linux-kernel - Re: [PATCH v2 1/3] dt-bindings: net: fsl-fec add mdc/mdio bitbang option

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Message-ID: <20201130222645.GG2073444@lunn.ch>
Date:   Mon, 30 Nov 2020 23:26:45 +0100
From:   Andrew Lunn <andrew@...n.ch>
To:     Florian Fainelli <f.fainelli@...il.com>
Cc:     Adrien Grassein <adrien.grassein@...il.com>, fugang.duan@....com,
        davem@...emloft.net, kuba@...nel.org, robh+dt@...nel.org,
        netdev@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/3] dt-bindings: net: fsl-fec add mdc/mdio bitbang
 option

> >> I am currently upstreaming the "Nitrogen 8m Mini board" that seems to not use a
> >> "normal" mdio bus but a "bitbanged" one with the fsl fec driver.
> > 
> > Any idea why?
> > 
> > Anyway, you should not replicate code, don't copy bitbanging code into
> > the FEC. Just use the existing bit-banger MDIO bus master driver.
> 
> Right there should be no need for you to modify the FEC driver at all,
> there is an existing generic bitbanged MDIO bus driver here:

Hi Florian

Speculation on my part, until i hear back on the Why? question, but
i'm guessing the board has a wrong pullup on the MDIO line. It takes
too long for the PHY/FEC to pull the line low at the default
2.5MHz. bit-banging is much slower, so it works.

If i'm right, there is a much simpler fix for this. Use the
clock-frequency property for the MDIO bus to slow the clock down.

	Andrew