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

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Date:   Mon, 15 Aug 2022 19:00:42 +0100
From:   "Russell King (Oracle)" <linux@...linux.org.uk>
To:     Florian Fainelli <f.fainelli@...il.com>
Cc:     netdev@...r.kernel.org,
        Broadcom internal kernel review list 
        <bcm-kernel-feedback-list@...adcom.com>,
        Andrew Lunn <andrew@...n.ch>,
        Heiner Kallweit <hkallweit1@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net-next] net: phy: broadcom: Implement suspend/resume
 for AC131 and BCM5241

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?

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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