netdev - Re: [PATCH net-next 8/8] net: phy: marvell10g: select host interface configuration

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Date:   Thu, 18 Nov 2021 13:22:18 +0000
From:   "Russell King (Oracle)" <linux@...linux.org.uk>
To:     Vladimir Oltean <olteanv@...il.com>
Cc:     Marek Behún <kabel@...nel.org>,
        netdev@...r.kernel.org, Andrew Lunn <andrew@...n.ch>,
        Jakub Kicinski <kuba@...nel.org>,
        David Miller <davem@...emloft.net>,
        Rob Herring <robh+dt@...nel.org>, devicetree@...r.kernel.org
Subject: Re: [PATCH net-next 8/8] net: phy: marvell10g: select host interface
 configuration

On Thu, Nov 18, 2021 at 02:03:34PM +0200, Vladimir Oltean wrote:
> On Wed, Nov 17, 2021 at 11:50:50PM +0100, Marek Behún wrote:
> > +static int mv3310_select_mactype(unsigned long *interfaces)
> > +{
> > +	if (test_bit(PHY_INTERFACE_MODE_USXGMII, interfaces))
> > +		return MV_V2_33X0_PORT_CTRL_MACTYPE_USXGMII;
> > +	else if (test_bit(PHY_INTERFACE_MODE_SGMII, interfaces) &&
> > +		 test_bit(PHY_INTERFACE_MODE_10GBASER, interfaces))
> > +		return MV_V2_33X0_PORT_CTRL_MACTYPE_10GBASER;
> > +	else if (test_bit(PHY_INTERFACE_MODE_SGMII, interfaces) &&
> > +		 test_bit(PHY_INTERFACE_MODE_RXAUI, interfaces))
> > +		return MV_V2_33X0_PORT_CTRL_MACTYPE_RXAUI;
> > +	else if (test_bit(PHY_INTERFACE_MODE_SGMII, interfaces) &&
> > +		 test_bit(PHY_INTERFACE_MODE_XAUI, interfaces))
> > +		return MV_V2_3310_PORT_CTRL_MACTYPE_XAUI;
> > +	else if (test_bit(PHY_INTERFACE_MODE_10GBASER, interfaces))
> > +		return MV_V2_33X0_PORT_CTRL_MACTYPE_10GBASER_RATE_MATCH;
> > +	else if (test_bit(PHY_INTERFACE_MODE_RXAUI, interfaces))
> > +		return MV_V2_33X0_PORT_CTRL_MACTYPE_RXAUI_RATE_MATCH;
> > +	else if (test_bit(PHY_INTERFACE_MODE_XAUI, interfaces))
> > +		return MV_V2_3310_PORT_CTRL_MACTYPE_XAUI_RATE_MATCH;
> > +	else if (test_bit(PHY_INTERFACE_MODE_SGMII, interfaces))
> > +		return MV_V2_33X0_PORT_CTRL_MACTYPE_10GBASER;
> > +	else
> > +		return -1;
> > +}
> > +
> 
> I would like to understand this heuristic better. Both its purpose and
> its implementation.
> 
> It says:
> (a) If the intersection between interface modes supported by the MAC and
>     the PHY contains USXGMII, then use USXGMII as a MACTYPE
> (b) Otherwise, if the intersection contains both 10GBaseR and SGMII, then
>     use 10GBaseR as MACTYPE
> (...)
> (c) Otherwise, if the intersection contains just 10GBaseR (no SGMII), then
>     use 10GBaseR with rate matching as MACTYPE
> (...)
> (d) Otherwise, if the intersection contains just SGMII (no 10GBaseR), then
>     use 10GBaseR as MACTYPE (no rate matching).

What is likely confusing you is a misinterpretation of the constant.
MV_V2_33X0_PORT_CTRL_MACTYPE_10GBASER actually means the PHY will
choose between 10GBASE-R, 5GBASE-R, 2500BASE-X, and SGMII depending
on the speed negotiated by the media. In this setting, the PHY
dictates which interface mode will be used.

I could have named "MV_V2_33X0_PORT_CTRL_MACTYPE_10GBASER" as
"MV_V2_33X0_PORT_CTRL_MACTYPE_10GBASER_5GBASER_2500BASEX_SGMII_AUTONEG_ON".
Similar with "MV_V2_33X0_PORT_CTRL_MACTYPE_10GBASER_NO_SGMII_AN", which
would be
"MV_V2_33X0_PORT_CTRL_MACTYPE_10GBASER_5GBASER_2500BASEX_SGMII_AUTONEG_OFF".
And "MV_V2_3310_PORT_CTRL_MACTYPE_XAUI" would be
"MV_V2_3310_PORT_CTRL_MACTYPE_XAUI_5GBASER_2500BASEX_SGMII_AUTONEG_ON".

Clearly using such long identifiers would have been rediculous,
especially the second one at 74 characters.

> First of all, what is MACTYPE exactly? And what is the purpose of
> changing it? What would happen if this configuration remained fixed, as
> it were?

The PHY defines the MAC interface mode depending on the MACTYPE
setting selected and the results of the media side negotiation.

I think the above answers your remaining questions.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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