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Date:   Sat, 15 Jul 2023 00:27:20 +0200
From:   Andrew Lunn <andrew@lunn.ch>
To:     Alexandru Ardelean <alex@shruggie.ro>
Cc:     netdev@vger.kernel.org, devicetree@vger.kernel.org,
        linux-kernel@vger.kernel.org, davem@davemloft.net,
        edumazet@google.com, kuba@kernel.org, pabeni@redhat.com,
        robh+dt@kernel.org, krzysztof.kozlowski+dt@linaro.org,
        conor+dt@kernel.org, hkallweit1@gmail.com, linux@armlinux.org.uk,
        olteanv@gmail.com, marius.muresan@mxt.ro
Subject: Re: [PATCH v2 1/2 net-next] net: phy: mscc: add support for CLKOUT
 ctrl reg for VSC8531 and similar
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 <CAH3L5QrtFwTqqFKjPrMFCz4JgUWOFWFUJXpN71Gyprcd33A7hg@mail.gmail.com>
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On Fri, Jul 14, 2023 at 09:09:14AM +0300, Alexandru Ardelean wrote:
> On Thu, Jul 13, 2023 at 11:35 PM Andrew Lunn <andrew@lunn.ch> wrote:
> >
> > > +set_reg:
> > > +     mutex_lock(&phydev->lock);
> > > +     rc = phy_modify_paged(phydev, MSCC_PHY_PAGE_EXTENDED_GPIO,
> > > +                           VSC8531_CLKOUT_CNTL, mask, set);
> > > +     mutex_unlock(&phydev->lock);
> >
> > What is this mutex protecting?
> 
> This was inspired by vsc85xx_edge_rate_cntl_set().
> Which has the same format.

phy_modify_paged() locks the MDIO bus while it swaps the page, so
nothing else can use it. That also protects the read/modify/write.

Nothing is modifying phydev, so the lock is not needed for that
either.

> I'll re-test with this lock removed.
> I may be misremembering (or maybe I did something silly at some
> point), but there was a weird stack-trace warning before adding this
> lock there.
> This was with a 5.10.116 kernel version.

This patch is for net-next, please test there.

When testing for locking issues, and when doing development in
general, it is a good idea to turn on CONFIG_PROVE_LOCKING and
CONFIG_DEBUG_ATOMIC_SLEEP.

	Andrew