static int unimac_mdio_probe(struct platform_device *pdev)
{
	struct unimac_mdio_pdata *pdata = pdev->dev.platform_data;
	struct unimac_mdio_priv *priv;
	struct device_node *np;
	struct mii_bus *bus;
	struct resource *r;
	int ret;

	np = pdev->dev.of_node;

	priv = devm_kzalloc(&pdev->dev, sizeof(*priv), GFP_KERNEL);
	if (!priv)
		return -ENOMEM;

	r = platform_get_resource(pdev, IORESOURCE_MEM, 0);
	if (!r)
		return -EINVAL;

	/* Just ioremap, as this MDIO block is usually integrated into an
	 * Ethernet MAC controller register range
	 */
	priv->base = devm_ioremap(&pdev->dev, r->start, resource_size(r));
	if (!priv->base) {
		dev_err(&pdev->dev, "failed to remap register\n");
		return -ENOMEM;
	}

	priv->mii_bus = mdiobus_alloc();
	if (!priv->mii_bus)
		return -ENOMEM;

	bus = priv->mii_bus;
	bus->priv = priv;
	if (pdata) {
		bus->name = pdata->bus_name;
		priv->wait_func = pdata->wait_func;
		priv->wait_func_data = pdata->wait_func_data;
		bus->phy_mask = ~pdata->phy_mask;
	} else {
		bus->name = "unimac MII bus";
		priv->wait_func_data = priv;
		priv->wait_func = unimac_mdio_poll;
	}
	bus->parent = &pdev->dev;
	bus->read = unimac_mdio_read;
	bus->write = unimac_mdio_write;
	bus->reset = unimac_mdio_reset;
	snprintf(bus->id, MII_BUS_ID_SIZE, "%s-%d", pdev->name, pdev->id);

	ret = of_mdiobus_register(bus, np);
	if (ret) {
		dev_err(&pdev->dev, "MDIO bus registration failed\n");
		goto out_mdio_free;
	}

	platform_set_drvdata(pdev, priv);

	dev_info(&pdev->dev, "Broadcom UniMAC MDIO bus at 0x%p\n", priv->base);

	return 0;

out_mdio_free:
	mdiobus_free(bus);
	return ret;
}
A NULL pointer dereference occurs when the application dereferences a pointer that it expects to be valid, but is NULL, typically causing a crash or exit.