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 11:09:11 -0700
From:   Florian Fainelli <f.fainelli@...il.com>
To:     "Russell King (Oracle)" <linux@...linux.org.uk>,
        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 8/15/22 11:00, Russell King (Oracle) wrote:
> 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.

What read-modify-write problem are you referring to, that is, are you 
talking about my statement about setting BMCR.PDOWN only or something else?

I could use phy_modify(), sure.
-- 
Florian