netdev - Re: high latency with imx8mm compared to imx6q

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Date:   Sat, 18 Feb 2023 14:20:53 +0100 (CET)
From:   Richard Weinberger <richard@....at>
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>,
        xiaoning wang <xiaoning.wang@....com>,
        linux-imx <linux-imx@....com>
Subject: Re: high latency with imx8mm compared to imx6q

----- Ursprüngliche Mail -----
> Von: "wei fang" <wei.fang@....com>
> 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.

Disabling interrupt coalescing seems to make things much better. :-)
 
>> >>
>> >> 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.

Now I don't see the outlines anymore. Maybe the test from before was really wonky. :-S
Next week I'll do a bigger test on the testbed with interrupt coalescing
disabled at driver level.

Thanks a lot for all the great input so far!
//richard