1===========
2NTB Drivers
3===========
4
5NTB (Non-Transparent Bridge) is a type of PCI-Express bridge chip that connects
6the separate memory systems of two or more computers to the same PCI-Express
7fabric. Existing NTB hardware supports a common feature set: doorbell
8registers and memory translation windows, as well as non common features like
9scratchpad and message registers. Scratchpad registers are read-and-writable
10registers that are accessible from either side of the device, so that peers can
11exchange a small amount of information at a fixed address. Message registers can
12be utilized for the same purpose. Additionally they are provided with with
13special status bits to make sure the information isn't rewritten by another
14peer. Doorbell registers provide a way for peers to send interrupt events.
15Memory windows allow translated read and write access to the peer memory.
16
17NTB Core Driver (ntb)
18=====================
19
20The NTB core driver defines an api wrapping the common feature set, and allows
21clients interested in NTB features to discover NTB the devices supported by
22hardware drivers.  The term "client" is used here to mean an upper layer
23component making use of the NTB api.  The term "driver," or "hardware driver,"
24is used here to mean a driver for a specific vendor and model of NTB hardware.
25
26NTB Client Drivers
27==================
28
29NTB client drivers should register with the NTB core driver.  After
30registering, the client probe and remove functions will be called appropriately
31as ntb hardware, or hardware drivers, are inserted and removed.  The
32registration uses the Linux Device framework, so it should feel familiar to
33anyone who has written a pci driver.
34
35NTB Typical client driver implementation
36----------------------------------------
37
38Primary purpose of NTB is to share some peace of memory between at least two
39systems. So the NTB device features like Scratchpad/Message registers are
40mainly used to perform the proper memory window initialization. Typically
41there are two types of memory window interfaces supported by the NTB API:
42inbound translation configured on the local ntb port and outbound translation
43configured by the peer, on the peer ntb port. The first type is
44depicted on the next figure
45
46Inbound translation:
47 Memory:              Local NTB Port:      Peer NTB Port:      Peer MMIO:
48  ____________
49 | dma-mapped |-ntb_mw_set_trans(addr)  |
50 | memory     |        _v____________   |   ______________
51 | (addr)     |<======| MW xlat addr |<====| MW base addr |<== memory-mapped IO
52 |------------|       |--------------|  |  |--------------|
53
54So typical scenario of the first type memory window initialization looks:
551) allocate a memory region, 2) put translated address to NTB config,
563) somehow notify a peer device of performed initialization, 4) peer device
57maps corresponding outbound memory window so to have access to the shared
58memory region.
59
60The second type of interface, that implies the shared windows being
61initialized by a peer device, is depicted on the figure:
62
63Outbound translation:
64 Memory:        Local NTB Port:    Peer NTB Port:      Peer MMIO:
65  ____________                      ______________
66 | dma-mapped |                |   | MW base addr |<== memory-mapped IO
67 | memory     |                |   |--------------|
68 | (addr)     |<===================| MW xlat addr |<-ntb_peer_mw_set_trans(addr)
69 |------------|                |   |--------------|
70
71Typical scenario of the second type interface initialization would be:
721) allocate a memory region, 2) somehow deliver a translated address to a peer
73device, 3) peer puts the translated address to NTB config, 4) peer device maps
74outbound memory window so to have access to the shared memory region.
75
76As one can see the described scenarios can be combined in one portable
77algorithm.
78 Local device:
79  1) Allocate memory for a shared window
80  2) Initialize memory window by translated address of the allocated region
81     (it may fail if local memory window initialization is unsupported)
82  3) Send the translated address and memory window index to a peer device
83 Peer device:
84  1) Initialize memory window with retrieved address of the allocated
85     by another device memory region (it may fail if peer memory window
86     initialization is unsupported)
87  2) Map outbound memory window
88
89In accordance with this scenario, the NTB Memory Window API can be used as
90follows:
91 Local device:
92  1) ntb_mw_count(pidx) - retrieve number of memory ranges, which can
93     be allocated for memory windows between local device and peer device
94     of port with specified index.
95  2) ntb_get_align(pidx, midx) - retrieve parameters restricting the
96     shared memory region alignment and size. Then memory can be properly
97     allocated.
98  3) Allocate physically contiguous memory region in compliance with
99     restrictions retrieved in 2).
100  4) ntb_mw_set_trans(pidx, midx) - try to set translation address of
101     the memory window with specified index for the defined peer device
102     (it may fail if local translated address setting is not supported)
103  5) Send translated base address (usually together with memory window
104     number) to the peer device using, for instance, scratchpad or message
105     registers.
106 Peer device:
107  1) ntb_peer_mw_set_trans(pidx, midx) - try to set received from other
108     device (related to pidx) translated address for specified memory
109     window. It may fail if retrieved address, for instance, exceeds
110     maximum possible address or isn't properly aligned.
111  2) ntb_peer_mw_get_addr(widx) - retrieve MMIO address to map the memory
112     window so to have an access to the shared memory.
113
114Also it is worth to note, that method ntb_mw_count(pidx) should return the
115same value as ntb_peer_mw_count() on the peer with port index - pidx.
116
117NTB Transport Client (ntb\_transport) and NTB Netdev (ntb\_netdev)
118------------------------------------------------------------------
119
120The primary client for NTB is the Transport client, used in tandem with NTB
121Netdev.  These drivers function together to create a logical link to the peer,
122across the ntb, to exchange packets of network data.  The Transport client
123establishes a logical link to the peer, and creates queue pairs to exchange
124messages and data.  The NTB Netdev then creates an ethernet device using a
125Transport queue pair.  Network data is copied between socket buffers and the
126Transport queue pair buffer.  The Transport client may be used for other things
127besides Netdev, however no other applications have yet been written.
128
129NTB Ping Pong Test Client (ntb\_pingpong)
130-----------------------------------------
131
132The Ping Pong test client serves as a demonstration to exercise the doorbell
133and scratchpad registers of NTB hardware, and as an example simple NTB client.
134Ping Pong enables the link when started, waits for the NTB link to come up, and
135then proceeds to read and write the doorbell scratchpad registers of the NTB.
136The peers interrupt each other using a bit mask of doorbell bits, which is
137shifted by one in each round, to test the behavior of multiple doorbell bits
138and interrupt vectors.  The Ping Pong driver also reads the first local
139scratchpad, and writes the value plus one to the first peer scratchpad, each
140round before writing the peer doorbell register.
141
142Module Parameters:
143
144* unsafe - Some hardware has known issues with scratchpad and doorbell
145	registers.  By default, Ping Pong will not attempt to exercise such
146	hardware.  You may override this behavior at your own risk by setting
147	unsafe=1.
148* delay\_ms - Specify the delay between receiving a doorbell
149	interrupt event and setting the peer doorbell register for the next
150	round.
151* init\_db - Specify the doorbell bits to start new series of rounds.  A new
152	series begins once all the doorbell bits have been shifted out of
153	range.
154* dyndbg - It is suggested to specify dyndbg=+p when loading this module, and
155	then to observe debugging output on the console.
156
157NTB Tool Test Client (ntb\_tool)
158--------------------------------
159
160The Tool test client serves for debugging, primarily, ntb hardware and drivers.
161The Tool provides access through debugfs for reading, setting, and clearing the
162NTB doorbell, and reading and writing scratchpads.
163
164The Tool does not currently have any module parameters.
165
166Debugfs Files:
167
168* *debugfs*/ntb\_tool/*hw*/
169	A directory in debugfs will be created for each
170	NTB device probed by the tool.  This directory is shortened to *hw*
171	below.
172* *hw*/db
173	This file is used to read, set, and clear the local doorbell.  Not
174	all operations may be supported by all hardware.  To read the doorbell,
175	read the file.  To set the doorbell, write `s` followed by the bits to
176	set (eg: `echo 's 0x0101' > db`).  To clear the doorbell, write `c`
177	followed by the bits to clear.
178* *hw*/mask
179	This file is used to read, set, and clear the local doorbell mask.
180	See *db* for details.
181* *hw*/peer\_db
182	This file is used to read, set, and clear the peer doorbell.
183	See *db* for details.
184* *hw*/peer\_mask
185	This file is used to read, set, and clear the peer doorbell
186	mask.  See *db* for details.
187* *hw*/spad
188	This file is used to read and write local scratchpads.  To read
189	the values of all scratchpads, read the file.  To write values, write a
190	series of pairs of scratchpad number and value
191	(eg: `echo '4 0x123 7 0xabc' > spad`
192	# to set scratchpads `4` and `7` to `0x123` and `0xabc`, respectively).
193* *hw*/peer\_spad
194	This file is used to read and write peer scratchpads.  See
195	*spad* for details.
196
197NTB Hardware Drivers
198====================
199
200NTB hardware drivers should register devices with the NTB core driver.  After
201registering, clients probe and remove functions will be called.
202
203NTB Intel Hardware Driver (ntb\_hw\_intel)
204------------------------------------------
205
206The Intel hardware driver supports NTB on Xeon and Atom CPUs.
207
208Module Parameters:
209
210* b2b\_mw\_idx
211	If the peer ntb is to be accessed via a memory window, then use
212	this memory window to access the peer ntb.  A value of zero or positive
213	starts from the first mw idx, and a negative value starts from the last
214	mw idx.  Both sides MUST set the same value here!  The default value is
215	`-1`.
216* b2b\_mw\_share
217	If the peer ntb is to be accessed via a memory window, and if
218	the memory window is large enough, still allow the client to use the
219	second half of the memory window for address translation to the peer.
220* xeon\_b2b\_usd\_bar2\_addr64
221	If using B2B topology on Xeon hardware, use
222	this 64 bit address on the bus between the NTB devices for the window
223	at BAR2, on the upstream side of the link.
224* xeon\_b2b\_usd\_bar4\_addr64 - See *xeon\_b2b\_bar2\_addr64*.
225* xeon\_b2b\_usd\_bar4\_addr32 - See *xeon\_b2b\_bar2\_addr64*.
226* xeon\_b2b\_usd\_bar5\_addr32 - See *xeon\_b2b\_bar2\_addr64*.
227* xeon\_b2b\_dsd\_bar2\_addr64 - See *xeon\_b2b\_bar2\_addr64*.
228* xeon\_b2b\_dsd\_bar4\_addr64 - See *xeon\_b2b\_bar2\_addr64*.
229* xeon\_b2b\_dsd\_bar4\_addr32 - See *xeon\_b2b\_bar2\_addr64*.
230* xeon\_b2b\_dsd\_bar5\_addr32 - See *xeon\_b2b\_bar2\_addr64*.
231