DE5-Net uses only 4 PCIe lanes

Hi everyone,

after installing a DE5-Net I ran the aocl diagnose acl0 command and I saw that the top transfer speed was of only 1124.51 mb/s and 1559.28 mb/s for writes and reads respectively.

It looked odd since the beginning because with 8 lanes that transfer at 500MB/s/lane I have a nominal speed of 4000 MB/s and the results I got show only a 38% of efficiency in the driver.

Thereafter, I wrote my own OpenCL benchmark increasing the block size and I got similar results. Still perplexed, I looked into the driver and I found the following statement:

--- Quote Start ---

The driver achieves approximately 3100 MB/sec on gen2 x 8 PCIe core with SG DMA on Stratix IV GX FPGA (77.5% efficiency)

--- Quote End ---

This showed me that I'm getting only half of the achievable bandwidth.

Finally, using the PCI utilities I found out that even if the PCI slot is a x8 the card has negotiated a x4 width (see output below).

Is it a driver problem?

Am I missing something?

Does somebody know how to use all the 8 lanes?

p.s: I don't know if it's relevant or not, but I also made sure that PCI Express Control DIP Switch on the bottom is set as suggested in the user manual: SW7.1 and SW7.2 off, and SW7.3 on.

# lspci -vv -s 0a:00.0

0a:00.0 Unassigned class : Altera Corporation Device ab00 (rev 01)
    Subsystem: Altera Corporation Device 0004
    Control: 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
    Interrupt: pin A routed to IRQ 16
    Region 0: Memory at f2100000 (64-bit, prefetchable) 
    Capabilities:  MSI: Enable- Count=1/4 Maskable- 64bit+
        Address: 0000000000000000  Data: 0000
    Capabilities:  Power Management version 3
        Flags: PMEClk- DSI- D1- D2- AuxCurrent=0mA PME(D0-,D1-,D2-,D3hot-,D3cold-)
        Status: D0 NoSoftRst- PME-Enable- DSel=0 DScale=0 PME-
    Capabilities:  Express (v2) Endpoint, MSI 00
        DevCap:    MaxPayload 256 bytes, PhantFunc 0, Latency L0s <64ns, L1 <1us
            ExtTag- AttnBtn- AttnInd- PwrInd- RBE+ FLReset-
        DevCtl:    Report errors: Correctable- Non-Fatal- Fatal- Unsupported-
            RlxdOrd- ExtTag- PhantFunc- AuxPwr- NoSnoop+
            MaxPayload 128 bytes, MaxReadReq 512 bytes
        DevSta:    CorrErr- UncorrErr- FatalErr- UnsuppReq- AuxPwr- TransPend-
        LnkCap:    Port# 1, Speed 5GT/s, Width x8, ASPM not supported, Exit Latency L0s <4us, L1 <1us
            ClockPM- Surprise- LLActRep- BwNot-
    LnkCtl:    ASPM Disabled; RCB 64 bytes Disabled- CommClk+
            ExtSynch- ClockPM- AutWidDis- BWInt- AutBWInt-
        LnkSta:    Speed 5GT/s, Width x4, TrErr- Train- SlotClk+ DLActive- BWMgmt- ABWMgmt-
        DevCap2: Completion Timeout: Range ABCD, TimeoutDis+, LTR-, OBFF Not Supported
        DevCtl2: Completion Timeout: 50us to 50ms, TimeoutDis-, LTR-, OBFF Disabled
        LnkCtl2: Target Link Speed: 5GT/s, EnterCompliance- SpeedDis-
             Transmit Margin: Normal Operating Range, EnterModifiedCompliance- ComplianceSOS-
             Compliance De-emphasis: -6dB
        LnkSta2: Current De-emphasis Level: -6dB, EqualizationComplete-, EqualizationPhase1-
             EqualizationPhase2-, EqualizationPhase3-, LinkEqualizationRequest-
    Capabilities:  Virtual Channel
        Caps:    LPEVC=0 RefClk=100ns PATEntryBits=1
        Arb:    Fixed- WRR32- WRR64- WRR128-
        Ctrl:    ArbSelect=Fixed
        Status:    InProgress-
        VC0:    Caps:    PATOffset=00 MaxTimeSlots=1 RejSnoopTrans-
            Arb:    Fixed- WRR32- WRR64- WRR128- TWRR128- WRR256-
            Ctrl:    Enable+ ID=0 ArbSelect=Fixed TC/VC=01
            Status:    NegoPending- InProgress-
    Capabilities:  Vendor Specific Information: ID=a000 Rev=0 Len=044 <?>
    Kernel driver in use: aclpci