netdev - Re: [PATCH RFC net-next 1/6] net: phy: realtek: configure SerDes mode for rtl822x/8251b PHYs

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Date: Tue, 27 Feb 2024 14:34:41 +0100
From: Eric Woudstra <ericwouds@...il.com>
To: "Russell King (Oracle)" <linux@...linux.org.uk>
Cc: Andrew Lunn <andrew@...n.ch>, Heiner Kallweit <hkallweit1@...il.com>,
 "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
 Marek Behún <kabel@...nel.org>,
 Frank Wunderlich <frank-w@...lic-files.de>,
 Daniel Golle <daniel@...rotopia.org>, netdev@...r.kernel.org,
 Alexander Couzens <lynxis@...0.eu>
Subject: Re: [PATCH RFC net-next 1/6] net: phy: realtek: configure SerDes mode
 for rtl822x/8251b PHYs

On 2/27/24 11:36, Russell King (Oracle) wrote:

> It would be nice to fill phydev->possible_interfaces even if
> phydev->host_interfaces has not been populated. That means that the
> "newer" paths in phylink can be always used during validation.
> 
> In other words, move the if() test just above this to below it.
> 

I will change it accordingly.

>> +	ret = phy_write_mmd(phydev, MDIO_MMD_VEND1, 0x75f3, 0);
>> +	if (ret < 0)
>> +		return ret;
> 
> It would be nice to know what this is doing.
>> +
>> +	ret = phy_modify_mmd_changed(phydev, MDIO_MMD_VEND1,
>> +				     RTL822X_VND1_SERDES_OPTION,
>> +				     RTL822X_VND1_SERDES_OPTION_MODE_MASK,
>> +				     mode);
>> +	if (ret < 0)
>> +		return ret;
>> +
>> +	/* the following 3 writes into SerDes control are needed for 2500base-x
>> +	 * mode to work properly
>> +	 */
>> +	ret = phy_write_mmd(phydev, MDIO_MMD_VEND1, 0x6a04, 0x0503);
>> +	if (ret < 0)
>> +		return ret;
>> +
>> +	ret = phy_write_mmd(phydev, MDIO_MMD_VEND1, 0x6f10, 0xd455);
>> +	if (ret < 0)
>> +		return ret;
>> +
>> +	return phy_write_mmd(phydev, MDIO_MMD_VEND1, 0x6f11, 0x8020);
> 
> Also for these. "to work properly" is too vague - is it to do with the
> inband signalling?
> 
> 
> Just to confirm that this doesn't change existing device behaviour?

This is a magic sequence all needed to change serdes-option-mode and only
that. This patch (series) does not change inband negotiation behavior of
the phy (there is another sequence of magic numbers to disable it).

I will change the comment to describe it better.

Regards,

Eric