ice: Add documentation for devlink-rate implementation

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    $ devlink region delete pci/0000:01:00.0/device-caps snapshot 1

Devlink Rate

============

The ``ice`` driver implements devlink-rate API. It allows for offload of

the Hierarchical QoS to the hardware. It enables user to group Virtual

Functions in a tree structure and assign supported parameters: tx_share,

tx_max, tx_priority and tx_weight to each node in a tree. So effectively

user gains an ability to control how much bandwidth is allocated for each

VF group. This is later enforced by the HW.

It is assumed that this feature is mutually exclusive with DCB performed

in FW and ADQ, or any driver feature that would trigger changes in QoS,

for example creation of the new traffic class. The driver will prevent DCB

or ADQ configuration if user started making any changes to the nodes using

devlink-rate API. To configure those features a driver reload is necessary.

Correspondingly if ADQ or DCB will get configured the driver won't export

hierarchy at all, or will remove the untouched hierarchy if those

features are enabled after the hierarchy is exported, but before any

changes are made.

This feature is also dependent on switchdev being enabled in the system.

It's required bacause devlink-rate requires devlink-port objects to be

present, and those objects are only created in switchdev mode.

If the driver is set to the switchdev mode, it will export internal

hierarchy the moment VF's are created. Root of the tree is always

represented by the node_0. This node can't be deleted by the user. Leaf

nodes and nodes with children also can't be deleted.

.. list-table:: Attributes supported

    :widths: 15 85

    * - Name

      - Description

    * - ``tx_max``

      - maximum bandwidth to be consumed by the tree Node. Rate Limit is

        an absolute number specifying a maximum amount of bytes a Node may

        consume during the course of one second. Rate limit guarantees

        that a link will not oversaturate the receiver on the remote end

        and also enforces an SLA between the subscriber and network

        provider.

    * - ``tx_share``

      - minimum bandwidth allocated to a tree node when it is not blocked.

        It specifies an absolute BW. While tx_max defines the maximum

        bandwidth the node may consume, the tx_share marks committed BW

        for the Node.

    * - ``tx_priority``

      - allows for usage of strict priority arbiter among siblings. This

        arbitration scheme attempts to schedule nodes based on their

        priority as long as the nodes remain within their bandwidth limit.

        Range 0-7. Nodes with priority 7 have the highest priority and are

        selected first, while nodes with priority 0 have the lowest

        priority. Nodes that have the same priority are treated equally.

    * - ``tx_weight``

      - allows for usage of Weighted Fair Queuing arbitration scheme among

        siblings. This arbitration scheme can be used simultaneously with

        the strict priority. Range 1-200. Only relative values mater for

        arbitration.

``tx_priority`` and ``tx_weight`` can be used simultaneously. In that case

nodes with the same priority form a WFQ subgroup in the sibling group

and arbitration among them is based on assigned weights.

.. code:: shell

    # enable switchdev

    $ devlink dev eswitch set pci/0000:4b:00.0 mode switchdev

    # at this point driver should export internal hierarchy

    $ echo 2 > /sys/class/net/ens785np0/device/sriov_numvfs

    $ devlink port function rate show

    pci/0000:4b:00.0/node_25: type node parent node_24

    pci/0000:4b:00.0/node_24: type node parent node_0

    pci/0000:4b:00.0/node_32: type node parent node_31

    pci/0000:4b:00.0/node_31: type node parent node_30

    pci/0000:4b:00.0/node_30: type node parent node_16

    pci/0000:4b:00.0/node_19: type node parent node_18

    pci/0000:4b:00.0/node_18: type node parent node_17

    pci/0000:4b:00.0/node_17: type node parent node_16

    pci/0000:4b:00.0/node_14: type node parent node_5

    pci/0000:4b:00.0/node_5: type node parent node_3

    pci/0000:4b:00.0/node_13: type node parent node_4

    pci/0000:4b:00.0/node_12: type node parent node_4

    pci/0000:4b:00.0/node_11: type node parent node_4

    pci/0000:4b:00.0/node_10: type node parent node_4

    pci/0000:4b:00.0/node_9: type node parent node_4

    pci/0000:4b:00.0/node_8: type node parent node_4

    pci/0000:4b:00.0/node_7: type node parent node_4

    pci/0000:4b:00.0/node_6: type node parent node_4

    pci/0000:4b:00.0/node_4: type node parent node_3

    pci/0000:4b:00.0/node_3: type node parent node_16

    pci/0000:4b:00.0/node_16: type node parent node_15

    pci/0000:4b:00.0/node_15: type node parent node_0

    pci/0000:4b:00.0/node_2: type node parent node_1

    pci/0000:4b:00.0/node_1: type node parent node_0

    pci/0000:4b:00.0/node_0: type node

    pci/0000:4b:00.0/1: type leaf parent node_25

    pci/0000:4b:00.0/2: type leaf parent node_25

    # let's create some custom node

    $ devlink port function rate add pci/0000:4b:00.0/node_custom parent node_0

    # second custom node

    $ devlink port function rate add pci/0000:4b:00.0/node_custom_1 parent node_custom

    # reassign second VF to newly created branch

    $ devlink port function rate set pci/0000:4b:00.0/2 parent node_custom_1

    # assign tx_weight to the VF

    $ devlink port function rate set pci/0000:4b:00.0/2 tx_weight 5

    # assign tx_share to the VF

    $ devlink port function rate set pci/0000:4b:00.0/2 tx_share 500Mbps