Re: [PATCH v2 1/5] driver core: add a faux bus for use when a simple device/bus is needed

[Rob Herring <robh@kernel.org>]

On Tue, Feb 04, 2025 at 10:46:50AM -0600, Rob Herring wrote:
> On Tue, Feb 04, 2025 at 12:09:13PM +0100, Greg Kroah-Hartman wrote:
> > Many drivers abuse the platform driver/bus system as it provides a
> > simple way to create and bind a device to a driver-specific set of
> > probe/release functions.  Instead of doing that, and wasting all of the
> > memory associated with a platform device, here is a "faux" bus that
> > can be used instead.
> > 
> > Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> > ---
> >  v2: - renamed bus and root device to just "faux" thanks to Thomas
> >      - removed the one-driver-per-device and now just have one driver
> >        entirely thanks to Danilo
> >      - kerneldoc fixups and additions and string handling bounds checks
> >        hanks to Andy
> >      - coding style fix thanks to Jonathan
> >      - tested that the destroy path actually works
> > 
> >  drivers/base/Makefile       |   2 +-
> >  drivers/base/base.h         |   1 +
> >  drivers/base/faux.c         | 196 ++++++++++++++++++++++++++++++++++++
> >  drivers/base/init.c         |   1 +
> >  include/linux/device/faux.h |  31 ++++++
> >  5 files changed, 230 insertions(+), 1 deletion(-)
> >  create mode 100644 drivers/base/faux.c
> >  create mode 100644 include/linux/device/faux.h
> > 
> > diff --git a/drivers/base/Makefile b/drivers/base/Makefile
> > index 7fb21768ca36..8074a10183dc 100644
> > --- a/drivers/base/Makefile
> > +++ b/drivers/base/Makefile
> > @@ -6,7 +6,7 @@ obj-y			:= component.o core.o bus.o dd.o syscore.o \
> >  			   cpu.o firmware.o init.o map.o devres.o \
> >  			   attribute_container.o transport_class.o \
> >  			   topology.o container.o property.o cacheinfo.o \
> > -			   swnode.o
> > +			   swnode.o faux.o
> >  obj-$(CONFIG_AUXILIARY_BUS) += auxiliary.o
> >  obj-$(CONFIG_DEVTMPFS)	+= devtmpfs.o
> >  obj-y			+= power/
> > diff --git a/drivers/base/base.h b/drivers/base/base.h
> > index 8cf04a557bdb..0042e4774b0c 100644
> > --- a/drivers/base/base.h
> > +++ b/drivers/base/base.h
> > @@ -137,6 +137,7 @@ int hypervisor_init(void);
> >  static inline int hypervisor_init(void) { return 0; }
> >  #endif
> >  int platform_bus_init(void);
> > +int faux_bus_init(void);
> >  void cpu_dev_init(void);
> >  void container_dev_init(void);
> >  #ifdef CONFIG_AUXILIARY_BUS
> > diff --git a/drivers/base/faux.c b/drivers/base/faux.c
> > new file mode 100644
> > index 000000000000..9b28643afc45
> > --- /dev/null
> > +++ b/drivers/base/faux.c
> > @@ -0,0 +1,196 @@
> > +// SPDX-License-Identifier: GPL-2.0-only
> > +/*
> > + * Copyright (c) 2025 Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> > + * Copyright (c) 2025 The Linux Foundation
> > + *
> > + * A "simple" faux bus that allows devices to be created and added
> > + * automatically to it.  This is to be used whenever you need to create a
> > + * device that is not associated with any "real" system resources, and do
> > + * not want to have to deal with a bus/driver binding logic.  It is
> > + * intended to be very simple, with only a create and a destroy function
> > + * available.
> > + */
> > +#include <linux/err.h>
> > +#include <linux/init.h>
> > +#include <linux/slab.h>
> > +#include <linux/string.h>
> > +#include <linux/container_of.h>
> > +#include <linux/device/faux.h>
> > +#include "base.h"
> > +
> > +#define MAX_NAME_SIZE	256	/* Max size of a faux_device name */
> > +
> > +/*
> > + * Internal wrapper structure so we can hold the memory
> > + * for the driver and the name string of the faux device.
> > + */
> > +struct faux_object {
> > +	struct faux_device faux_dev;
> > +	const struct faux_driver_ops *faux_ops;
> > +	char name[];
> > +};
> > +#define to_faux_object(dev) container_of_const(dev, struct faux_object, faux_dev.dev)
> > +
> > +static struct device faux_bus_root = {
> > +	.init_name	= "faux",
> > +};
> > +
> > +static int faux_match(struct device *dev, const struct device_driver *drv)
> > +{
> > +	/* Match always succeeds, we only have one driver */
> > +	return 1;
> > +}
> > +
> > +static int faux_probe(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;
> > +	int ret = 0;
> > +
> > +	if (faux_ops && faux_ops->probe)
> 
> Is there any use for faux_ops being NULL (or probe being NULL for that 
> matter)? I can't think of one. So faux_device_create should check that 
> and fail instead of checking here.

NM, I see your converted cases do just that. Weird.

I suppose you could still say if faux_ops is not NULL, then probe must 
not be NULL.

Rob