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: <Z6H9g_bvvMf5pPyc@cassiopeiae>
Date: Tue, 4 Feb 2025 12:44:03 +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 12:09:13PM +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?

> + */
> +void faux_device_destroy(struct faux_device *faux_dev)
> +{
> +	struct device *dev = &faux_dev->dev;
> +
> +	if (IS_ERR_OR_NULL(faux_dev))
> +		return;
> +
> +	device_del(dev);
> +
> +	/* The final put_device() will clean up the driver we created for this device. */
> +	put_device(dev);

Same here, how do we know it's the final one? I also think the "clean up the
driver we created for this device" part isn't true any longer.

> +}
> +EXPORT_SYMBOL_GPL(faux_device_destroy);