linux-kernel - Re: [PATCH] net: phy: mscc: enable MAC SerDes autonegotiation

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Date:   Sun, 27 Mar 2022 14:02:29 +0530
From:   Raag Jadav <raagjadav@...il.com>
To:     "Russell King (Oracle)" <linux@...linux.org.uk>
Cc:     Andrew Lunn <andrew@...n.ch>,
        Heiner Kallweit <hkallweit1@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Steen Hegelund <steen.hegelund@...rochip.com>,
        Bjarni Jonasson <bjarni.jonasson@...rochip.com>,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] net: phy: mscc: enable MAC SerDes autonegotiation

On Sat, Feb 26, 2022 at 04:31:57PM +0000, Russell King (Oracle) wrote:
> On Sat, Feb 26, 2022 at 12:53:27PM +0530, Raag Jadav wrote:
> > On Thu, Feb 24, 2022 at 10:48:57AM +0000, Russell King (Oracle) wrote:
> > > Sorry for the late comment on this patch.
> > > 
> > > On Sat, Feb 05, 2022 at 12:14:52PM +0530, Raag Jadav wrote:
> > > > +static int vsc85xx_config_inband_aneg(struct phy_device *phydev, bool enabled)
> > > > +{
> > > > +	int rc;
> > > > +	u16 reg_val = 0;
> > > > +
> > > > +	if (enabled)
> > > > +		reg_val = MSCC_PHY_SERDES_ANEG;
> > > > +
> > > > +	mutex_lock(&phydev->lock);
> > > > +
> > > > +	rc = phy_modify_paged(phydev, MSCC_PHY_PAGE_EXTENDED_3,
> > > > +			      MSCC_PHY_SERDES_PCS_CTRL, MSCC_PHY_SERDES_ANEG,
> > > > +			      reg_val);
> > > > +
> > > > +	mutex_unlock(&phydev->lock);
> > > 
> > > What is the reason for the locking here?
> > > 
> > > phy_modify_paged() itself is safe due to the MDIO bus lock, so you
> > > shouldn't need locking around it.
> > > 
> > 
> > True.
> > 
> > My initial thought was to have serialized access at PHY level,
> > as we have multiple ports to work with.
> > But I guess MDIO bus lock could do the job as well.
> 
> The MDIO bus lock is the only lock that will guarantee that no other
> users can nip onto the bus and possibly access your PHY in the middle
> of an operation that requires more than one access to complete. Adding
> local locking at PHY driver level does not give you those guarantees.
> This is exactly why phy_modify() etc was added - because phy_read()..
> phy_write() does not give that guarantee.
> 
> As an example of something that could interfere - the userspace MII
> ioctls.
> 
> > I've gone through Vladimir's patches and they look more promising
> > than this approach.
> > Let me know if I could be of any help.
> 
> I haven't seen them - so up to you.
> 

Definitely worth a look.

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