devlink: add a devlink-resource.rst documentation file

Take the little bit of documentation for resources from various commit
messages and combine it into a new devlink-resource.rst file.

This could probably be expanded on even further by someone with more
knowledge of how the devlink resources work.
Signed-off-by: NJacob Keller <jacob.e.keller@intel.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Documentation/networking/devlink/devlink-resource.rst

+.. SPDX-License-Identifier: GPL-2.0
+
+================
+Devlink Resource
+================
+
+``devlink`` provides the ability for drivers to register resources, which
+can allow administrators to see the device restrictions for a given
+resource, as well as how much of the given resource is currently
+in use. Additionally, these resources can optionally have configurable size.
+This could enable the administrator to limit the number of resources that
+are used.
+
+For example, the ``netdevsim`` driver enables ``/IPv4/fib`` and
+``/IPv4/fib-rules`` as resources to limit the number of IPv4 FIB entries and
+rules for a given device.
+
+Resource Ids
+============
+
+Each resource is represented by an id, and contains information about its
+current size and related sub resources. To access a sub resource, you
+specify the path of the resource. For example ``/IPv4/fib`` is the id for
+the ``fib`` sub-resource under the ``IPv4`` resource.
+
+example usage
+-------------
+
+The resources exposed by the driver can be observed, for example:
+
+.. code:: shell
+
+    $devlink resource show pci/0000:03:00.0
+    pci/0000:03:00.0:
+      name kvd size 245760 unit entry
+        resources:
+          name linear size 98304 occ 0 unit entry size_min 0 size_max 147456 size_gran 128
+          name hash_double size 60416 unit entry size_min 32768 size_max 180224 size_gran 128
+          name hash_single size 87040 unit entry size_min 65536 size_max 212992 size_gran 128
+
+Some resource's size can be changed. Examples:
+
+.. code:: shell
+
+    $devlink resource set pci/0000:03:00.0 path /kvd/hash_single size 73088
+    $devlink resource set pci/0000:03:00.0 path /kvd/hash_double size 74368
+
+The changes do not apply immediately, this can be validated by the 'size_new'
+attribute, which represents the pending change in size. For example:
+
+.. code:: shell
+
+    $devlink resource show pci/0000:03:00.0
+    pci/0000:03:00.0:
+      name kvd size 245760 unit entry size_valid false
+      resources:
+        name linear size 98304 size_new 147456 occ 0 unit entry size_min 0 size_max 147456 size_gran 128
+        name hash_double size 60416 unit entry size_min 32768 size_max 180224 size_gran 128
+        name hash_single size 87040 unit entry size_min 65536 size_max 212992 size_gran 128
+
+Note that changes in resource size may require a device reload to properly
+take effect.

Documentation/networking/devlink/index.rst

@@ -17,6 +17,7 @@ general.
    devlink-info
    devlink-params
    devlink-region
+   devlink-resource
    devlink-trap
 
 Driver-specific documentation