Subject: Re: [PATCH v2 net-next 02/15] net: mdio: add driver for NXP SJA1110 100BASE-T1 embedded PHYs

From: Andy Shevchenko <[email-masked]>
Date: Thu, 22 Jan 2026 16:44:47 +0200

On Thu, Jan 22, 2026 at 02:47:08PM +0200, Vladimir Oltean wrote:
> On Thu, Jan 22, 2026 at 02:12:21PM +0200, Andy Shevchenko wrote:
> > On Thu, Jan 22, 2026 at 12:56:41PM +0200, Vladimir Oltean wrote:

...

> > > +static int sja1110_base_t1_mdio_read_c22(struct mii_bus *bus, int phy, int reg)
> > > +{
> > > +	struct sja1110_base_t1_private *priv = bus->priv;
> > > +	struct regmap *regmap = priv->regmap;
> > > +	unsigned int addr, val;
> > > +	int err;
> > > +
> > > +	addr = sja1110_base_t1_encode_addr(phy, SJA1110_C22, reg & 0x1f);
> > 
> > GENMASK() ? Or do you have already a defined mask for this?
> 
> Hmm, I can't find a definition for this. In the MDIO world it is
> "well known" that clause 22 offers a 5-bit register address space.
> So the 0x1f number doesn't seem too magical to me.
> 
> But I think my assumptions date since before the MDIO bus API was split
> between separate clause 22 and clause 45 reads/writes. I don't know
> whether masking reg & 0x1f is the best practice. I'm surprised that
> __mdiobus_read() doesn't enforce a limit on "regnum", and I don't see
> other MDIO bus drivers explicitly C22 registers >= 32. I really don't
> know what is the best practice.

Me neither. At bare minimum to check / perform two things:
- make sure this approach is consistent across the kernel
- define the magic with meaningful name

Maybe (assuming second one is done) fix the rest in the future
via some helper function?

...

> > > +static int sja1110_base_t1_mdio_probe(struct platform_device *pdev)
> > > +{
> > > +	struct sja1110_base_t1_private *priv;
> > > +	struct device *dev = &pdev->dev;
> > > +	struct regmap *regmap;
> > > +	struct resource *res;
> > > +	struct mii_bus *bus;
> > > +	int err;
> > 
> > > +	if (!dev->of_node || !dev->parent)
> > 
> > Can we avoid dereferencing? And perhaps dev_fwnode(dev)?
> 
> Avoid dereferencing what?

of_node

> > > +		return -ENODEV;
> > > +
> > > +	regmap = dev_get_regmap(dev->parent, NULL);
> > > +	if (!regmap)
> > > +		return -ENODEV;
> > > +
> > > +	bus = mdiobus_alloc_size(sizeof(*priv));
> > > +	if (!bus)
> > > +		return -ENOMEM;
> > > +
> > > +	bus->name = "SJA1110 100base-T1 MDIO bus";
> > > +	snprintf(bus->id, MII_BUS_ID_SIZE, "%s", dev_name(dev));
> > > +	bus->read = sja1110_base_t1_mdio_read_c22;
> > > +	bus->write = sja1110_base_t1_mdio_write_c22;
> > > +	bus->read_c45 = sja1110_base_t1_mdio_read_c45;
> > > +	bus->write_c45 = sja1110_base_t1_mdio_write_c45;
> > > +	bus->parent = dev;
> > > +	priv = bus->priv;
> > > +	priv->regmap = regmap;
> > > +
> > > +	res = platform_get_resource(pdev, IORESOURCE_REG, 0);
> > > +	if (res)
> > > +		priv->base = res->start;
> > > +
> > > +	err = of_mdiobus_register(bus, dev->of_node);
> 
> Why would I use dev_fwnode() if I need to pass it as OF to
> of_mdiobus_register() here?

dev_of_node() then. Wondering if we can use fwnode_mdiobus_register_phy() here
(I remember that OF/fwnode code in MDIO/PHY is not trivial, but I don't know
 all the details).

> > > +	if (err)
> > > +		goto err_free_bus;
> > > +
> > > +	priv->bus = bus;
> > > +	platform_set_drvdata(pdev, priv);
> > > +
> > > +	return 0;
> > > +
> > > +err_free_bus:
> > > +	mdiobus_free(bus);
> > > +
> > > +	return err;
> > > +}

-- 
With Best Regards,
Andy Shevchenko