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

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Message-ID: <2025020106-avert-senorita-4181@gregkh>
Date: Sat, 1 Feb 2025 09:00:00 +0100
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: Danilo Krummrich <dakr@...nel.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 07:43:07PM +0100, Danilo Krummrich wrote:
> 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.

I hope so as the original C code for this is less than 200 lines of code :)

I wonder what it would look like to do a "real" bus in rust, maybe I'll
try that someday, but for now, I want this to be used by C 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().

What artificial match?  Ah, you mean they would both be in the same
"object"?

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

I don't really understand the objection here.  Oooh, you want the C code
to both create/manage the driver AND the device at the same time?  Hey I
like that, it would make the interface to it even simpler!  Let me go
try that, and see if it is what you are thinking of here...

thanks!

greg k-h