netdev - Re: [PATCH v3 net-next 05/10] net: dsa: microchip: add DSA support for microchip lan937x

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Message-ID: <20210807170009.q2gjoraaheps2tub@skbuf>
Date:   Sat, 7 Aug 2021 20:00:09 +0300
From:   Vladimir Oltean <olteanv@...il.com>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     "Russell King (Oracle)" <linux@...linux.org.uk>,
        Prasanna Vengateshan <prasanna.vengateshan@...rochip.com>,
        netdev@...r.kernel.org, robh+dt@...nel.org,
        UNGLinuxDriver@...rochip.com, Woojung.Huh@...rochip.com,
        hkallweit1@...il.com, davem@...emloft.net, kuba@...nel.org,
        linux-kernel@...r.kernel.org, vivien.didelot@...il.com,
        f.fainelli@...il.com, devicetree@...r.kernel.org
Subject: Re: [PATCH v3 net-next 05/10] net: dsa: microchip: add DSA support
 for microchip lan937x

On Sat, Aug 07, 2021 at 05:40:35PM +0200, Andrew Lunn wrote:
> I suspect it is a 50/50 roll of a dice what rx and tx actually
> mean. Is it from the perspective of the MAC or the PHY? Luckily,
> rgmii-rxid and rgmii-txid don't appear in DT very often.

I checked an NXP board schematic which has both, and:

When you connect a MAC to a PHY using RGMII, the RXD[3:0] pins of the
MAC connect to the RXD[3:0] pins of the PHY, and the TXD[3:0] goes to
TXD[3:0]. So it is neither the perspective of the MAC nor of the PHY.

When you connect a MAC to a MAC using RGMII, the RXD[3:0] of one MAC
goes to the TXD[3:0] of the other, and vice versa.

Nonetheless, a phy-mode of "rgmii-rxid" always means to the local MAC
that "the RX delays have been dealt with" - either by PCB traces, or a
remote MAC applying TX delay, or the PHY applying RX delay.