doc/mctp: Add a little detail about kernel internals

Describe common flows and refcounting behaviour.

Signed-off-by: Jeremy Kerr <jk@codeconstruct.com.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/Documentation/networking/mctp.rst b/Documentation/networking/mctp.rst
index 6100cdc..2c54b02 100644 (file)
--- a/Documentation/networking/mctp.rst
+++ b/Documentation/networking/mctp.rst
@@ -211,3 +211,62 @@ remote address is already known, or the message does not require a reply.
 
 Like the send calls, sockets will only receive responses to requests they have
 sent (TO=1) and may only respond (TO=0) to requests they have received.
+
+Kernel internals
+================
+
+There are a few possible packet flows in the MCTP stack:
+
+1. local TX to remote endpoint, message <= MTU::
+
+       sendmsg()
+        -> mctp_local_output()
+           : route lookup
+           -> rt->output() (== mctp_route_output)
+              -> dev_queue_xmit()
+
+2. local TX to remote endpoint, message > MTU::
+
+       sendmsg()
+       -> mctp_local_output()
+           -> mctp_do_fragment_route()
+              : creates packet-sized skbs. For each new skb:
+              -> rt->output() (== mctp_route_output)
+                 -> dev_queue_xmit()
+
+3. remote TX to local endpoint, single-packet message::
+
+       mctp_pkttype_receive()
+       : route lookup
+       -> rt->output() (== mctp_route_input)
+          : sk_key lookup
+          -> sock_queue_rcv_skb()
+
+4. remote TX to local endpoint, multiple-packet message::
+
+       mctp_pkttype_receive()
+       : route lookup
+       -> rt->output() (== mctp_route_input)
+          : sk_key lookup
+          : stores skb in struct sk_key->reasm_head
+
+       mctp_pkttype_receive()
+       : route lookup
+       -> rt->output() (== mctp_route_input)
+          : sk_key lookup
+          : finds existing reassembly in sk_key->reasm_head
+          : appends new fragment
+          -> sock_queue_rcv_skb()
+
+Key refcounts
+-------------
+
+ * keys are refed by:
+
+   - a skb: during route output, stored in ``skb->cb``.
+
+   - netns and sock lists.
+
+ * keys can be associated with a device, in which case they hold a
+   reference to the dev (set through ``key->dev``, counted through
+   ``dev->key_count``). Multiple keys can reference the device.