[ofa-general] [PATCH] doc/ipoib: document CM, offloads, interrupt moderation

[Or Gerlitz]

Update the documentation to include connected mode, stateless offloads
and interrupt moderation.

Signed-off-by: Or Gerlitz <ogerlitz at voltaire.com>

Index: linus-linux-2.6/Documentation/infiniband/ipoib.txt
===================================================================
--- linus-linux-2.6.orig/Documentation/infiniband/ipoib.txt
+++ linus-linux-2.6/Documentation/infiniband/ipoib.txt
@@ -24,6 +24,46 @@ Partitions and P_Keys
   The P_Key for any interface is given by the "pkey" file, and the
   main interface for a subinterface is in "parent."

+Datagram vs Connected modes
+
+ The driver supports two modes of operation - datagram and connected.
+ The mode is set/read through the /sys/class/net/ibXXX/mode device entry.
+
+ For datagram mode, the IB UD (Unrelaible Datagram) transport is used
+ and as such the device MTU has very close proximity to the IB L2 MTU
+ which is typically 2k (larger MTU of 4K supported by the code).
+
+ For connected mode, the IB RC (Reliable Connection) transport is used.
+ The main reasoning behind connected mode is to take advantage of the
+ connected nature of the IB transport and use MTUs up to the maximal
+ IP packet size of 64K - which reduces the number of IP packets needed for
+ handling large UDP datagrams, TCP segments, etc and increases the performance.
+
+ Under connected mode, the device UD QP is still used for multicast and
+ communication with peers who don't support connected mode. In this case,
+ RX emulation of ICMP PMTU packets is used for having the stack use
+ the smaller UD MTU for these neighbours.
+
+Stateless offloads
+
+  In case the underlying IB HW supports that, IPoIB advertizes TCP checksum
+  and/or Large Send (LSO) offloading capability to the network stack. This
+  is done by the device setting the relevant bits on its features bitmask.
+
+  Large Receive (LRO) offloading is also implemented and may be turned on/off
+  using ethtool calls, for the time being its supported only for checksum
+  offload capable devices.
+
+  Offloads are supported only under datagram mode.
+
+Interrupt moderation
+
+  In case the underlying IB device supports CQ event moderation, one can
+  have run time control on the rate of interrupt being generated and thus
+  reduce the overhead incurred by handling interrupts. For the most case
+  IPoIB doesn't use events for TX completion signaling and as such, only
+  RX moderation is supported.
+
 Debugging Information

   By compiling the IPoIB driver with CONFIG_INFINIBAND_IPOIB_DEBUG set
@@ -55,3 +95,5 @@ References
     http://ietf.org/rfc/rfc4391.txt
   IP over InfiniBand (IPoIB) Architecture (RFC 4392)
     http://ietf.org/rfc/rfc4392.txt
+  IP over InfiniBand: Connected Mode (RFC 4755)
+    http://ietf.org/rfc/rfc4755.txt