linux-kernel - Re: [PATCH net-next v7 3/3] net: dsa: yt921x: Add support for Motorcomm YT921x

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Message-ID: <CAAXyoMMEUeqxJaAYb8fbeACp7N=hFOQrPbtk4LDJM4CZw7n6mA@mail.gmail.com>
Date: Mon, 8 Sep 2025 21:10:36 +0800
From: Yangfl <mmyangfl@...il.com>
To: Andrew Lunn <andrew@...n.ch>
Cc: netdev@...r.kernel.org, Vladimir Oltean <olteanv@...il.com>, 
	"David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, 
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, Rob Herring <robh@...nel.org>, 
	Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley <conor+dt@...nel.org>, 
	Simon Horman <horms@...nel.org>, Russell King <linux@...linux.org.uk>, devicetree@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next v7 3/3] net: dsa: yt921x: Add support for
 Motorcomm YT921x

On Mon, Sep 8, 2025 at 9:00 PM Andrew Lunn <andrew@...n.ch> wrote:
>
> > +/* Prepare for read/write operations. Not a lock primitive despite underlying
> > + * implementations may perform a lock (could be a no-op if the bus supports
> > + * native atomic operations on internal ASIC registers).
>
> It is more than atomic operations. Look at how long you hold the
> lock. It is not a simple read/modify/write, you hold it over multiple
> reads and writes. If the ASIC provided some sort of locking, it would
> be available for MDIO, I2C, and SPI, and probably mean additional bus
> transactions.
>
> > + *
> > + * To serialize register operations, use yt921x_lock() instead.
> > + */
> > +static void yt921x_reg_acquire(struct yt921x_priv *priv)
> > +{
> > +     if (priv->smi_ops->acquire)
> > +             priv->smi_ops->acquire(priv->smi_ctx);
> > +}
>
> So, as i said in my review to previous versions, skip the if and just
> take the mutex. KISS. I would not even call mutex_lock(priv->lock);
> Don't over engineer the solution, this will probably work for I2C and
> SPI as well.
>
> > +/* You should manage the bus ownership yourself and use yt921x_reg_read()
> > + * directly, except for register polling with read_poll_timeout(); see examples
> > + * below.
> > + */
> > +static int yt921x_reg_read_managed(struct yt921x_priv *priv, u32 reg, u32 *valp)
> > +{
> > +     int res;
> > +
> > +     yt921x_reg_acquire(priv);
> > +     res = yt921x_reg_read(priv, reg, valp);
> > +     yt921x_reg_release(priv);
> > +
> > +     return res;
> > +}
>
> Sorry, i missed your reply to my comment to the previous version. You
> said:
>
> > The driver itself does not need an explicit lock (so long as dsa
> > framework does not call two conflicting methods on the same port),
>
> The DSA framework makes no such guarantees. The DSA framework is also
> not the only entry point into the driver, phylink will directly call
> into the driver, and if you implement things like LEDs, they will have
> direct access to the driver.
>
> So i suggest only having a high level lock, acquired on entry,
> released on exit, e.g. as mv88e6xxx does. KISS.
>
>     Andrew
>
> ---
> pw-bot: cr

So you mean holding bus->mdio_lock during any operations instead of
implementing driver's own lock? Wouldn't other bus participants starve
if I want to poll a register for like 100ms?