netdev - Re: [PATCH v2] net/phy/vitesse: Configure RGMII skew on VSC8601, if needed

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Date:   Wed, 16 Nov 2016 17:54:37 +0100
From:   Andrew Lunn <andrew@...n.ch>
To:     Alex <alex.g@...ptrum.com>
Cc:     f.fainelli@...il.com, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, davem@...emloft.net,
        Gokhan Cosgul <gokhan@...ptrum.com>
Subject: Re: [PATCH v2] net/phy/vitesse: Configure RGMII skew on VSC8601, if
 needed

On Wed, Nov 16, 2016 at 08:44:30AM -0800, Alex wrote:
> 
> 
> On 11/16/2016 05:50 AM, Andrew Lunn wrote:
> >On Wed, Nov 16, 2016 at 01:02:33AM -0800, Alexandru Gagniuc wrote:
> >>With RGMII, we need a 1.5 to 2ns skew between clock and data lines. The
> >>VSC8601 can handle this internally. While the VSC8601 can set more
> >>fine-grained delays, the standard skew settings work out of the box.
> >>The same heuristic is used to determine when this skew should be enabled
> >>as in vsc824x_config_init().
> >>
> >>+/* This adds a skew for both TX and RX clocks, so the skew should only be
> >>+ * applied to "rgmii-id" interfaces. It may not work as expected
> >>+ * on "rgmii-txid", "rgmii-rxid" or "rgmii" interfaces. */
> >
> >Hi Alexandru
> >
> >You should be able to make "rgmii" work as expected. If that is the
> >phy mode, disable the skew.
> 
> And that's exactly the implemented behavior. See
> vsc8601_config_init() below.

I don't think so. vsc8601_config_init() will not cause the skew to be
cleared if the phy-mode is "rgmii" and something else like the
bootloader could of set the skew. So saying that "rgmii" might not
work as expected is true. But with a minor change, you can make it
work as expected.

	Andrew