Re: [PATCH] rust/kernel: Add faux device bindings

[Lyude Paul <lyude@redhat.com>]

On Fri, 2025-02-07 at 01:16 +0100, Danilo Krummrich wrote:
> On Thu, Feb 06, 2025 at 06:04:03PM -0500, Lyude Paul wrote:
> > On Thu, 2025-02-06 at 23:30 +0100, Danilo Krummrich wrote:
> > > > +//! Abstractions for the faux bus.
> > > > +//!
> > > > +//! This crate provides bindings for working with faux devices in kernel modules. It should be
> > > > +//! preferred for creating virtual devices over the platform API.
> > > 
> > > "preferred" implies a bit that platform devices are still an option for that
> > > (even if not preferred). Maybe just not mention it at all. But if you want to,
> > > maybe something along the lines of "faux devices are the solution for the
> > > historical abuse of platform devices as virtual devices"?
> > > 
> > > > +//!
> > > > +//! C header: [`include/linux/device/faux.h`]
> > > > +use crate::{bindings, device, error::from_err_ptr, prelude::*};
> > > > +use core::ptr::{addr_of_mut, null, NonNull};
> > > > +
> > > > +/// The faux device representation.
> > > > +///
> > > > +/// This type represents the registration of a [`struct faux_device`]. When an instance of this type
> > > > +/// is dropped, its respective faux device will be unregistered from the system.
> > > 
> > > Ultimately, this will be used to be passed to C APIs, such as drm_dev_alloc(),
> > > which increment the reference count of the underlying struct device.
> > > 
> > > Should we consider that in Rust we may have a need to take additional references
> > > in the future too?
> > > 
> > > Maybe it would be more future proof to call this structure `Registration` and
> > > leave us the option to define faux::Device for reference counting later on.
> > 
> > Yeah I was considering calling this Registration rather than Device, but
> > mainly for the reason that a device registration (at least to me) is a unique
> > resource.
> 
> What about the fact that your comment says "This type represents the
> registration [...]"? :-)
> 
> > I think actually taking references to the Device should be the job
> > of the kernel device core though
> 
> Everyone who stores a pointer to a reference counted thing has to take a
> reference.
> 
> drm_dev_init() for instance, takes a refernece because a drm_device can outlive
> the parent device' (in this case the faux device') registration.
> 
> Once we get to native Rust APIs of this kind in the future, we'd need to take
> our own reference of this device.
> 
> The `Registration` structure's lifetime should represent the time in which a
> device is registered in the system.
> 
> Whereas the `Device` structure's lifetime should represent the lifetime of a
> single reference to the device. This is exactly what pci::Device,
> platform::Device, and the base device::Device do.
> 
> For the faux device it's that faux_device_create() allocates, initializes and
> registers the device at once and faux_device_destroy() unregisters the device
> and drops the initial reference from faux_device_create() at once. After that,
> the device is unregistered, but depending on whether there are still references
> held to the device, it can still be alive.

Whoops! i think I may have misunderstood what you said - I am aware create()
takes a device reference and destroy() drops one. I thought you were saying we
should make `Registration` itself have a separate reference count.

But yeah - I mean, faux::Registration::as_ref::<device::Device>() lets you get
a device::Device which you can take a reference on using ARef. So you still
can take a reference count to the device without us adding support for it
explicitly was what I was getting at.

> 
> I also suggest to have a look at `MiscDeviceRegistration` registration, which is
> similar from the registration side of things.
> 

-- 
Cheers,
 Lyude Paul (she/her)
 Software Engineer at Red Hat

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