linux-kernel - Re: [PATCH 1/3] driver core: add a faux bus for use when a simple device/bus is needed

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Message-ID: <21fb6681-c214-4841-9cc6-45da3f965066@t-8ch.de>
Date: Tue, 4 Feb 2025 11:44:41 +0100
From: Thomas Weißschuh <linux@...ssschuh.net>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: "Rafael J. Wysocki" <rafael@...nel.org>, 
	Danilo Krummrich <dakr@...nel.org>, Lyude Paul <lyude@...hat.com>, 
	Alexander Lobakin <aleksander.lobakin@...el.com>, Andy Shevchenko <andriy.shevchenko@...ux.intel.com>, 
	Bjorn Helgaas <bhelgaas@...gle.com>, Jonathan Cameron <Jonathan.Cameron@...wei.com>, 
	Liam Girdwood <lgirdwood@...il.com>, Lukas Wunner <lukas@...ner.de>, Mark Brown <broonie@...nel.org>, 
	Maíra Canal <mairacanal@...eup.net>, Robin Murphy <robin.murphy@....com>, 
	Simona Vetter <simona.vetter@...ll.ch>, Zijun Hu <quic_zijuhu@...cinc.com>, linux-kernel@...r.kernel.org, 
	linux-usb@...r.kernel.org, rust-for-linux@...r.kernel.org
Subject: Re: [PATCH 1/3] driver core: add a faux bus for use when a simple
 device/bus is needed

On 2025-02-04 11:20:43+0100, Greg Kroah-Hartman wrote:
> On Tue, Feb 04, 2025 at 11:08:11AM +0100, Thomas Weißschuh wrote:
> > On 2025-02-03 15:25:17+0100, Greg Kroah-Hartman wrote:
> > > +static void faux_remove(struct device *dev)
> > > +{
> > > +	struct faux_object *faux_obj = to_faux_object(dev);
> > > +	struct faux_device *faux_dev = &faux_obj->faux_dev;
> > > +	const struct faux_driver_ops *faux_ops = faux_obj->faux_ops;
> > > +
> > > +	if (faux_ops && faux_ops->remove)
> > > +		faux_ops->remove(faux_dev);
> > > +}
> > > +
> > > +static const struct bus_type faux_bus_type = {
> > > +	.name		= "faux_bus",
> > 
> > Is the _bus suffix intentional?
> 
> It was intentional.
> 
> > Other busses don't have it.
> 
> True.  Naming is hard.  I guess /sys/bus/faux/ makes sense, I will go
> rename it.
> 
> But for the "root" device, does /sys/devices/faux_bus/ make sense, or
> should it be /sys/devices/faux/ as well?  I'm now leaning toward the
> latter...

I'm leaning slightly towards the former.
But my naming skills are beyond limited.

> > > +	.match		= faux_match,
> > > +	.probe		= faux_probe,
> > > +	.remove		= faux_remove,
> > > +};
> > > +
> > > +static void faux_device_release(struct device *dev)
> > > +{
> > > +	struct faux_object *faux_obj = to_faux_object(dev);
> > > +	struct device_driver *drv = &faux_obj->driver;
> > > +
> > > +	/*
> > > +	 * Now that the device is going away, it has been unbound from the
> > > +	 * driver we created for it, so it is safe to unregister the driver from
> > > +	 * the system.
> > > +	 */
> > > +	driver_unregister(drv);
> > > +
> > > +	kfree(faux_obj);
> > > +}
> > > +
> > > +/**
> > > + * __faux_device_create - create and register a faux device and driver
> > > + * @name: name of the device and driver we are adding
> > > + * @faux_ops: struct faux_driver_ops that the new device will call back into, can be NULL
> > > + * @owner: module owner of the device/driver
> > > + *
> > > + * Create a new faux device and driver, both with the same name, and register
> > > + * them in the driver core properly.  The probe() callback of @faux_ops will be
> > > + * called with the new device that is created for the caller to do something
> > > + * with.
> > > + */
> > > +struct faux_device *__faux_device_create(const char *name,
> > > +					       struct faux_driver_ops *faux_ops,
> > 
> > const
> > 
> > > +					       struct module *owner)
> > 
> > What about attributes?
> 
> What in-kernel user of this wants an attribute for such a device?

It was mostly a guess.
However drivers/video/fbdev/gbefb.c seems to be an example.

> And again, if we find one, we can make a faux_device_create_groups()
> call that takes a pointer to an attribute group structure if it's really
> needed.

Fair enough.

> > > +{
> > > +	struct device_driver *drv;
> > > +	struct device *dev;
> > > +	struct faux_object *faux_obj;
> > > +	struct faux_device *faux_dev;
> > > +	int ret;
> > > +
> > > +	faux_obj = kzalloc(sizeof(*faux_obj) + strlen(name) + 1, GFP_KERNEL);
> > > +	if (!faux_obj)
> > > +		return NULL;
> > > +
> > > +	/* Save off the name of the object into local memory */
> > > +	strcpy(faux_obj->name, name);
> > > +
> > > +	/* Initialize the driver portion and register it with the driver core */
> > > +	faux_obj->faux_ops = faux_ops;
> > > +	drv = &faux_obj->driver;
> > > +
> > > +	drv->owner = owner;
> > > +	drv->name = faux_obj->name;
> > 
> > Assuming most names are constant, this would be better with kstrdup_const().
> > Which is also used by dev_set_name() under the hood.
> 
> I've now removed the additional driver, but note that this is just a
> pointer assignment, which is fine to do here as the lifespan of
> faux_obj->name outlived the driver structure's lifespan.

It outlives it because there is extra space allocated for it in faux_obj.
With kstrdup_const() that space would not be needed.
In the end it shouldn't really matter one way or another.


Thomas