Re: MDIO over I2C driver driver probe dependency issue

On 1/7/2021 6:22 PM, Brian Silverman wrote:

I've written a very small generic MDIO driver that uses the existing
mdio-i2c.c library in drivers/net/phy.  The driver allows
communication to the PHY's MDIO interface as using I2C, as supported by
PHYs like the BCM54616S.  This is working on my hardware.  

The one issue I have is that I2C is not up and available (i.e. probed)
at the time that the MDIO interface comes up.  To fix this, I've changed
the device order in drivers/Makefile to put "obj-y += i2c/"
before "obj-y += net/".

While that works, I prefer not to have to keep that difference from
mainline Linux.  Also, I don't understand why i2c drivers occur
arbitrarily late in the Makefile - surely there are other devices
drivers that need i2c to be enabled when they are probed?

Is there a way to do this that doesn't change probe order?  Or is there
a way to change probe order without patching mainline Linux?

Linux supports probe deferral so when a consumer of a resource finds
that said resource's provider is not available, it should return
-EPROBE_DEFER which puts the driver's probe routine onto a list of
driver's probe function to retry at a later time.

In your case the GEM Ethernet driver should get an -EPROBE_DEFER while
the Ethernet PHY device tree node is looked up via
phylink_of_phy_connect() because the mdio-i2c-gen i2c client has not had
a chance to register the MDIO bus yet. Have you figured out the call
path that does not work for you?

Which version of the kernel are you using? What I am referring to is
assuming mainline, but maybe this is not your case?

---

For reference, here's the driver (excluding headers and footers):

static int mdio_i2c_gen_probe(struct i2c_client *client,
    const struct i2c_device_id *id)
{
struct mii_bus *bus;

bus = mdio_i2c_alloc(&client->dev, client->adapter);
if (IS_ERR(bus)){
return PTR_ERR(bus);
}
bus->name = "Generic MDIO bus over I2C";
bus->parent = &client->dev;

return of_mdiobus_register(bus, client->dev.of_node);
}

static int mdio_i2c_gen_remove(struct i2c_client *client)
{
return 0;
}

static const struct of_device_id mdio_i2c_gen_of_match[] = {
{ .compatible = "mdio-i2c-gen", },
{ }
};
MODULE_DEVICE_TABLE(of, mdio_i2c_gen_of_match);

static struct i2c_driver mdio_i2c_gen_driver = {
.driver = {
.name= "mdio-i2c-gen",
.of_match_table = of_match_ptr(mdio_i2c_gen_of_match),
},
.probe= mdio_i2c_gen_probe,
.remove= mdio_i2c_gen_remove,
};

module_i2c_driver(mdio_i2c_gen_driver);


---

And here's a device-tree snippet:

&gem3 {
    status = "okay";
    phy-handle =  <&phy0>;
};

&i2c0 {
    mdio@40 {
        compatible = "mdio-i2c-gen";
        reg = <0x40>;
        #address-cells = <1>;
        #size-cells = <0>;

        phy0: phy@0 {
            reg = <0>;
        };
    };
};

-- 
Florian