docs: better docs, mv doxygen to sphinx

This patch refactors the VPP sphinx docs
in order to make it easier to consume
for external readers as well as VPP developers.

It also makes sphinx the single source
of documentation, which simplifies maintenance
and operation.

Most important updates are:

- reformat the existing documentation as rst
- split RELEASE.md and move it into separate rst files
- remove section 'events'
- remove section 'archive'
- remove section 'related projects'
- remove section 'feature by release'
- remove section 'Various links'
- make (Configuration reference, CLI docs,
  developer docs) top level items in the list
- move 'Use Cases' as part of 'About VPP'
- move 'Troubleshooting' as part of 'Getting Started'
- move test framework docs into 'Developer Documentation'
- add a 'Contributing' section for gerrit,
  docs and other contributer related infos
- deprecate doxygen and test-docs targets
- redirect the "make doxygen" target to "make docs"

Type: refactor

Change-Id: I552a5645d5b7964d547f99b1336e2ac24e7c209f
Signed-off-by: Nathan Skrzypczak <nathan.skrzypczak@gmail.com>
Signed-off-by: Andrew Yourtchenko <ayourtch@gmail.com>

1 files changed, 225 insertions, 0 deletions

diff --git a/docs/developer/corearchitecture/featurearcs.rst b/docs/developer/corearchitecture/featurearcs.rst
new file mode 100644
index 00000000000..89c50e38dce
--- /dev/null
+++ b/docs/developer/corearchitecture/featurearcs.rst
@@ -0,0 +1,225 @@
+Feature Arcs
+============
+
+A significant number of vpp features are configurable on a per-interface
+or per-system basis. Rather than ask feature coders to manually
+construct the required graph arcs, we built a general mechanism to
+manage these mechanics.
+
+Specifically, feature arcs comprise ordered sets of graph nodes. Each
+feature node in an arc is independently controlled. Feature arc nodes
+are generally unaware of each other. Handing a packet to “the next
+feature node” is quite inexpensive.
+
+The feature arc implementation solves the problem of creating graph arcs
+used for steering.
+
+At the beginning of a feature arc, a bit of setup work is needed, but
+only if at least one feature is enabled on the arc.
+
+On a per-arc basis, individual feature definitions create a set of
+ordering dependencies. Feature infrastructure performs a topological
+sort of the ordering dependencies, to determine the actual feature
+order. Missing dependencies **will** lead to runtime disorder. See
+https://gerrit.fd.io/r/#/c/12753 for an example.
+
+If no partial order exists, vpp will refuse to run. Circular dependency
+loops of the form “a then b, b then c, c then a” are impossible to
+satisfy.
+
+Adding a feature to an existing feature arc
+-------------------------------------------
+
+To nobody’s great surprise, we set up feature arcs using the typical
+“macro -> constructor function -> list of declarations” pattern:
+
+.. code:: c
+
+       VNET_FEATURE_INIT (mactime, static) =
+       {
+         .arc_name = "device-input",
+         .node_name = "mactime",
+         .runs_before = VNET_FEATURES ("ethernet-input"),
+       };
+
+This creates a “mactime” feature on the “device-input” arc.
+
+Once per frame, dig up the vnet_feature_config_main_t corresponding to
+the “device-input” feature arc:
+
+.. code:: c
+
+       vnet_main_t *vnm = vnet_get_main ();
+       vnet_interface_main_t *im = &vnm->interface_main;
+       u8 arc = im->output_feature_arc_index;
+       vnet_feature_config_main_t *fcm;
+
+       fcm = vnet_feature_get_config_main (arc);
+
+Note that in this case, we’ve stored the required arc index - assigned
+by the feature infrastructure - in the vnet_interface_main_t. Where to
+put the arc index is a programmer’s decision when creating a feature
+arc.
+
+Per packet, set next0 to steer packets to the next node they should
+visit:
+
+.. code:: c
+
+       vnet_get_config_data (&fcm->config_main,
+                             &b0->current_config_index /* value-result */,
+                             &next0, 0 /* # bytes of config data */);
+
+Configuration data is per-feature arc, and is often unused. Note that
+it’s normal to reset next0 to divert packets elsewhere; often, to drop
+them for cause:
+
+.. code:: c
+
+       next0 = MACTIME_NEXT_DROP;
+       b0->error = node->errors[DROP_CAUSE];
+
+Creating a feature arc
+----------------------
+
+Once again, we create feature arcs using constructor macros:
+
+.. code:: c
+
+       VNET_FEATURE_ARC_INIT (ip4_unicast, static) =
+       {
+         .arc_name = "ip4-unicast",
+         .start_nodes = VNET_FEATURES ("ip4-input", "ip4-input-no-checksum"),
+         .arc_index_ptr = &ip4_main.lookup_main.ucast_feature_arc_index,
+       };
+
+In this case, we configure two arc start nodes to handle the
+“hardware-verified ip checksum or not” cases. During initialization, the
+feature infrastructure stores the arc index as shown.
+
+In the head-of-arc node, do the following to send packets along the
+feature arc:
+
+.. code:: c
+
+       ip_lookup_main_t *lm = &im->lookup_main;
+       arc = lm->ucast_feature_arc_index;
+
+Once per packet, initialize packet metadata to walk the feature arc:
+
+.. code:: c
+
+   vnet_feature_arc_start (arc, sw_if_index0, &next, b0);
+
+Enabling / Disabling features
+-----------------------------
+
+Simply call vnet_feature_enable_disable to enable or disable a specific
+feature:
+
+.. code:: c
+
+       vnet_feature_enable_disable ("device-input", /* arc name */
+                                    "mactime",      /* feature name */
+                                sw_if_index,    /* Interface sw_if_index */
+                                    enable_disable, /* 1 => enable */
+                                    0 /* (void *) feature_configuration */,
+                                    0 /* feature_configuration_nbytes */);
+
+The feature_configuration opaque is seldom used.
+
+If you wish to make a feature a *de facto* system-level concept, pass
+sw_if_index=0 at all times. Sw_if_index 0 is always valid, and
+corresponds to the “local” interface.
+
+Related “show” commands
+-----------------------
+
+To display the entire set of features, use “show features [verbose]”.
+The verbose form displays arc indices, and feature indicies within the
+arcs
+
+::
+
+   $ vppctl show features verbose
+   Available feature paths
+   <snip>
+   [14] ip4-unicast:
+     [ 0]: nat64-out2in-handoff
+     [ 1]: nat64-out2in
+     [ 2]: nat44-ed-hairpin-dst
+     [ 3]: nat44-hairpin-dst
+     [ 4]: ip4-dhcp-client-detect
+     [ 5]: nat44-out2in-fast
+     [ 6]: nat44-in2out-fast
+     [ 7]: nat44-handoff-classify
+     [ 8]: nat44-out2in-worker-handoff
+     [ 9]: nat44-in2out-worker-handoff
+     [10]: nat44-ed-classify
+     [11]: nat44-ed-out2in
+     [12]: nat44-ed-in2out
+     [13]: nat44-det-classify
+     [14]: nat44-det-out2in
+     [15]: nat44-det-in2out
+     [16]: nat44-classify
+     [17]: nat44-out2in
+     [18]: nat44-in2out
+     [19]: ip4-qos-record
+     [20]: ip4-vxlan-gpe-bypass
+     [21]: ip4-reassembly-feature
+     [22]: ip4-not-enabled
+     [23]: ip4-source-and-port-range-check-rx
+     [24]: ip4-flow-classify
+     [25]: ip4-inacl
+     [26]: ip4-source-check-via-rx
+     [27]: ip4-source-check-via-any
+     [28]: ip4-policer-classify
+     [29]: ipsec-input-ip4
+     [30]: vpath-input-ip4
+     [31]: ip4-vxlan-bypass
+     [32]: ip4-lookup
+   <snip>
+
+Here, we learn that the ip4-unicast feature arc has index 14, and that
+e.g. ip4-inacl is the 25th feature in the generated partial order.
+
+To display the features currently active on a specific interface, use
+“show interface features”:
+
+::
+
+   $ vppctl show interface GigabitEthernet3/0/0 features
+   Feature paths configured on GigabitEthernet3/0/0...
+   <snip>
+   ip4-unicast:
+     nat44-out2in
+   <snip>
+
+Table of Feature Arcs
+---------------------
+
+Simply search for name-strings to track down the arc definition,
+location of the arc index, etc.
+
+::
+
+               |    Arc Name      |
+               |------------------|
+               | device-input     |
+               | ethernet-output  |
+               | interface-output |
+               | ip4-drop         |
+               | ip4-local        |
+               | ip4-multicast    |
+               | ip4-output       |
+               | ip4-punt         |
+               | ip4-unicast      |
+               | ip6-drop         |
+               | ip6-local        |
+               | ip6-multicast    |
+               | ip6-output       |
+               | ip6-punt         |
+               | ip6-unicast      |
+               | mpls-input       |
+               | mpls-output      |
+               | nsh-output       |