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

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Message-ID: <2025022616-refute-unmarked-a4be@gregkh>
Date: Wed, 26 Feb 2025 11:32:38 -0800
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: Danilo Krummrich <dakr@...nel.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 12:24:33PM +0100, Danilo Krummrich wrote:
> 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.

Yes.

> > +		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>

Thanks for the review!

greg k-h