Re: [PATCH net-next] net: phy: broadcom: Implement suspend/resume for AC131 and BCM5241

[Russell King (Oracle)]

On Mon, Aug 15, 2022 at 10:43:56AM -0700, Florian Fainelli wrote:
> +	/* We cannot use a read/modify/write here otherwise the PHY continues
> +	 * to drive LEDs which defeats the purpose of low power mode.
> +	 */
...
> +	/* Set standby mode */
> +	reg = phy_read(phydev, MII_BRCM_FET_SHDW_AUXMODE4);
> +	if (reg < 0) {
> +		err = reg;
> +		goto done;
> +	}
> +
> +	reg |= MII_BRCM_FET_SHDW_AM4_STANDBY;
> +
> +	err = phy_write(phydev, MII_BRCM_FET_SHDW_AUXMODE4, reg);

Does the read-modify-write problem extend to this register? Why would
the PHY behave differently whether you used phy_modify() here or not?
On the mdio bus, it should be exactly the same - the only difference
is that we're guaranteed to hold the lock over the sequence whereas
this drops and re-acquires the lock.

If it's sensitive to the timing of the read and the write, it suggests
the above code is fragile - maybe there needs to be a minimum delay
inserted between the read and the write?

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