netdev - Re: [PATCH net-next 1/2] net: phy: Add Speed downshift set driver for Microsemi PHYs.

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Date:   Tue, 18 Oct 2016 16:11:48 +0530
From:   Raju Lakkaraju <Raju.Lakkaraju@...rosemi.com>
To:     Florian Fainelli <f.fainelli@...il.com>
CC:     Andrew Lunn <andrew@...n.ch>, <netdev@...r.kernel.org>,
        <devicetree@...r.kernel.org>, <Allan.Nielsen@...rosemi.com>
Subject: Re: [PATCH net-next 1/2] net: phy: Add Speed downshift set driver
 for Microsemi PHYs.

Hi Florian,

Thank you for review comments.

On Mon, Oct 17, 2016 at 05:38:46AM -0700, Florian Fainelli wrote:
> EXTERNAL EMAIL
> 
> 
> On October 17, 2016 12:31:54 AM PDT, Raju Lakkaraju <Raju.Lakkaraju@...rosemi.com> wrote:
> >Hi Andrew,
> >
> >Thank you for code review and comments.
> >
> >On Fri, Oct 14, 2016 at 02:12:32PM +0200, Andrew Lunn wrote:
> >> EXTERNAL EMAIL
> >>
> >>
> >> On Fri, Oct 14, 2016 at 05:10:32PM +0530, Raju Lakkaraju wrote:
> >> > From: Raju Lakkaraju <Raju.Lakkaraju@...rosemi.com>
> >> >
> >> > For operation in cabling environments that are incompatible with
> >> > 1000BAST-T, VSC8531 device provides an automatic link speed
> >> > downshift operation. When enabled, the device automatically changes
> >> > its 1000BAST-T auto-negotiation to the next slower speed after
> >> > a configured number of failed attempts at 1000BAST-T.
> >> > This feature is useful in setting up in networks using older cable
> >> > installations that include only pairs A and B, and not pairs C and
> >D.
> >>
> >> Any reason not to just turn this on by default when auto-neg is
> >> enabled?
> >>
> >Downshift can enable by default when auto-neg enabled. This is good
> >idea.
> >But we would like to provide option to customer can choose whether this
> >feature need to enable or disable and also configure failure attempts.
> >
> >Do you have any other suggestion how to configure failure attempts?
> 
> Is the speed downshift feature similar to what Intel and Broadcom refer to as wirespeed? I have seen cases with Broadcom PHYs where we had to turn such a feature on to allow auto-negotiation to complete with 4-wire cables, but this had the downside of impacting normal autoneg, so it is left disabled.
> 

Yes. I check the Broadcom wirespeed code. Downshift is similar to wirespeed.
But Broadcom wirespeed configuration in Ethernet controller. 

> I would expect the number of customers using this feature to be fairly limited, so having a tunable to turn this downshift on/off may be acceptable. Ethtool supports a number of tunable parameters now (such as rx_copybreak), there may be room for using something similar for boolean flags like these.
> 

This implementation shows little bit specific to Ethernet controller.
Do you have any PHY specific examples?

In another mail thread, you proposed similar to net device features.
Shall i implement that suggestion here?

> --
> Florian

---
Thanks,
Raju.