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