netdev - Re: [PATCH net-next] net: dsa: sja1105: enable internal pull-down for RX_DV/CRS_DV/RX_CTL and RX

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Message-Id: <20200420.110038.1108481207729207065.davem@davemloft.net>
Date:   Mon, 20 Apr 2020 11:00:38 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     olteanv@...il.com
Cc:     f.fainelli@...il.com, vivien.didelot@...il.com, andrew@...n.ch,
        netdev@...r.kernel.org
Subject: Re: [PATCH net-next] net: dsa: sja1105: enable internal pull-down
 for RX_DV/CRS_DV/RX_CTL and RX_ER

From: Vladimir Oltean <olteanv@...il.com>
Date: Fri, 17 Apr 2020 22:50:52 +0300

> From: Vladimir Oltean <vladimir.oltean@....com>
> 
> Some boards do not have the RX_ER MII signal connected. Normally in such
> situation, those pins would be grounded, but then again, some boards
> left it electrically floating.
> 
> When sending traffic to those switch ports, one can see that the
> N_SOFERR statistics counter is incrementing once per each packet. The
> user manual states for this counter that it may count the number of
> frames "that have the MII error input being asserted prior to or
> up to the SOF delimiter byte". So the switch MAC is sampling an
> electrically floating signal, and preventing proper traffic reception
> because of that.
> 
> As a workaround, enable the internal weak pull-downs on the input pads
> for the MII control signals. This way, a floating signal would be
> internally tied to ground.
> 
> The logic levels of signals which _are_ externally driven should not be
> bothered by this 40-50 KOhm internal resistor. So it is not an issue to
> enable the internal pull-down unconditionally, irrespective of PHY
> interface type (MII, RMII, RGMII, SGMII) and of board layout.
> 
> Signed-off-by: Vladimir Oltean <vladimir.oltean@....com>

Applied, thanks.