linux-kernel - Re: [PATCH v2 1/1] net: stmmac: dwmac-meson8b: Fix the RGMII TX delay on Meson8b/8m2 SoCs

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Message-ID: <20191226213614.GC32477@lunn.ch>
Date:   Thu, 26 Dec 2019 22:36:14 +0100
From:   Andrew Lunn <andrew@...n.ch>
To:     Martin Blumenstingl <martin.blumenstingl@...glemail.com>
Cc:     f.fainelli@...il.com, davem@...emloft.net, netdev@...r.kernel.org,
        linux-amlogic@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v2 1/1] net: stmmac: dwmac-meson8b: Fix the RGMII TX
 delay on Meson8b/8m2 SoCs

On Thu, Dec 26, 2019 at 08:01:01PM +0100, Martin Blumenstingl wrote:
> GXBB and newer SoCs use the fixed FCLK_DIV2 (1GHz) clock as input for
> the m250_sel clock. Meson8b and Meson8m2 use MPLL2 instead, whose rate
> can be adjusted at runtime.
> 
> So far we have been running MPLL2 with ~250MHz (and the internal
> m250_div with value 1), which worked enough that we could transfer data
> with an TX delay of 4ns. Unfortunately there is high packet loss with
> an RGMII PHY when transferring data (receiving data works fine though).
> Odroid-C1's u-boot is running with a TX delay of only 2ns as well as
> the internal m250_div set to 2 - no lost (TX) packets can be observed
> with that setting in u-boot.
> 
> Manual testing has shown that the TX packet loss goes away when using
> the following settings in Linux (the vendor kernel uses the same
> settings):
> - MPLL2 clock set to ~500MHz
> - m250_div set to 2
> - TX delay set to 2ns on the MAC side
> 
> Update the m250_div divider settings to only accept dividers greater or
> equal 2 to fix the TX delay generated by the MAC.
> 
> iperf3 results before the change:
> [ ID] Interval           Transfer     Bitrate         Retr
> [  5]   0.00-10.00  sec   182 MBytes   153 Mbits/sec  514      sender
> [  5]   0.00-10.00  sec   182 MBytes   152 Mbits/sec           receiver
> 
> iperf3 results after the change (including an updated TX delay of 2ns):
> [ ID] Interval           Transfer     Bitrate         Retr  Cwnd
> [  5]   0.00-10.00  sec   927 MBytes   778 Mbits/sec    0      sender
> [  5]   0.00-10.01  sec   927 MBytes   777 Mbits/sec           receiver
> 
> Fixes: 4f6a71b84e1afd ("net: stmmac: dwmac-meson8b: fix internal RGMII clock configuration")
> Signed-off-by: Martin Blumenstingl <martin.blumenstingl@...glemail.com>

Reviewed-by: Andrew Lunn <andrew@...n.ch>

    Andrew