Commit cdcca896 authored by Serge Semin's avatar Serge Semin Committed by Jon Mason

NTB: Add new Memory Windows API documentation

Since the new API slightly changes the way a typical NTB client driver
works, the documentation file needs to be appropriately updated.
Signed-off-by: default avatarSerge Semin <fancer.lancer@gmail.com>
Acked-by: default avatarAllen Hubbe <Allen.Hubbe@dell.com>
Signed-off-by: default avatarJon Mason <jdmason@kudzu.us>
parent bc3e49ad
# 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
memory.
the separate memory systems of two or more computers to the same PCI-Express
fabric. Existing NTB hardware supports a common feature set: doorbell
registers and memory translation windows, as well as non common features like
scratchpad and message registers. 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. Message registers can
be utilized for the same purpose. Additionally they are provided with with
special status bits to make sure the information isn't rewritten by another
peer. Doorbell registers provide a way for peers to send interrupt events.
Memory windows allow translated read and write access to the peer memory.
## NTB Core Driver (ntb)
......@@ -26,6 +28,87 @@ 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 Typical client driver implementation
Primary purpose of NTB is to share some peace of memory between at least two
systems. So the NTB device features like Scratchpad/Message registers are
mainly used to perform the proper memory window initialization. Typically
there are two types of memory window interfaces supported by the NTB API:
inbound translation configured on the local ntb port and outbound translation
configured by the peer, on the peer ntb port. The first type is
depicted on the next figure
Inbound translation:
Memory: Local NTB Port: Peer NTB Port: Peer MMIO:
____________
| dma-mapped |-ntb_mw_set_trans(addr) |
| memory | _v____________ | ______________
| (addr) |<======| MW xlat addr |<====| MW base addr |<== memory-mapped IO
|------------| |--------------| | |--------------|
So typical scenario of the first type memory window initialization looks:
1) allocate a memory region, 2) put translated address to NTB config,
3) somehow notify a peer device of performed initialization, 4) peer device
maps corresponding outbound memory window so to have access to the shared
memory region.
The second type of interface, that implies the shared windows being
initialized by a peer device, is depicted on the figure:
Outbound translation:
Memory: Local NTB Port: Peer NTB Port: Peer MMIO:
____________ ______________
| dma-mapped | | | MW base addr |<== memory-mapped IO
| memory | | |--------------|
| (addr) |<===================| MW xlat addr |<-ntb_peer_mw_set_trans(addr)
|------------| | |--------------|
Typical scenario of the second type interface initialization would be:
1) allocate a memory region, 2) somehow deliver a translated address to a peer
device, 3) peer puts the translated address to NTB config, 4) peer device maps
outbound memory window so to have access to the shared memory region.
As one can see the described scenarios can be combined in one portable
algorithm.
Local device:
1) Allocate memory for a shared window
2) Initialize memory window by translated address of the allocated region
(it may fail if local memory window initialization is unsupported)
3) Send the translated address and memory window index to a peer device
Peer device:
1) Initialize memory window with retrieved address of the allocated
by another device memory region (it may fail if peer memory window
initialization is unsupported)
2) Map outbound memory window
In accordance with this scenario, the NTB Memory Window API can be used as
follows:
Local device:
1) ntb_mw_count(pidx) - retrieve number of memory ranges, which can
be allocated for memory windows between local device and peer device
of port with specified index.
2) ntb_get_align(pidx, midx) - retrieve parameters restricting the
shared memory region alignment and size. Then memory can be properly
allocated.
3) Allocate physically contiguous memory region in compliance with
restrictions retrieved in 2).
4) ntb_mw_set_trans(pidx, midx) - try to set translation address of
the memory window with specified index for the defined peer device
(it may fail if local translated address setting is not supported)
5) Send translated base address (usually together with memory window
number) to the peer device using, for instance, scratchpad or message
registers.
Peer device:
1) ntb_peer_mw_set_trans(pidx, midx) - try to set received from other
device (related to pidx) translated address for specified memory
window. It may fail if retrieved address, for instance, exceeds
maximum possible address or isn't properly aligned.
2) ntb_peer_mw_get_addr(widx) - retrieve MMIO address to map the memory
window so to have an access to the shared memory.
Also it is worth to note, that method ntb_mw_count(pidx) should return the
same value as ntb_peer_mw_count() on the peer with port index - pidx.
### NTB Transport Client (ntb\_transport) and NTB Netdev (ntb\_netdev)
The primary client for NTB is the Transport client, used in tandem with NTB
......
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