Date:	Tue,  5 Oct 2010 08:55:47 +0200
From:	Bernard Metzler <bmt@...ich.ibm.com>
To:	netdev@...r.kernel.org
Cc:	linux-rdma@...r.kernel.org, Bernard Metzler <bmt@...ich.ibm.com>
Subject: [PATCH] SIW: Documentation (initial)

---
 Documentation/networking/siw.txt |   91 ++++++++++++++++++++++++++++++++++++++
 1 files changed, 91 insertions(+), 0 deletions(-)
 create mode 100644 Documentation/networking/siw.txt

diff --git a/Documentation/networking/siw.txt b/Documentation/networking/siw.txt
new file mode 100644
index 0000000..f051d8b
--- /dev/null
+++ b/Documentation/networking/siw.txt
@@ -0,0 +1,91 @@
+SoftiWARP: Software iWARP kernel driver module.
+
+General
+-------
+SoftiWARP (siw) implements the iWARP protocol suite (MPA/DDP/RDMAP,
+IETF-RFC 5044/5041/5040) completely in software as a Linux kernel module.
+siw runs on top of TCP kernel sockets and exports the Linux kernel ibvers
+RDMA interface. siw interfaces with the iwcm connection manager.
+
+
+Transmit Path
+-------------
+If a send queue (SQ) work queue element gets posted, siw tries to send
+it directly out of the application context. If the SQ was non-empty,
+SQ processing is done asynchronously by a kernel worker thread. This
+thread gets scheduled, if the TCP socket signals new write space to
+be available. If during send operation the socket send space get
+exhausted, SQ processing is abandoned until new socket write space
+becomes available.
+
+
+Receive Path
+------------
+All application data is placed into target buffers within softirq
+socket callback. Application notification is asynchronous.
+
+
+User Interface
+--------------
+All fast path operations such as posting of work requests and
+reaping of work completions currently involve a system call into
+the siw module. Kernel/user-mapped send and receive as well as 
+completion queues are not part of the current code. In
+particular, mapped completion queues may improve performance,
+since reaping completion queue entries as well as re-arming
+the completion queue could be done more efficiently.
+
+
+Memory Management
+-----------------
+siw currently uses kernels ib_umem_get() function to pin memory for later
+use in data transfer operations. Transmit and receive memory is checked
+against correct access permissions only in the moment of access by the
+network input path or before pushing it to the socket for transmission.
+ib_umem_get() provides DMA mappings for the requested address space which
+is not used by siw.
+
+
+Module Parameters
+-----------------
+The following siw module parameters are recognized.
+loopback_enabled:
+	If set, siw attaches also to the looback device. Checked only
+	during module insertion.
+
+mpa_crc_enabled:
+	If set, the MPA CRC gets generated and checked both in tx and rx
+	path. Without hardware support, setting this flag will severely
+	hurt throughput. 
+
+zcopy_tx:
+	If set, payload of non signalled work requests
+	(such as non signalled WRITE or SEND as well as all READ
+	responses) are transferred using the TCP sockets
+	sendpage interface. This parameter can be switched on and
+	off dynamically (echo 1 >> /sys/module/siw/parameters/zcopy_tx
+	for enablement, 0 for disabling). System load may benefits from
+	using 0copy data transmission. 0copy is not enabled if
+	mpa_crc_enabled is set.
+
+
+Compile Time Flags:
+-DCHECK_DMA_CAPABILITIES
+	Checks if the device siw wants to attach to provides
+	DMA capabilities. While DMA capabilities are currently not
+	needed (siw works on top of a kernel TCP socket), siw
+	uses ib_umem_get() which performs a (not used) DMA address
+	translation. Writing a siw private memory reservation and
+	pinning routine would solve the issue.
+
+-DSIW_TX_FULLSEGS
+	Experimental, not enabled by default. If set,
+	siw tries not to overrun the socket (not sending until
+	-EAGAIN retrun), but stops sending if the current segment
+	would not fit into the socket's estimated tx buffer. With that,
+	wire FPDUs may get truncated by the TCP stack far less often.
+	Since this feature manipulates the sock's SOCK_NOSPACE
+	bit, it violates strict layering and is therefore considered
+	proprietary.
+	Since TCP is a byte stream protocol, no guarantee can be given
+	if FPDU's are not fragmented.
-- 
1.5.4.3

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