linux-kernel - Re: [PATCH v5 10/10] net: stmmac: dwmac-sun8i: Handle integrated/external MDIOs

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Message-ID: <20170908141736.GF25219@lunn.ch>
Date:   Fri, 8 Sep 2017 16:17:36 +0200
From:   Andrew Lunn <andrew@...n.ch>
To:     Corentin Labbe <clabbe.montjoie@...il.com>
Cc:     robh+dt@...nel.org, mark.rutland@....com,
        maxime.ripard@...e-electrons.com, wens@...e.org,
        linux@...linux.org.uk, catalin.marinas@....com,
        will.deacon@....com, peppe.cavallaro@...com,
        alexandre.torgue@...com, f.fainelli@...il.com,
        netdev@...r.kernel.org, devicetree@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 10/10] net: stmmac: dwmac-sun8i: Handle
 integrated/external MDIOs

> > Do you know why the reset times out/fails?
> > 
> 
> Because there are nothing connected to it.

That should not be an issue. A read should just return 0xffff.  And it
should return 0xffff fast. The timing of the MDIO protocol is fixed. A
read or a write takes a fixed number of cycles, independent of if
there is a device there or not. The bus data line has a pullup, so if
you try to access a missing device, you automatically read 0xffff.

       Andrew