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Valued Contributor III
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device tree source ehternet and skew timing => performance issue ?

hello, 

i ve some problem with the network performance and the cyclone 5 on linux. 

i try different kernel versions, after some error research i found 

the solution, it was/is the ethernet skew. 

 

with the socfpga default device tree source @github (socfpga_cyclone5_sockit.dts), i get only ~10-40 Mbits/s with iperf. 

i change the skew at the dts and the speed is now ~600Mbits/s. 

 

That was the original stuff: 

&gmac1 { status = "okay"; phy-mode = "rgmii"; rxd0-skew-ps = <0>; rxd1-skew-ps = <0>; rxd2-skew-ps = <0>; rxd3-skew-ps = <0>; txen-skew-ps = <0>; txc-skew-ps = <2600>; rxdv-skew-ps = <0>; rxc-skew-ps = <2000>; }; 

 

 

i change it to: 

&gmac1 { status = "okay"; phy-mode = "rgmii"; txd0-skew-ps = <0>; /* -420ps */ txd1-skew-ps = <0>; /* -420ps */ txd2-skew-ps = <0>; /* -420ps */ txd3-skew-ps = <0>; /* -420ps */ rxd0-skew-ps = <420>; /* 0ps */ rxd1-skew-ps = <420>; /* 0ps */ rxd2-skew-ps = <420>; /* 0ps */ rxd3-skew-ps = <420>; /* 0ps */ txen-skew-ps = <0>; /* -420ps */ txc-skew-ps = <1860>; /* 960ps */ rxdv-skew-ps = <420>; /* 0ps */ rxc-skew-ps = <1680>; /* 780ps */ max-frame-size = <3800>; status = "okay"; }; 

 

 

 

Where can i get the skew settings? 

i dont want to roll the dice. 

 

we use an custom design board with cylcone 5 and an micrel ksz9031 phy. 

 

 

thanks for the support
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Valued Contributor III
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Hello roll3, 

 

I stumbled over the same question. So how to find/calculate the correct PHY skew settings? I found the following two sources which deals with the topic: 

1.“Cyclone V and Arria V SoC Device Design Guideline -> Board Design Guideline for SoC FPGAs -> Design Guideline for HPS Interfaces -> HPS EMAC PHY Interface: https://www.altera.com/documentation/doq1481305867183.html#fvj1481304448658 

2. KSZ9031RNX Datasheet -> 3.9.3 RGMII PAD SKEW REGISTERS: http://ww1.microchip.com/downloads/en/devicedoc/00002117f.pdf 

 

So the skew settings depends on the used MAC (here Cyclone V), the board traces (your layout) and the PHY (here the KSZ9031). However, I did not figure out how to apply the given information correctly. Do you have newer information about the topic? 

 

Best regards, 

niosIIuser
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