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Date:   Mon, 29 Aug 2022 15:09:55 +0200
From:   Andrew Lunn <andrew@lunn.ch>
To:     Anand Moon <anand@edgeble.ai>
Cc:     Giuseppe Cavallaro <peppe.cavallaro@st.com>,
        Alexandre Torgue <alexandre.torgue@foss.st.com>,
        Jose Abreu <joabreu@synopsys.com>,
        "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        Maxime Coquelin <mcoquelin.stm32@gmail.com>,
        Sugar Zhang <sugar.zhang@rock-chips.com>,
        David Wu <david.wu@rock-chips.com>,
        Jagan Teki <jagan@edgeble.ai>, netdev@vger.kernel.org,
        linux-stm32@st-md-mailman.stormreply.com,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/2] net: ethernet: stmicro: stmmac: dwmac-rk: Add rv1126
 support
Message-ID: <Ywy6o2d9j4Z7+WYX@lunn.ch>
References: <20220829065044.1736-1-anand@edgeble.ai>
 <20220829065044.1736-2-anand@edgeble.ai>
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On Mon, Aug 29, 2022 at 06:50:42AM +0000, Anand Moon wrote:
> Rockchip RV1126 has GMAC 10/100/1000M ethernet controller
> via RGMII and RMII interfaces are configured via M0 and M1 pinmux.
> 
> This patch adds rv1126 support by adding delay lines of M0 and M1
> simultaneously.

What does 'delay lines' mean with respect to RGMII?

The RGMII signals need a 2ns delay between the clock and the data
lines. There are three places this can happen:

1) In the PHY
2) Extra long lines on the PCB
3) In the MAC

Generally, 1) is used, and controlled via phy-mode. A value of
PHY_INTERFACE_MODE_RGMII_ID passed to the PHY driver means it will add
these delays.

You don't want both the MAC and the PHY adding delays.

    Andrew