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

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Date:   Wed, 16 Nov 2016 09:10:02 -0800
From:   Alex <alex.g@...ptrum.com>
To:     Andrew Lunn <andrew@...n.ch>
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 11/16/2016 08:54 AM, Andrew Lunn wrote:
> 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.

That's not within the scope of this change. The scope is to make 
rgmii-id work. Any additional changes would be untested.

Alex