netdev - Re: MDIO over I2C driver driver probe dependency issue

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Date:   Thu, 7 Jan 2021 19:05:21 -0800
From:   Florian Fainelli <f.fainelli@...il.com>
To:     Brian Silverman <silvermanbri@...il.com>, netdev@...r.kernel.org,
        andrew@...n.ch
Subject: 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