netdev - Re: [PATCH v2] drivers/net/phy: add driver for Microchip LAN867x 10BASE-T1S PHY

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Message-ID: <de2435f5-cbb0-4303-ad6a-878cf008074c@lunn.ch>
Date:   Wed, 19 Apr 2023 17:27:10 +0200
From:   Andrew Lunn <andrew@...n.ch>
To:     Vladimir Oltean <olteanv@...il.com>
Cc:     Ramón Nordin Rodriguez 
        <ramon.nordin.rodriguez@...roamp.se>,
        Heiner Kallweit <hkallweit1@...il.com>,
        Russell King <linux@...linux.org.uk>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org,
        Parthiban.Veerasooran@...rochip.com
Subject: Re: [PATCH v2] drivers/net/phy: add driver for Microchip LAN867x
 10BASE-T1S PHY

> > +static int lan867x_read_status(struct phy_device *phydev)
> > +{
> > +	/* The phy has some limitations, namely:
> > +	 *  - always reports link up
> > +	 *  - only supports 10MBit half duplex
> > +	 *  - does not support auto negotiate
> > +	 */
> > +	phydev->link = 1;
> > +	phydev->duplex = DUPLEX_HALF;
> > +	phydev->speed = SPEED_10;
> > +	phydev->autoneg = AUTONEG_DISABLE;
> 
> Sounds really suboptimal if phylib has to poll once per second to find
> out static information. Does the PHY have an architectural limitation in
> that it does not report link status? Is it a T1S thing? Something should
> change here, but I'm not really sure what. A PHY that doesn't report
> link status seems... strange?

Hi Vladimir

I thought that as well. I skimmed the data sheet, and i don't see
anything. You can maybe imply there is no link by looking for PLCA
beacons, but i don't know how reliable that is. You are not forced to
use PLCA, so there might not be any, but communication is still
possible.

The other T1S PHY, NCN26000, does have link status. So i don't think
it is a T1S thing.

Also, the typical use case of T1S is automotive. There are no RJ45
plug/sockets to be unplugged. It is an industrial connector. And i
expect such applications don't really care about L2 connectivity.  L7
either works, or it does not.

My thinking was, the overhead of a pointless poll once per second is
minimal, not enough to justify a change to the core.

	 Andrew