# 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
## 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 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.
* 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
* 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.