linux-kernel - Re: [PATCH v4 1/9] driver core: add a faux bus for use when a simple device/bus is needed

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Message-ID: <2025021209-ultimate-dazzler-0e81@gregkh>
Date: Wed, 12 Feb 2025 08:39:12 +0100
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: Kurt Borja <kuurtb@...il.com>
Cc: Zijun Hu <zijun_hu@...oud.com>, linux-kernel@...r.kernel.org,
	Lyude Paul <lyude@...hat.com>,
	"Rafael J. Wysocki" <rafael@...nel.org>,
	Danilo Krummrich <dakr@...nel.org>,
	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,
	Thomas Weißschuh <thomas.weissschuh@...utronix.de>
Subject: Re: [PATCH v4 1/9] driver core: add a faux bus for use when a simple
 device/bus is needed

On Tue, Feb 11, 2025 at 10:49:34AM -0500, Kurt Borja wrote:
> On Tue Feb 11, 2025 at 10:29 AM -05, Zijun Hu wrote:
> > On 2025/2/10 22:29, Kurt Borja wrote:
> >>> +
> >>> +	ret = device_add(dev);
> >>> +	if (ret) {
> >>> +		pr_err("%s: device_add for faux device '%s' failed with %d\n",
> >>> +		       __func__, name, ret);
> >>> +		put_device(dev);
> >>> +		return NULL;
> >>> +	}
> >> Now that the probe is synchronous, what do you think about returning
> >> -ENODEV if the device failed to bind to the driver?
> >> 
> >
> > Result of device registering @ret is not, should not be, effected by
> > "device binding driver (probe result)"
> >
> > if device binging driver failed, you may return -ENODEV in
> > faux_ops->probe(). not here.
> 
> After thinking about this discussion, I understand why this might be the
> expected behavior. I'm thinking about very simple modules that would
> remain loaded even if the probe fails. But of course this may cause
> problems to other modules.
> 
> In the end, this is just my opinion so it would be up to Greg to decide.
> However, there is still an issue with the groups added to the device,
> which a user might expect they are tied to an "attached" device
> lifetime and this currently not the case.

I agree with you here, this could be confusing and cause problems, and
we should be creating apis that "work properly and simply".  Having a
probe callback is good to add device data like you mention, so that you
can properly add the information before the sysfs files are accessed,
removing that race condition.

> >> This would be useful for modules that may want to unload if the probe
> >> fails.
> >
> > may need to root cause if probe failure happens.
> >
> > how to unload module automatically if probe() failure ?
> 
> If we check for !dev->driver, a module might propagate an error to the
> module_init, thus making it fail to load.

Agreed.  Thanks so much for your review comments, they are greatly
appreciated.  When I get time next week I'll make these changes and send
out some patches.

thanks,

greg k-h