napi_complete_done() instead.
+ * Mark NAPI processing as complete. Should only be called if poll budget
+ * has not been completely consumed.
+ * Prefer over napi_complete().
  * Return false if device should avoid rearming interrupts.
  */
+bool napi_complete_done(struct napi_struct *n, int work_done);
+
 static inline bool napi_complete(struct napi_struct *n)
 {
 	return napi_complete_done(n, 0);