linux-kernel - Re: [PATCH net-next v1 3/3] net: dsa: microchip: Fix PHY loopback configuration for KSZ8794 and KSZ8873

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Message-ID: <20231207051502.GB1324895@pengutronix.de>
Date:   Thu, 7 Dec 2023 06:15:02 +0100
From:   Oleksij Rempel <o.rempel@...gutronix.de>
To:     Vladimir Oltean <olteanv@...il.com>
Cc:     Woojung Huh <woojung.huh@...rochip.com>,
        Andrew Lunn <andrew@...n.ch>,
        Arun Ramadoss <arun.ramadoss@...rochip.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        UNGLinuxDriver@...rochip.com, Eric Dumazet <edumazet@...gle.com>,
        kernel@...gutronix.de, Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH net-next v1 3/3] net: dsa: microchip: Fix PHY loopback
 configuration for KSZ8794 and KSZ8873

On Thu, Dec 07, 2023 at 02:28:23AM +0200, Vladimir Oltean wrote:
> On Tue, Nov 21, 2023 at 04:24:26PM +0100, Oleksij Rempel wrote:
> > Correct the PHY loopback bit handling in the ksz8_w_phy_bmcr and
> > ksz8_r_phy_bmcr functions for KSZ8794 and KSZ8873 variants in the ksz8795
> > driver. Previously, the code erroneously used Bit 7 of port register 0xD
> > for both chip variants, which is actually for LED configuration. This
> > update ensures the correct registers and bits are used for the PHY
> > loopback feature:
> > 
> > - For KSZ8794: Use 0xF / Bit 7.
> > - For KSZ8873: Use 0xD / Bit 0.
> > 
> > Signed-off-by: Oleksij Rempel <o.rempel@...gutronix.de>
> > ---
> 
> How did you find, and how did you test this, and on which one of the switches?

I tested it by using "ethtool -t lanX" command on KSZ8873. Before this
patch the link will stop to work _after_ end of the selftest. The
selftest will fail too.

After this patch, the selftest is passed, except of the TCP test. And
link is working _after_ the selftest,

> Opening the KSZ8873 datasheet, I am confused about their description of
> the "far-end loopback". They make it sound as if this loops the packets
> _received_ from the media side of PHY port A back to the transmit side
> of PHY port A. But the route that these packets take is through the MAC
> of PHY port A, then the switching fabric, then PHY port B which reflects
> them back to PHY port A, where they finally egress.
> 
> Actually, they even go as far as saying that if you set the loopback bit
> of port 1, the packets that will be looped back will be from port 2's
> RXP/RXM to TXP/TXM pins, and viceversa.
> 
> If true, I believe this isn't the behavior expected by phy_loopback(),
> where the TX signals from the media side of the PHY are looped back into
> the RX side.
> 
> 

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