1# NTB Drivers
  2
  3NTB (Non-Transparent Bridge) is a type of PCI-Express bridge chip that connects
  4the separate memory systems of two computers to the same PCI-Express fabric.
  5Existing NTB hardware supports a common feature set, including scratchpad
  6registers, doorbell registers, and memory translation windows.  Scratchpad
  7registers are read-and-writable registers that are accessible from either side
  8of the device, so that peers can exchange a small amount of information at a
  9fixed address.  Doorbell registers provide a way for peers to send interrupt
 10events.  Memory windows allow translated read and write access to the peer
 11memory.
 12
 13## NTB Core Driver (ntb)
 14
 15The NTB core driver defines an api wrapping the common feature set, and allows
 16clients interested in NTB features to discover NTB the devices supported by
 17hardware drivers.  The term "client" is used here to mean an upper layer
 18component making use of the NTB api.  The term "driver," or "hardware driver,"
 19is used here to mean a driver for a specific vendor and model of NTB hardware.
 20
 21## NTB Client Drivers
 22
 23NTB client drivers should register with the NTB core driver.  After
 24registering, the client probe and remove functions will be called appropriately
 25as ntb hardware, or hardware drivers, are inserted and removed.  The
 26registration uses the Linux Device framework, so it should feel familiar to
 27anyone who has written a pci driver.
 28
 29### NTB Transport Client (ntb\_transport) and NTB Netdev (ntb\_netdev)
 30
 31The primary client for NTB is the Transport client, used in tandem with NTB
 32Netdev.  These drivers function together to create a logical link to the peer,
 33across the ntb, to exchange packets of network data.  The Transport client
 34establishes a logical link to the peer, and creates queue pairs to exchange
 35messages and data.  The NTB Netdev then creates an ethernet device using a
 36Transport queue pair.  Network data is copied between socket buffers and the
 37Transport queue pair buffer.  The Transport client may be used for other things
 38besides Netdev, however no other applications have yet been written.
 39
 40### NTB Ping Pong Test Client (ntb\_pingpong)
 41
 42The Ping Pong test client serves as a demonstration to exercise the doorbell
 43and scratchpad registers of NTB hardware, and as an example simple NTB client.
 44Ping Pong enables the link when started, waits for the NTB link to come up, and
 45then proceeds to read and write the doorbell scratchpad registers of the NTB.
 46The peers interrupt each other using a bit mask of doorbell bits, which is
 47shifted by one in each round, to test the behavior of multiple doorbell bits
 48and interrupt vectors.  The Ping Pong driver also reads the first local
 49scratchpad, and writes the value plus one to the first peer scratchpad, each
 50round before writing the peer doorbell register.
 51
 52Module Parameters:
 53
 54* unsafe - Some hardware has known issues with scratchpad and doorbell
 55	registers.  By default, Ping Pong will not attempt to exercise such
 56	hardware.  You may override this behavior at your own risk by setting
 57	unsafe=1.
 58* delay\_ms - Specify the delay between receiving a doorbell
 59	interrupt event and setting the peer doorbell register for the next
 60	round.
 61* init\_db - Specify the doorbell bits to start new series of rounds.  A new
 62	series begins once all the doorbell bits have been shifted out of
 63	range.
 64* dyndbg - It is suggested to specify dyndbg=+p when loading this module, and
 65	then to observe debugging output on the console.
 66
 67### NTB Tool Test Client (ntb\_tool)
 68
 69The Tool test client serves for debugging, primarily, ntb hardware and drivers.
 70The Tool provides access through debugfs for reading, setting, and clearing the
 71NTB doorbell, and reading and writing scratchpads.
 72
 73The Tool does not currently have any module parameters.
 74
 75Debugfs Files:
 76
 77* *debugfs*/ntb\_tool/*hw*/ - A directory in debugfs will be created for each
 78	NTB device probed by the tool.  This directory is shortened to *hw*
 79	below.
 80* *hw*/db - This file is used to read, set, and clear the local doorbell.  Not
 81	all operations may be supported by all hardware.  To read the doorbell,
 82	read the file.  To set the doorbell, write `s` followed by the bits to
 83	set (eg: `echo 's 0x0101' > db`).  To clear the doorbell, write `c`
 84	followed by the bits to clear.
 85* *hw*/mask - This file is used to read, set, and clear the local doorbell mask.
 86	See *db* for details.
 87* *hw*/peer\_db - This file is used to read, set, and clear the peer doorbell.
 88	See *db* for details.
 89* *hw*/peer\_mask - This file is used to read, set, and clear the peer doorbell
 90	mask.  See *db* for details.
 91* *hw*/spad - This file is used to read and write local scratchpads.  To read
 92	the values of all scratchpads, read the file.  To write values, write a
 93	series of pairs of scratchpad number and value
 94	(eg: `echo '4 0x123 7 0xabc' > spad`
 95	# to set scratchpads `4` and `7` to `0x123` and `0xabc`, respectively).
 96* *hw*/peer\_spad - This file is used to read and write peer scratchpads.  See
 97	*spad* for details.
 98
 99## NTB Hardware Drivers
100
101NTB hardware drivers should register devices with the NTB core driver.  After
102registering, clients probe and remove functions will be called.
103
104### NTB Intel Hardware Driver (ntb\_hw\_intel)
105
106The Intel hardware driver supports NTB on Xeon and Atom CPUs.
107
108Module Parameters:
109
110* b2b\_mw\_idx - If the peer ntb is to be accessed via a memory window, then use
111	this memory window to access the peer ntb.  A value of zero or positive
112	starts from the first mw idx, and a negative value starts from the last
113	mw idx.  Both sides MUST set the same value here!  The default value is
114	`-1`.
115* b2b\_mw\_share - If the peer ntb is to be accessed via a memory window, and if
116	the memory window is large enough, still allow the client to use the
117	second half of the memory window for address translation to the peer.
118* xeon\_b2b\_usd\_bar2\_addr64 - If using B2B topology on Xeon hardware, use
119	this 64 bit address on the bus between the NTB devices for the window
120	at BAR2, on the upstream side of the link.
121* xeon\_b2b\_usd\_bar4\_addr64 - See *xeon\_b2b\_bar2\_addr64*.
122* xeon\_b2b\_usd\_bar4\_addr32 - See *xeon\_b2b\_bar2\_addr64*.
123* xeon\_b2b\_usd\_bar5\_addr32 - See *xeon\_b2b\_bar2\_addr64*.
124* xeon\_b2b\_dsd\_bar2\_addr64 - See *xeon\_b2b\_bar2\_addr64*.
125* xeon\_b2b\_dsd\_bar4\_addr64 - See *xeon\_b2b\_bar2\_addr64*.
126* xeon\_b2b\_dsd\_bar4\_addr32 - See *xeon\_b2b\_bar2\_addr64*.
127* xeon\_b2b\_dsd\_bar5\_addr32 - See *xeon\_b2b\_bar2\_addr64*.