On Wed, May 29, 2024 at 06:17:22PM +0800, Yanteng Si wrote:

 3. Our priv->dma_cap.pcs is false, so let's use PHY_INTERFACE_MODE_NA;

Useful to note.

 4. Our GMAC does not support Delay, so let's use PHY_INTERFACE_MODE_RGMII_ID,
    the current dts is wrong, a fix patch will be sent to the LoongArch list
    later.

RGMII requires a delay somewhere in the system, and there are three
options: at the MAC, in the board traces, or at the PHY. The
PHY_INTERFACE_MODE_RGMII* passed to the PHY determines what delays the
PHY uses, and thus what the GMAC uses has no bearing on this - if the
board traces are adding the required delay, then
PHY_INTERFACE_MODE_RGMII is sufficient.

If the board traces do not add the required delay, and the GMAC doesn't
add a delay, then it is necessary to add the delay at the PHY, so
using PHY_INTERFACE_MODE_RGMII_ID is appropriate.

It's all detailed in Documentation/networking/phy.rst

What isn't documented there (and arguably should be) is if the MAC
adjusts its delays according to the PHY interface mode, then the MAC
should pass PHY_INTERFACE_MODE_RGMII to the PHY _irrespective_ of
which _ID/_TXID/ _RXID has been selected by firmware (since we don't
want the PHY to be adding its own delays if they've already been taken
care of by the MAC.)

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