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

Danilo Krummrich
2025-02-04T12:04:20Z

On Tue, Feb 04, 2025 at 12:52:34PM +0100, Greg Kroah-Hartman wrote:
> On Tue, Feb 04, 2025 at 12:44:03PM +0100, Danilo Krummrich 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
> >
> > I really like it, it's as simply as it can be.
> >
> > Please find one nit below, otherwise
> >
> > Reviewed-by: Danilo Krummrich <dakr@kernel.org>
> >
> > >
> > > +/**
> > > + * faux_device_destroy - destroy a faux device
> > > + * @faux_dev: faux device to destroy
> > > + *
> > > + * Unregister and free all memory associated with a faux device that was
> > > + * previously created with a call to faux_device_create().
> >
> > Can we really claim that this frees all memory? Someone can still have a
> > reference to the underlying struct device, right?
>
> That "someone" is the person that had the original device pointer passed
> to it, so if that person then calls faux_device_destroy(), yes, that
> should all be properly cleaned up.
>
> But even if it isn't, the device is destroyed and gone from sysfs, and
> whenever that final final put_device() is called, the memory will then
> be freed by the driver core itself.

Oh indeed, the code here is perfectly fine. I just wanted to say that calling
faux_device_destroy() is not a guarantee that "all memory associated with a
faux device" is actually freed, as the kernel-doc comment above says (or at
least implies).

So, the concern only was that the comment could be confusing, as in "How can
faux_device_destroy() free the memory, if I still have a separate reference to
this thing?" (which it clearly would not).