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Subject[PATCH v2 7/9] NTB: Add new Memory Windows API documentation
Date
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: Serge Semin <fancer.lancer@gmail.com>

---
Documentation/ntb.txt | 99 ++++++++++++++++++++++++++++++++++++++++++++++-----
1 file changed, 91 insertions(+), 8 deletions(-)

diff --git a/Documentation/ntb.txt b/Documentation/ntb.txt
index 1d9bbab..d01bb69 100644
--- a/Documentation/ntb.txt
+++ b/Documentation/ntb.txt
@@ -1,14 +1,16 @@
# 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 utialized 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 regiesters 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 initialzation 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 complience 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
--
2.6.6
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