# 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*.