linux-kernel - Re: [PATCH] WIP: drivers/base: Add virtual_device

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Message-ID: <Z50ZuzrDn-GQWOuP@cassiopeiae>
Date: Fri, 31 Jan 2025 19:43:07 +0100
From: Danilo Krummrich <dakr@...nel.org>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: Lyude Paul <lyude@...hat.com>, rust-for-linux@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Maíra Canal <mairacanal@...eup.net>,
	"Rafael J. Wysocki" <rafael@...nel.org>,
	Jonathan Cameron <Jonathan.Cameron@...wei.com>,
	Zijun Hu <quic_zijuhu@...cinc.com>,
	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
	Robin Murphy <robin.murphy@....com>,
	Alexander Lobakin <aleksander.lobakin@...el.com>,
	Lukas Wunner <lukas@...ner.de>, Bjorn Helgaas <bhelgaas@...gle.com>
Subject: Re: [PATCH] WIP: drivers/base: Add virtual_device_create()

On Fri, Jan 31, 2025 at 05:40:01PM +0100, Greg Kroah-Hartman wrote:
> On Fri, Jan 31, 2025 at 09:00:32AM +0100, Greg Kroah-Hartman wrote:
> > On Thu, Jan 30, 2025 at 04:28:26PM -0500, Lyude Paul wrote:
> > > As Greg KH pointed out, we have a nice /sys/devices/virtual directory free
> > > for the taking - but the vast majority of device drivers concerned with
> > > virtual devices do not use this and instead misuse the platform device API.
> > > 
> > > To fix this, let's start by adding a simple function that can be used for
> > > creating virtual devices - virtual_device_create().
> > > 
> > > Signed-off-by: Lyude Paul <lyude@...hat.com>
> > > 
> > > ---
> > > 
> > > So, WIP obviously because I wrote this up in a few minutes - but this goes
> > > off the idea that Danilo suggested to me off-list of coming up with a
> > > simple API for handling virtual devices that's a little more obvious to
> > > use. I wanted to get people's feedback and if we're happy with this idea,
> > > I'm willing to go through and add some pointers to this function in various
> > > platform API docs - along with porting over the C version of VKMS over to
> > > this API.
> > 
> > This is a big better, but not quite.  Let me carve out some time today
> > to knock something a bit nicer together...
> 
> Ok, here's a rough first-cut.  It builds, and boots, and I've converted
> a driver to use the api to prove it works here.  I'll add a bunch more
> documentation before turning it into a "real" patch, but this should
> give you something to work off of.
> 
> I've run out of time for tonight (dinner is calling), but I think you
> get the idea, right?  If you want to knock up a rust binding for this
> api, it should almost be identical to the platform api you were trying
> to use before, right?

Yes, additionally, since this can't use the existing platform abstractions any
more, we need the bus abstraction for the virtual bus, i.e. the corresponding
driver::RegistrationOps implementation, module_virtual_driver macro, etc. Should
be a little less than 200 lines of code.

Other than in C, in Rust we don't need the "artificial" match between a virtual
device and a virtual driver to have automatic cleanup through things like
devm_kzalloc().

But I guess we want it for consistency and to have the corresponding sysfs
entries and uevents. OOC, are there any other reasons?