PCIe MPS参数介绍及如何更改

1.简介

2.主要功能作用

3.MPS控制策略

4.如何更改

1.简介

MPS

该参数含义是一个TLP包里携带的有效净荷的最大值是多少字节（该限制条件同时适用于写操作和读操作）。

MRRS

该参数含义是一个TLP读请求包，一次最多能向接收端请求读出多少字节。（MRRS参数出现的原因大家有没有想过……其实设定MRRS的原因是为了避免某个设备长时间占用总线）

2.主要功能作用

在MPS在PCIe整体性能中，有至关重要的作用。随着MPS大小的增加，PCIe传输效率也在不断的提升。不过，在x86的机器中，RC端的MPS通常是128B/256B。在ARM CPU中，为了追求高效性能，部分场景也会设置为512B。

整个PCIe系统中，MPS的大小，跟RC、PCIe Switch、Endpoint都有相互的影响，最终TLP传输的数据大小取决与MPS最小的一个设备。比如下图示例，RC MPS=256B，PCIe Switch MPS=512B，但是EP3 MPS=128B。所以最终数据传输的大小采用的是MPS=128B。

以下是一个实例

lspci -s 01:00.0 -vvvv
01:00.0 Non-Volatile memory controller: Toshiba Corporation Device 0116 (prog-if 02 [NVM Express])Subsystem: Toshiba Corporation Device 0001Control: I/O- Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR- FastB2B- DisINTx+Status: Cap+ 66MHz- UDF- FastB2B- ParErr- DEVSEL=fast >TAbort- <TAbort- <MAbort- >SERR- <PERR- INTx-Latency: 0, Cache Line Size: 64 bytesInterrupt: pin A routed to IRQ 16NUMA node: 0Region 0: Memory at 96200000 (64-bit, non-prefetchable) [size=16K]Capabilities: [40] Express (v2) Endpoint, MSI 00DevCap:	MaxPayload 256 bytes, PhantFunc 0, Latency L0s unlimited, L1 unlimitedExtTag- AttnBtn- AttnInd- PwrInd- RBE+ FLReset+ SlotPowerLimit 25.000WDevCtl:	Report errors: Correctable+ Non-Fatal+ Fatal+ Unsupported+RlxdOrd- ExtTag- PhantFunc- AuxPwr- NoSnoop- FLReset-MaxPayload 256 bytes, MaxReadReq 512 bytesDevSta:	CorrErr- UncorrErr- FatalErr- UnsuppReq- AuxPwr- TransPend-LnkCap:	Port #0, Speed 8GT/s, Width x4, ASPM L1, Exit Latency L0s <2us, L1 <32usClockPM- Surprise- LLActRep- BwNot- ASPMOptComp+LnkCtl:	ASPM L1 Enabled; RCB 64 bytes Disabled- CommClk+ExtSynch- ClockPM- AutWidDis- BWInt- AutBWInt-LnkSta:	Speed 8GT/s, Width x4, TrErr- Train- SlotClk+ DLActive- BWMgmt- ABWMgmt-DevCap2: Completion Timeout: Range AB, TimeoutDis+, LTR+, OBFF Not SupportedDevCtl2: Completion Timeout: 50us to 50ms, TimeoutDis-, LTR+, OBFF DisabledLnkCtl2: Target Link Speed: 8GT/s, EnterCompliance- SpeedDis-Transmit Margin: Normal Operating Range, EnterModifiedCompliance- ComplianceSOS-Compliance De-emphasis: -6dBLnkSta2: Current De-emphasis Level: -6dB, EqualizationComplete+, EqualizationPhase1+EqualizationPhase2+, EqualizationPhase3+, LinkEqualizationRequest-Capabilities: [80] Power Management version 3Flags: PMEClk- DSI- D1- D2- AuxCurrent=0mA PME(D0-,D1-,D2-,D3hot-,D3cold-)Status: D0 NoSoftRst+ PME-Enable- DSel=0 DScale=0 PME-Capabilities: [90] MSI: Enable- Count=1/32 Maskable+ 64bit+Address: 0000000000000000  Data: 0000Masking: 00000000  Pending: 00000000Capabilities: [b0] MSI-X: Enable+ Count=32 Masked-Vector table: BAR=0 offset=00002000PBA: BAR=0 offset=00003000

3.MPS控制策略

pcie_bus_tune_off   Disable PCIe MPS (Max Payload Size)tuning and use the BIOS-configured MPS defaults.pcie_bus_safe   Set every device's MPS to the largest valuesupported by all devices below the root complex.pcie_bus_perf   Set device MPS to the largest allowable MPSbased on its parent bus. Also set MRRS (Max Read Request Size) to the largest supportedvalue (no larger than the MPS that the deviceor bus can support) for best performance.pcie_bus_peer2peer  Set every device's MPS to 128B, whichevery device is guaranteed to support. This configuration allows peer-to-peer DMA betweenany pair of devices, possibly at the cost ofreduced performance.  This also guaranteesthat hot-added devices will work.