linux-kernel - Re: [PATCH] driver core: faux: only create the device if probe() succeeds

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Message-ID: <Z72oceZFfJcMAf-D@pollux>
Date: Tue, 25 Feb 2025 12:24:33 +0100
From: Danilo Krummrich <dakr@...nel.org>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org,
	Kurt Borja <kuurtb@...il.com>,
	"Rafael J. Wysocki" <rafael@...nel.org>
Subject: Re: [PATCH] driver core: faux: only create the device if probe()
 succeeds

On Tue, Feb 25, 2025 at 07:35:46AM +0100, Greg Kroah-Hartman wrote:
> It's really hard to know if a faux device properly passes the callback
> to probe() without having to poke around in the faux_device structure
> and then clean up.  Instead of having to have every user of the api do
> this logic, just do it in the faux device core itself.
> 
> This makes the use of a custom probe() callback for a faux device much
> simpler overall.
> 
> Suggested-by: Kurt Borja <kuurtb@...il.com>
> Cc: "Rafael J. Wysocki" <rafael@...nel.org>
> Cc: Danilo Krummrich <dakr@...nel.org>
> Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> ---
>  drivers/base/faux.c |   15 ++++++++++++++-
>  1 file changed, 14 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/base/faux.c b/drivers/base/faux.c
> index 531e9d789ee0..407c1d1aad50 100644
> --- a/drivers/base/faux.c
> +++ b/drivers/base/faux.c
> @@ -102,7 +102,9 @@ static void faux_device_release(struct device *dev)
>   *
>   * Note, when this function is called, the functions specified in struct
>   * faux_ops can be called before the function returns, so be prepared for
> - * everything to be properly initialized before that point in time.
> + * everything to be properly initialized before that point in time.  If the
> + * probe callback (if one is present) does NOT succeed, the creation of the
> + * device will fail and NULL will be returned.
>   *
>   * Return:
>   * * NULL if an error happened with creating the device
> @@ -147,6 +149,17 @@ struct faux_device *faux_device_create_with_groups(const char *name,
>  		return NULL;
>  	}
>  
> +	/*
> +	 * Verify that we did bind the driver to the device (i.e. probe worked),
> +	 * if not, let's fail the creation as trying to guess if probe was
> +	 * successful is almost impossible to determine by the caller.
> +	 */
> +	if (!dev->driver) {

Seems like really_probe() cleans things up properly through
device_unbind_cleanup(), such that dev->driver is guaranteed to be NULL on
failure.

> +		dev_err(dev, "probe did not succeed, tearing down the device\n");
> +		faux_device_destroy(faux_dev);
> +		faux_dev = NULL;

NIT: Maybe return NULL directly (like above) in case a subsequent change adds
more code to the end of this function and does not consider that we fall
through.

Either way,

Reviewed-by: Danilo Krummrich <dakr@...nel.org>

> +	}
> +
>  	return faux_dev;
>  }
>  EXPORT_SYMBOL_GPL(faux_device_create_with_groups);