netdev - RE: high latency with imx8mm compared to imx6q

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Date:   Sat, 18 Feb 2023 12:28:20 +0000
From:   Wei Fang <wei.fang@....com>
To:     Richard Weinberger <richard@....at>
CC:     Andrew Lunn <andrew@...n.ch>,
        David Laight <David.Laight@...lab.com>,
        netdev <netdev@...r.kernel.org>,
        Shenwei Wang <shenwei.wang@....com>,
        Clark Wang <xiaoning.wang@....com>,
        dl-linux-imx <linux-imx@....com>
Subject: RE: high latency with imx8mm compared to imx6q


> -----Original Message-----
> From: Richard Weinberger <richard@....at>
> Sent: 2023年2月18日 20:03
> To: Wei Fang <wei.fang@....com>
> Cc: Andrew Lunn <andrew@...n.ch>; David Laight
> <David.Laight@...lab.com>; netdev <netdev@...r.kernel.org>; Shenwei
> Wang <shenwei.wang@....com>; Clark Wang <xiaoning.wang@....com>;
> dl-linux-imx <linux-imx@....com>
> Subject: Re: high latency with imx8mm compared to imx6q
> 
> ----- Ursprüngliche Mail -----
> > Von: "wei fang" <wei.fang@....com>
> >> Hm, I thought my settings are fine (IOW no coalescing at all).
> >> Coalesce parameters for eth0:
> >> Adaptive RX: n/a  TX: n/a
> >> rx-usecs: 0
> >> rx-frames: 0
> >> tx-usecs: 0
> >> tx-frames: 0
> >>
> > Unfortunately, the fec driver does not support to set
> > rx-usecs/rx-frames/tx-usecs/tx-frames
> > to 0 to disable interrupt coalescing. 0 is an invalid parameters. :(
> 
> So setting all values to 1 is the most "no coalescing" setting i can get?
> 
If you use the ethtool cmd, the minimum can only be set to 1.
But you can set the coalescing registers directly on your console,
ENET_RXICn[ICEN] (addr: base + F0h offset + (4d × n) where n=0,1,2) and
ENET_TXICn[ICEN] (addr: base + 100h offset + (4d × n), where n=0d to 2d)
set the ICEN bit (bit 31) to 0:
0 disable Interrupt coalescing.
1 disable Interrupt coalescing.
or modify you fec driver, but remember, the interrupt coalescing feature
can only be disable by setting the ICEN bit to 0, do not set the tx/rx usecs/frames
to 0.

> >>
> >> But I noticed something interesting this morning. When I set
> >> rx-usecs, tx-usecs, rx-frames and tx-frames to 1, *sometimes* the RTT is
> good.
> >>
> >> PING 192.168.0.52 (192.168.0.52) 56(84) bytes of data.
> >> 64 bytes from 192.168.0.52: icmp_seq=1 ttl=64 time=0.730 ms
> >> 64 bytes from 192.168.0.52: icmp_seq=2 ttl=64 time=0.356 ms
> >> 64 bytes from 192.168.0.52: icmp_seq=3 ttl=64 time=0.303 ms
> >> 64 bytes from 192.168.0.52: icmp_seq=4 ttl=64 time=2.22 ms
> >> 64 bytes from 192.168.0.52: icmp_seq=5 ttl=64 time=2.54 ms
> >> 64 bytes from 192.168.0.52: icmp_seq=6 ttl=64 time=0.354 ms
> >> 64 bytes from 192.168.0.52: icmp_seq=7 ttl=64 time=2.22 ms
> >> 64 bytes from 192.168.0.52: icmp_seq=8 ttl=64 time=2.54 ms
> >> 64 bytes from 192.168.0.52: icmp_seq=9 ttl=64 time=2.53 ms
> >>
> >> So coalescing plays a role but it looks like the ethernet controller
> >> does not always obey my settings.
> >> I didn't look into the configured registers so far, maybe ethtool
> >> does not set them correctly.
> >>
> > It look a bit weird. I did the same setting with my i.MX8ULP and
> > didn't have this issue. I'm not sure whether you network is stable or
> > network node devices also enable interrupt coalescing and the relevant
> > parameters are set to a bit high.
> 
> I'm pretty sure my network is good, I've tested also different locations.
> And as I said, with the imx6q on the very same network everything works as
> expected.
> 
> So, with rx-usecs/rx-frames/tx-usecs/tx-frames set to 1, you see a RTT smaller
> than 1ms?
> 
Yes, but my platform is i.MX8ULP not i.MX8MM, I'll check i.MX8MM next Monday.