264 lines
12 KiB
ReStructuredText
264 lines
12 KiB
ReStructuredText
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===========
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NTB Drivers
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===========
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NTB (Non-Transparent Bridge) is a type of PCI-Express bridge chip that connects
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the separate memory systems of two or more computers to the same PCI-Express
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fabric. Existing NTB hardware supports a common feature set: doorbell
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registers and memory translation windows, as well as non common features like
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scratchpad and message registers. Scratchpad registers are read-and-writable
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registers that are accessible from either side of the device, so that peers can
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exchange a small amount of information at a fixed address. Message registers can
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be utilized for the same purpose. Additionally they are provided with
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special status bits to make sure the information isn't rewritten by another
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peer. Doorbell registers provide a way for peers to send interrupt events.
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Memory windows allow translated read and write access to the peer memory.
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NTB Core Driver (ntb)
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=====================
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The NTB core driver defines an api wrapping the common feature set, and allows
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clients interested in NTB features to discover NTB the devices supported by
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hardware drivers. The term "client" is used here to mean an upper layer
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component making use of the NTB api. The term "driver," or "hardware driver,"
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is used here to mean a driver for a specific vendor and model of NTB hardware.
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NTB Client Drivers
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==================
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NTB client drivers should register with the NTB core driver. After
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registering, the client probe and remove functions will be called appropriately
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as ntb hardware, or hardware drivers, are inserted and removed. The
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registration uses the Linux Device framework, so it should feel familiar to
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anyone who has written a pci driver.
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NTB Typical client driver implementation
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----------------------------------------
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Primary purpose of NTB is to share some peace of memory between at least two
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systems. So the NTB device features like Scratchpad/Message registers are
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mainly used to perform the proper memory window initialization. Typically
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there are two types of memory window interfaces supported by the NTB API:
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inbound translation configured on the local ntb port and outbound translation
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configured by the peer, on the peer ntb port. The first type is
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depicted on the next figure::
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Inbound translation:
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Memory: Local NTB Port: Peer NTB Port: Peer MMIO:
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____________
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| dma-mapped |-ntb_mw_set_trans(addr) |
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| memory | _v____________ | ______________
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| (addr) |<======| MW xlat addr |<====| MW base addr |<== memory-mapped IO
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|------------| |--------------| | |--------------|
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So typical scenario of the first type memory window initialization looks:
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1) allocate a memory region, 2) put translated address to NTB config,
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3) somehow notify a peer device of performed initialization, 4) peer device
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maps corresponding outbound memory window so to have access to the shared
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memory region.
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The second type of interface, that implies the shared windows being
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initialized by a peer device, is depicted on the figure::
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Outbound translation:
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Memory: Local NTB Port: Peer NTB Port: Peer MMIO:
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____________ ______________
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| dma-mapped | | | MW base addr |<== memory-mapped IO
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| memory | | |--------------|
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| (addr) |<===================| MW xlat addr |<-ntb_peer_mw_set_trans(addr)
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|------------| | |--------------|
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Typical scenario of the second type interface initialization would be:
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1) allocate a memory region, 2) somehow deliver a translated address to a peer
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device, 3) peer puts the translated address to NTB config, 4) peer device maps
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outbound memory window so to have access to the shared memory region.
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As one can see the described scenarios can be combined in one portable
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algorithm.
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Local device:
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1) Allocate memory for a shared window
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2) Initialize memory window by translated address of the allocated region
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(it may fail if local memory window initialization is unsupported)
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3) Send the translated address and memory window index to a peer device
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Peer device:
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1) Initialize memory window with retrieved address of the allocated
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by another device memory region (it may fail if peer memory window
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initialization is unsupported)
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2) Map outbound memory window
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In accordance with this scenario, the NTB Memory Window API can be used as
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follows:
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Local device:
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1) ntb_mw_count(pidx) - retrieve number of memory ranges, which can
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be allocated for memory windows between local device and peer device
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of port with specified index.
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2) ntb_get_align(pidx, midx) - retrieve parameters restricting the
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shared memory region alignment and size. Then memory can be properly
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allocated.
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3) Allocate physically contiguous memory region in compliance with
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restrictions retrieved in 2).
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4) ntb_mw_set_trans(pidx, midx) - try to set translation address of
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the memory window with specified index for the defined peer device
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(it may fail if local translated address setting is not supported)
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5) Send translated base address (usually together with memory window
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number) to the peer device using, for instance, scratchpad or message
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registers.
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Peer device:
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1) ntb_peer_mw_set_trans(pidx, midx) - try to set received from other
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device (related to pidx) translated address for specified memory
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window. It may fail if retrieved address, for instance, exceeds
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maximum possible address or isn't properly aligned.
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2) ntb_peer_mw_get_addr(widx) - retrieve MMIO address to map the memory
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window so to have an access to the shared memory.
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Also it is worth to note, that method ntb_mw_count(pidx) should return the
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same value as ntb_peer_mw_count() on the peer with port index - pidx.
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NTB Transport Client (ntb\_transport) and NTB Netdev (ntb\_netdev)
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------------------------------------------------------------------
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The primary client for NTB is the Transport client, used in tandem with NTB
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Netdev. These drivers function together to create a logical link to the peer,
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across the ntb, to exchange packets of network data. The Transport client
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establishes a logical link to the peer, and creates queue pairs to exchange
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messages and data. The NTB Netdev then creates an ethernet device using a
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Transport queue pair. Network data is copied between socket buffers and the
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Transport queue pair buffer. The Transport client may be used for other things
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besides Netdev, however no other applications have yet been written.
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NTB Ping Pong Test Client (ntb\_pingpong)
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-----------------------------------------
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The Ping Pong test client serves as a demonstration to exercise the doorbell
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and scratchpad registers of NTB hardware, and as an example simple NTB client.
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Ping Pong enables the link when started, waits for the NTB link to come up, and
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then proceeds to read and write the doorbell scratchpad registers of the NTB.
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The peers interrupt each other using a bit mask of doorbell bits, which is
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shifted by one in each round, to test the behavior of multiple doorbell bits
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and interrupt vectors. The Ping Pong driver also reads the first local
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scratchpad, and writes the value plus one to the first peer scratchpad, each
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round before writing the peer doorbell register.
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Module Parameters:
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* unsafe - Some hardware has known issues with scratchpad and doorbell
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registers. By default, Ping Pong will not attempt to exercise such
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hardware. You may override this behavior at your own risk by setting
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unsafe=1.
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* delay\_ms - Specify the delay between receiving a doorbell
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interrupt event and setting the peer doorbell register for the next
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round.
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* init\_db - Specify the doorbell bits to start new series of rounds. A new
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series begins once all the doorbell bits have been shifted out of
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range.
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* dyndbg - It is suggested to specify dyndbg=+p when loading this module, and
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then to observe debugging output on the console.
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NTB Tool Test Client (ntb\_tool)
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--------------------------------
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The Tool test client serves for debugging, primarily, ntb hardware and drivers.
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The Tool provides access through debugfs for reading, setting, and clearing the
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NTB doorbell, and reading and writing scratchpads.
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The Tool does not currently have any module parameters.
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Debugfs Files:
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* *debugfs*/ntb\_tool/*hw*/
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A directory in debugfs will be created for each
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NTB device probed by the tool. This directory is shortened to *hw*
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below.
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* *hw*/db
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This file is used to read, set, and clear the local doorbell. Not
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all operations may be supported by all hardware. To read the doorbell,
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read the file. To set the doorbell, write `s` followed by the bits to
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set (eg: `echo 's 0x0101' > db`). To clear the doorbell, write `c`
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followed by the bits to clear.
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* *hw*/mask
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This file is used to read, set, and clear the local doorbell mask.
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See *db* for details.
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* *hw*/peer\_db
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This file is used to read, set, and clear the peer doorbell.
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See *db* for details.
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* *hw*/peer\_mask
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This file is used to read, set, and clear the peer doorbell
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mask. See *db* for details.
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* *hw*/spad
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This file is used to read and write local scratchpads. To read
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the values of all scratchpads, read the file. To write values, write a
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series of pairs of scratchpad number and value
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(eg: `echo '4 0x123 7 0xabc' > spad`
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# to set scratchpads `4` and `7` to `0x123` and `0xabc`, respectively).
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* *hw*/peer\_spad
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This file is used to read and write peer scratchpads. See
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*spad* for details.
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NTB MSI Test Client (ntb\_msi\_test)
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------------------------------------
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The MSI test client serves to test and debug the MSI library which
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allows for passing MSI interrupts across NTB memory windows. The
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test client is interacted with through the debugfs filesystem:
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* *debugfs*/ntb\_tool/*hw*/
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A directory in debugfs will be created for each
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NTB device probed by the tool. This directory is shortened to *hw*
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below.
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* *hw*/port
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This file describes the local port number
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* *hw*/irq*_occurrences
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One occurrences file exists for each interrupt and, when read,
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returns the number of times the interrupt has been triggered.
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* *hw*/peer*/port
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This file describes the port number for each peer
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* *hw*/peer*/count
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This file describes the number of interrupts that can be
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triggered on each peer
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* *hw*/peer*/trigger
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Writing an interrupt number (any number less than the value
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specified in count) will trigger the interrupt on the
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specified peer. That peer's interrupt's occurrence file
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should be incremented.
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NTB Hardware Drivers
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====================
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NTB hardware drivers should register devices with the NTB core driver. After
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registering, clients probe and remove functions will be called.
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NTB Intel Hardware Driver (ntb\_hw\_intel)
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------------------------------------------
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The Intel hardware driver supports NTB on Xeon and Atom CPUs.
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Module Parameters:
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* b2b\_mw\_idx
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If the peer ntb is to be accessed via a memory window, then use
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this memory window to access the peer ntb. A value of zero or positive
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starts from the first mw idx, and a negative value starts from the last
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mw idx. Both sides MUST set the same value here! The default value is
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`-1`.
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* b2b\_mw\_share
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If the peer ntb is to be accessed via a memory window, and if
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the memory window is large enough, still allow the client to use the
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second half of the memory window for address translation to the peer.
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* xeon\_b2b\_usd\_bar2\_addr64
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If using B2B topology on Xeon hardware, use
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this 64 bit address on the bus between the NTB devices for the window
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at BAR2, on the upstream side of the link.
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* xeon\_b2b\_usd\_bar4\_addr64 - See *xeon\_b2b\_bar2\_addr64*.
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* xeon\_b2b\_usd\_bar4\_addr32 - See *xeon\_b2b\_bar2\_addr64*.
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* xeon\_b2b\_usd\_bar5\_addr32 - See *xeon\_b2b\_bar2\_addr64*.
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* xeon\_b2b\_dsd\_bar2\_addr64 - See *xeon\_b2b\_bar2\_addr64*.
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* xeon\_b2b\_dsd\_bar4\_addr64 - See *xeon\_b2b\_bar2\_addr64*.
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* xeon\_b2b\_dsd\_bar4\_addr32 - See *xeon\_b2b\_bar2\_addr64*.
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* xeon\_b2b\_dsd\_bar5\_addr32 - See *xeon\_b2b\_bar2\_addr64*.
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