netdev - i.MX8MM Ethernet TX Bandwidth Fluctuations

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Message-ID: <421cc86c-b66f-b372-32f7-21e59f9a98bc@kontron.de>
Date:   Thu, 6 May 2021 16:45:38 +0200
From:   Frieder Schrempf <frieder.schrempf@...tron.de>
To:     NXP Linux Team <linux-imx@....com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>
Subject: i.MX8MM Ethernet TX Bandwidth Fluctuations

Hi,

we observed some weird phenomenon with the Ethernet on our i.MX8M-Mini boards. It happens quite often that the measured bandwidth in TX direction drops from its expected/nominal value to something like 50% (for 100M) or ~67% (for 1G) connections.

So far we reproduced this with two different hardware designs using two different PHYs (RGMII VSC8531 and RMII KSZ8081), two different kernel versions (v5.4 and v5.10) and link speeds of 100M and 1G.

To measure the throughput we simply run iperf3 on the target (with a short p2p connection to the host PC) like this:

	iperf3 -c 192.168.1.10 --bidir

But even something more simple like this can be used to get the info (with 'nc -l -p 1122 > /dev/null' running on the host):

	dd if=/dev/zero bs=10M count=1 | nc 192.168.1.10 1122

The results fluctuate between each test run and are sometimes 'good' (e.g. ~90 MBit/s for 100M link) and sometimes 'bad' (e.g. ~45 MBit/s for 100M link).
There is nothing else running on the system in parallel. Some more info is also available in this post: [1].

If there's anyone around who has an idea on what might be the reason for this, please let me know!
Or maybe someone would be willing to do a quick test on his own hardware. That would also be highly appreciated!

Thanks and best regards
Frieder

[1]: https://community.nxp.com/t5/i-MX-Processors/i-MX8MM-Ethernet-TX-Bandwidth-Fluctuations/m-p/1242467#M170563