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+# NTB Drivers
+
+NTB (Non-Transparent Bridge) is a type of PCI-Express bridge chip that connects
+the separate memory systems of two computers to the same PCI-Express fabric.
+Existing NTB hardware supports a common feature set, including scratchpad
+registers, doorbell registers, and memory translation windows.  Scratchpad
+registers are read-and-writable registers that are accessible from either side
+of the device, so that peers can exchange a small amount of information at a
+fixed address.  Doorbell registers provide a way for peers to send interrupt
+events.  Memory windows allow translated read and write access to the peer
+memory.
+
+## NTB Core Driver (ntb)
+
+The NTB core driver defines an api wrapping the common feature set, and allows
+clients interested in NTB features to discover NTB the devices supported by
+hardware drivers.  The term "client" is used here to mean an upper layer
+component making use of the NTB api.  The term "driver," or "hardware driver,"
+is used here to mean a driver for a specific vendor and model of NTB hardware.
+
+## NTB Client Drivers
+
+NTB client drivers should register with the NTB core driver.  After
+registering, the client probe and remove functions will be called appropriately
+as ntb hardware, or hardware drivers, are inserted and removed.  The
+registration uses the Linux Device framework, so it should feel familiar to
+anyone who has written a pci driver.
+
+### NTB Transport Client (ntb\_transport) and NTB Netdev (ntb\_netdev)
+
+The primary client for NTB is the Transport client, used in tandem with NTB
+Netdev.  These drivers function together to create a logical link to the peer,
+across the ntb, to exchange packets of network data.  The Transport client
+establishes a logical link to the peer, and creates queue pairs to exchange
+messages and data.  The NTB Netdev then creates an ethernet device using a
+Transport queue pair.  Network data is copied between socket buffers and the
+Transport queue pair buffer.  The Transport client may be used for other things
+besides Netdev, however no other applications have yet been written.
+
+### NTB Ping Pong Test Client (ntb\_pingpong)
+
+The Ping Pong test client serves as a demonstration to exercise the doorbell
+and scratchpad registers of NTB hardware, and as an example simple NTB client.
+Ping Pong enables the link when started, waits for the NTB link to come up, and
+then proceeds to read and write the doorbell scratchpad registers of the NTB.
+The peers interrupt each other using a bit mask of doorbell bits, which is
+shifted by one in each round, to test the behavior of multiple doorbell bits
+and interrupt vectors.  The Ping Pong driver also reads the first local
+scratchpad, and writes the value plus one to the first peer scratchpad, each
+round before writing the peer doorbell register.
+
+Module Parameters:
+
+* unsafe - Some hardware has known issues with scratchpad and doorbell
+        registers.  By default, Ping Pong will not attempt to exercise such
+        hardware.  You may override this behavior at your own risk by setting
+        unsafe=1.
+* delay\_ms - Specify the delay between receiving a doorbell
+        interrupt event and setting the peer doorbell register for the next
+        round.
+* init\_db - Specify the doorbell bits to start new series of rounds.  A new
+        series begins once all the doorbell bits have been shifted out of
+        range.
+* dyndbg - It is suggested to specify dyndbg=+p when loading this module, and
+        then to observe debugging output on the console.
+
+### NTB Tool Test Client (ntb\_tool)
+
+The Tool test client serves for debugging, primarily, ntb hardware and drivers.
+The Tool provides access through debugfs for reading, setting, and clearing the
+NTB doorbell, and reading and writing scratchpads.
+
+The Tool does not currently have any module parameters.
+
+Debugfs Files:
+
+* *debugfs*/ntb\_tool/*hw*/ - A directory in debugfs will be created for each
+        NTB device probed by the tool.  This directory is shortened to *hw*
+        below.
+* *hw*/db - This file is used to read, set, and clear the local doorbell.  Not
+        all operations may be supported by all hardware.  To read the doorbell,
+        read the file.  To set the doorbell, write `s` followed by the bits to
+        set (eg: `echo 's 0x0101' > db`).  To clear the doorbell, write `c`
+        followed by the bits to clear.
+* *hw*/mask - This file is used to read, set, and clear the local doorbell mask.
+        See *db* for details.
+* *hw*/peer\_db - This file is used to read, set, and clear the peer doorbell.
+        See *db* for details.
+* *hw*/peer\_mask - This file is used to read, set, and clear the peer doorbell
+        mask.  See *db* for details.
+* *hw*/spad - This file is used to read and write local scratchpads.  To read
+        the values of all scratchpads, read the file.  To write values, write a
+        series of pairs of scratchpad number and value
+        (eg: `echo '4 0x123 7 0xabc' > spad`
+        # to set scratchpads `4` and `7` to `0x123` and `0xabc`, respectively).
+* *hw*/peer\_spad - This file is used to read and write peer scratchpads.  See
+        *spad* for details.
+
+## NTB Hardware Drivers
+
+NTB hardware drivers should register devices with the NTB core driver.  After
+registering, clients probe and remove functions will be called.
+
+### NTB Intel Hardware Driver (ntb\_hw\_intel)
+
+The Intel hardware driver supports NTB on Xeon and Atom CPUs.
+
+Module Parameters:
+
+* b2b\_mw\_idx - If the peer ntb is to be accessed via a memory window, then use
+        this memory window to access the peer ntb.  A value of zero or positive
+        starts from the first mw idx, and a negative value starts from the last
+        mw idx.  Both sides MUST set the same value here!  The default value is
+        `-1`.
+* b2b\_mw\_share - If the peer ntb is to be accessed via a memory window, and if
+        the memory window is large enough, still allow the client to use the
+        second half of the memory window for address translation to the peer.
+* xeon\_b2b\_usd\_bar2\_addr64 - If using B2B topology on Xeon hardware, use
+        this 64 bit address on the bus between the NTB devices for the window
+        at BAR2, on the upstream side of the link.
+* xeon\_b2b\_usd\_bar4\_addr64 - See *xeon\_b2b\_bar2\_addr64*.
+* xeon\_b2b\_usd\_bar4\_addr32 - See *xeon\_b2b\_bar2\_addr64*.
+* xeon\_b2b\_usd\_bar5\_addr32 - See *xeon\_b2b\_bar2\_addr64*.
+* xeon\_b2b\_dsd\_bar2\_addr64 - See *xeon\_b2b\_bar2\_addr64*.
+* xeon\_b2b\_dsd\_bar4\_addr64 - See *xeon\_b2b\_bar2\_addr64*.
+* xeon\_b2b\_dsd\_bar4\_addr32 - See *xeon\_b2b\_bar2\_addr64*.
+* xeon\_b2b\_dsd\_bar5\_addr32 - See *xeon\_b2b\_bar2\_addr64*.