Commit f4d41c59 authored by Jeremy Kerr's avatar Jeremy Kerr Committed by David S. Miller
Browse files

doc/mctp: Add a little detail about kernel internals 



Describe common flows and refcounting behaviour.

Signed-off-by: default avatarJeremy Kerr <jk@codeconstruct.com.au>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent 97f09abf

Documentation/networking/mctp.rst

+59 −0
Original line number Diff line number Diff line
@@ -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.