From cdcca896aee19e338adf3000512cade4befa5c69 Mon Sep 17 00:00:00 2001 From: Serge Semin Date: Wed, 14 Dec 2016 02:49:19 +0300 Subject: 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: Serge Semin Acked-by: Allen Hubbe Signed-off-by: Jon Mason --- Documentation/ntb.txt | 99 ++++++++++++++++++++++++++++++++++++++++++++++----- 1 file changed, 91 insertions(+), 8 deletions(-) (limited to 'Documentation') diff --git a/Documentation/ntb.txt b/Documentation/ntb.txt index 1d9bbabb6c79..a5af4f0159f3 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 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 -- cgit v1.2.3