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

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Message-ID: <Z6ICPc3A5Cy1dgw_@cassiopeiae>
Date: Tue, 4 Feb 2025 13:04:13 +0100
From: Danilo Krummrich <dakr@...nel.org>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: linux-kernel@...r.kernel.org, "Rafael J. Wysocki" <rafael@...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-usb@...r.kernel.org,
	rust-for-linux@...r.kernel.org
Subject: Re: [PATCH v2 1/5] driver core: add a faux bus for use when a simple
 device/bus is needed

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@...uxfoundation.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@...nel.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).