[PATCH v2 16/26] i2c: mux: Create missing devlink between mux and adapter physical device

[Andy Shevchenko]

On Wed, May 07, 2025 at 09:12:58AM +0200, Herve Codina wrote:
> When removing an i2c controller device handling an i2c bus where an i2c
> mux is connected to, the removal process hangs and is stuck in the
> wait_completion() call done in i2c_del_adapter().
> 
> The i2c_del_adapter() tries to removed the i2c adapter related to the
> i2c controller device and the wait_completion() is waiting for the i2c
> adapter device release. This release is performed when the device is no
> more used (i.e. refcount reaches zero).
> 
> When an i2c mux is involved in an i2c path, the struct dev topology is
> the following:
>     +----------------+                +-------------------+
>     | i2c controller |                |      i2c mux      |
>     |     device     |                |      device       |
>     |       ^        |                |                   |
>     |       |        |                |                   |
>     |  dev's parent  |                |                   |
>     |       |        |                |                   |
>     |   i2c adapter  |                | i2c adapter chanX |
>     |     device  <---- dev's parent ------  device       |
>     |   (no driver)  |                |    (no driver)    |
>     +----------------+                +-------------------+
> 
> When an i2c mux device creates an i2c adapter for its downstream
> channel, a reference is taken to its adapter dev's parent. This parent
> is the i2c mux upstream adapter device.
> 
> No relationship exists between the i2c mux device itself and the i2c
> controller device (physical device) in order to have the i2c mux device
> calling i2c_del_adapter() to remove its downtream adapters and so,
> release references taken to the upstream adapter.
> 
> This consumer/supplier relationship is typically a devlink relationship.
> 
> Also, i2c muxes can be chained and so, the upstream adapter can be
> supplied by either an i2c controller device or an other i2c mux device.
> 
> In order to get the physical device of the adapter a mux is connected
> to, rely on the newly introduced i2c_adapter_get_physdev() and create
> the missing devlink between the i2c mux device and the physical
> device of the adapter the mux is connected to.
> 
> With that done, the i2c mux device is removed before the device
> handling the upstream i2c adapter (i2c controller device or i2c mux
> device). All references are released and the i2c_del_adapter() call
> performed by driver handling the upstream adapter device is not blocking
> anymore.

...

> +	/*
> +	 * There is no relationship set between the mux device and the physical
> +	 * device handling the parent adapter. Create this missing relationship
> +	 * in order to remove the i2c mux device (consumer) and so the dowstream
> +	 * channel adapters before removing the physical device (supplier) which
> +	 * handles the i2c mux upstream adapter.
> +	 */
> +	parent_physdev = i2c_get_adapter_physdev(parent);
> +	dl = device_link_add(muxc->dev, parent_physdev, DL_FLAG_AUTOREMOVE_CONSUMER);
> +	if (!dl) {
> +		dev_err(muxc->dev, "failed to create device link to %s\n",
> +			dev_name(parent_physdev));
> +		put_device(parent_physdev);
> +		ret = -EINVAL;
> +		goto err_free_priv;
> +	}
> +	put_device(parent_physdev);

Since you are not checking parent_physdev for NULL, the dev_name() can print a
"(null)" string. Is this by design?

-- 
With Best Regards,
Andy Shevchenko