linux-kernel - Re: [PATCH net-next] net: phy: intel-xway: Add RGMII internal delay configuration

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Date:   Fri, 9 Jul 2021 13:26:58 +0100
From:   "Russell King (Oracle)" <linux@...linux.org.uk>
To:     Martin Schiller <ms@....tdt.de>
Cc:     hauke@...ke-m.de, martin.blumenstingl@...glemail.com,
        f.fainelli@...il.com, andrew@...n.ch, hkallweit1@...il.com,
        davem@...emloft.net, kuba@...nel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next] net: phy: intel-xway: Add RGMII internal delay
 configuration

On Fri, Jul 09, 2021 at 01:57:26PM +0200, Martin Schiller wrote:
> +static int xway_gphy_of_reg_init(struct phy_device *phydev)
> +{
> +	struct device *dev = &phydev->mdio.dev;
> +	int delay_size = ARRAY_SIZE(xway_internal_delay);
> +	s32 rx_int_delay;
> +	s32 tx_int_delay;
> +	int err = 0;
> +	int val;
> +
> +	if (phy_interface_is_rgmii(phydev)) {
> +		val = phy_read(phydev, XWAY_MDIO_MIICTRL);
> +		if (val < 0)
> +			return val;
> +	}
> +
> +	/* Existing behavior was to use default pin strapping delay in rgmii
> +	 * mode, but rgmii should have meant no delay.  Warn existing users.
> +	 */
> +	if (phydev->interface == PHY_INTERFACE_MODE_RGMII) {
> +		const u16 txskew = (val & XWAY_MDIO_MIICTRL_TXSKEW_MASK) >>
> +				   XWAY_MDIO_MIICTRL_TXSKEW_SHIFT;
> +		const u16 rxskew = (val & XWAY_MDIO_MIICTRL_RXSKEW_MASK) >>
> +				   XWAY_MDIO_MIICTRL_RXSKEW_SHIFT;
> +
> +		if (txskew > 0 || rxskew > 0)
> +			phydev_warn(phydev,
> +				    "PHY has delays (e.g. via pin strapping), but phy-mode = 'rgmii'\n"
> +				    "Should be 'rgmii-id' to use internal delays txskew:%x rxskew:%x\n",
> +				    txskew, rxskew);
> +	}
> +
> +	/* RX delay *must* be specified if internal delay of RX is used. */
> +	if (phydev->interface == PHY_INTERFACE_MODE_RGMII_ID ||
> +	    phydev->interface == PHY_INTERFACE_MODE_RGMII_RXID) {
> +		rx_int_delay = phy_get_internal_delay(phydev, dev,
> +						      &xway_internal_delay[0],
> +						      delay_size, true);
> +
> +		if (rx_int_delay < 0) {
> +			phydev_err(phydev, "rx-internal-delay-ps must be specified\n");
> +			return rx_int_delay;
> +		}
> +
> +		val &= ~XWAY_MDIO_MIICTRL_RXSKEW_MASK;
> +		val |= rx_int_delay << XWAY_MDIO_MIICTRL_RXSKEW_SHIFT;
> +	}
> +
> +	/* TX delay *must* be specified if internal delay of TX is used. */
> +	if (phydev->interface == PHY_INTERFACE_MODE_RGMII_ID ||
> +	    phydev->interface == PHY_INTERFACE_MODE_RGMII_TXID) {
> +		tx_int_delay = phy_get_internal_delay(phydev, dev,
> +						      &xway_internal_delay[0],
> +						      delay_size, false);
> +
> +		if (tx_int_delay < 0) {
> +			phydev_err(phydev, "tx-internal-delay-ps must be specified\n");
> +			return tx_int_delay;
> +		}
> +
> +		val &= ~XWAY_MDIO_MIICTRL_TXSKEW_MASK;
> +		val |= tx_int_delay << XWAY_MDIO_MIICTRL_TXSKEW_SHIFT;
> +	}
> +
> +	if (phydev->interface == PHY_INTERFACE_MODE_RGMII_ID ||
> +	    phydev->interface == PHY_INTERFACE_MODE_RGMII_RXID ||
> +	    phydev->interface == PHY_INTERFACE_MODE_RGMII_TXID)
> +		err = phy_write(phydev, XWAY_MDIO_MIICTRL, val);
> +
> +	return err;
> +}

Please reconsider the above.  Maybe something like the following would
be better:

	u16 mask = 0;
	int val = 0;

	if (!phy_interface_is_rgmii(phydev))
		return;

	if (phydev->interface == PHY_INTERFACE_MODE_RGMII) {
		u16 txskew, rxskew;

		val = phy_read(phydev, XWAY_MDIO_MIICTRL);
		if (val < 0)
			return val;

		txskew = (val & XWAY_MDIO_MIICTRL_TXSKEW_MASK) >>
			 XWAY_MDIO_MIICTRL_TXSKEW_SHIFT;
		rxskew = (val & XWAY_MDIO_MIICTRL_RXSKEW_MASK) >>
			 XWAY_MDIO_MIICTRL_RXSKEW_SHIFT;

		if (txskew > 0 || rxskew > 0)
			phydev_warn(phydev,
				    "PHY has delays (e.g. via pin strapping), but phy-mode = 'rgmii'\n"
				    "Should be 'rgmii-id' to use internal delays txskew:%x rxskew:%x\n",
				    txskew, rxskew);
		return;
	}

	if (phydev->interface == PHY_INTERFACE_MODE_RGMII_ID ||
	    phydev->interface == PHY_INTERFACE_MODE_RGMII_RXID) {
		...
		mask |= XWAY_MDIO_MIICTRL_RXSKEW_MASK;
		val |= rx_int_delay << XWAY_MDIO_MIICTRL_RXSKEW_SHIFT;
	}

	if (phydev->interface == PHY_INTERFACE_MODE_RGMII_ID ||
	    phydev->interface == PHY_INTERFACE_MODE_RGMII_TXID) {
		...
		mask |= XWAY_MDIO_MIICTRL_TXSKEW_MASK;
		val |= rx_int_delay << XWAY_MDIO_MIICTRL_TXSKEW_SHIFT;
	}

	return phy_modify(phydev, XWAY_MDIO_MIICTRL, mask, val);

Using phy_modify() has the advantage that the read-modify-write is
done as a locked transaction on the bus, meaning that it is atomic.
There isn't a high cost to writing functions in a way that makes use
of that as can be seen from the above.

Thanks.

-- 
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