netdev - Re: [PATCH v2 1/2 net-next] net: phy: mscc: add support for CLKOUT ctrl reg for VSC8531 and similar

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Message-ID: <ab0ca942-5e84-4663-a0ed-689f023624b6@lunn.ch>
Date: Sat, 15 Jul 2023 00:27:20 +0200
From: Andrew Lunn <andrew@...n.ch>
To: Alexandru Ardelean <alex@...uggie.ro>
Cc: netdev@...r.kernel.org, devicetree@...r.kernel.org,
	linux-kernel@...r.kernel.org, davem@...emloft.net,
	edumazet@...gle.com, kuba@...nel.org, pabeni@...hat.com,
	robh+dt@...nel.org, krzysztof.kozlowski+dt@...aro.org,
	conor+dt@...nel.org, hkallweit1@...il.com, linux@...linux.org.uk,
	olteanv@...il.com, marius.muresan@....ro
Subject: Re: [PATCH v2 1/2 net-next] net: phy: mscc: add support for CLKOUT
 ctrl reg for VSC8531 and similar

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@...n.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