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

Lyude Paul
2025-02-06T23:04:10Z

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. I think actually taking references to the Device should be the job
of the kernel device core though and not subsystems like faux because there's
not really any operations we'd want to be dependent on registration besides
the obvious creating/destroying devices usecase. AFAIK this is pretty much how
most devices work, especially since with real devices unregistering likely
means the device has been physically removed from the system - an operation we
have no control over and want to act as a point to prevent new operations on
the device from starting and a chance to clean up existing operations.

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

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