linux-kernel - Re: [PATCH net 1/3] net: hns3: fix loopback test of serdes and phy is failed if duplex is half

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Message-ID: <a060e5cf-c1cf-4dfa-b534-ddb72e8652f8@lunn.ch>
Date: Wed, 17 Sep 2025 18:58:55 +0200
From: Andrew Lunn <andrew@...n.ch>
To: Jijie Shao <shaojijie@...wei.com>
Cc: davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org,
	pabeni@...hat.com, andrew+netdev@...n.ch, horms@...nel.org,
	shenjian15@...wei.com, liuyonglong@...wei.com,
	chenhao418@...wei.com, lantao5@...wei.com,
	huangdonghua3@...artners.com, yangshuaisong@...artners.com,
	huangdengdui@...artners.com, jonathan.cameron@...wei.com,
	salil.mehta@...wei.com, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH net 1/3] net: hns3: fix loopback test of serdes and phy
 is failed if duplex is half

On Wed, Sep 17, 2025 at 08:29:52PM +0800, Jijie Shao wrote:
> If duplex setting is half, mac and phy can not transmit and receive data
> at the same time.

Lets think about the fundamentals of Ethernet, MII and half duplex.

Is this specific to your MAC/PHY combination, or just generally true?

Should this is solved in phylib, because it is true for every MAC/PHY
combination? phylib returning -EINAL to phy_loopback() would seem like
the correct thing to do.

Is it specific to your PHY, but independent of the MAC?  Then the PHY
should return -EINVAL in its set_loopback() method.

> +	hdev->hw.mac.duplex_last = phydev->duplex;
> +
> +	ret = phy_set_bits(phydev, MII_BMCR, BMCR_FULLDPLX);

A MAC driver should not be doing this. What if the PHY is C45 only?
And Marvell PHYs need a soft reset before such an operation take
effect.

    Andrew

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