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

Danilo Krummrich
2025-02-25T11:24:37Z

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@gmail.com>
> Cc: "Rafael J. Wysocki" <rafael@kernel.org>
> Cc: Danilo Krummrich <dakr@kernel.org>
> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.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@kernel.org>

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