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: <Z6IcuEPSO3QwIWzW@pollux>
Date: Tue, 4 Feb 2025 14:57:12 +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 01:55:54PM +0100, Greg Kroah-Hartman wrote:
> On Tue, Feb 04, 2025 at 01:04:13PM +0100, Danilo Krummrich wrote:
> > 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).
> 
> Documentation is hard :)
> 
> Can you think of some wording here that would explain this better?
> Something like "after you call this, you can't touch the pointer you
> passed into here" is what I'm going for.

I would probably just say that it drops the initial reference created by
faux_device_create(), e.g.:

"Unregister a faux device and drop the initial reference obtained through
faux_device_create()."

--

>From the formal side of things:

The thing with "can't touch the pointer you passed into here" is that it
depends on the conditions and on the definition of what it means for a pointer
to be valid.

Let's say I have another pointer (B) to the device with (of course) a separate
reference.

Now, when I call faux_device_destroy(A), and given that I know for sure that the
reference of (B) does out-live this operation, I could also argue that I
downgraded my strong reference to a weak reference.

So, technically I could still touch the pointer, but formally it probably
wouldn't be valid anymore.