Re: [PATCH 7/7] rust: sample: misc: implement device driver sample

[Danilo Krummrich <dakr@kernel.org>]

On Fri, May 30, 2025 at 10:15:37PM +0200, Benno Lossin wrote:
> On Fri May 30, 2025 at 4:24 PM CEST, Danilo Krummrich wrote:
> > In order to demonstrate and test a MiscDeviceRegistration with a parent
> > device, introduce CONFIG_SAMPLE_RUST_MISC_DEVICE_WITH_PARENT.
> >
> > If CONFIG_SAMPLE_RUST_MISC_DEVICE_WITH_PARENT=y the misc device sample
> > is initialized with a parent device (faux), otherwise it is initialized
> > without a parent device, i.e. the exact same way as without this patch.
> >
> > Signed-off-by: Danilo Krummrich <dakr@kernel.org>
> > ---
> >  samples/rust/Kconfig             |  8 +++++
> >  samples/rust/rust_misc_device.rs | 50 +++++++++++++++++++++++++++++---
> >  2 files changed, 54 insertions(+), 4 deletions(-)
> >
> > diff --git a/samples/rust/Kconfig b/samples/rust/Kconfig
> > index b1006ab4bc3c..9948ec0939ef 100644
> > --- a/samples/rust/Kconfig
> > +++ b/samples/rust/Kconfig
> > @@ -30,6 +30,14 @@ config SAMPLE_RUST_MISC_DEVICE
> >  
> >  	  If unsure, say N.
> >  
> > +config SAMPLE_RUST_MISC_DEVICE_WITH_PARENT
> > +	bool "Create a misc device with a parent device"
> > +	depends on SAMPLE_RUST_MISC_DEVICE
> > +	default n
> > +	help
> > +	  Say Y here if you want the misc device sample to create a misc
> > +	  device with a parent device.
> > +
> 
> Why not create a separate file? The `cfg`s might confuse newcomers
> looking at the sample.

It would be a lot of duplicated code, unless we really *only* exercise the
device creation and registration part, which would be a bit unfortunate, given
that this sample is also a pretty good test.

> >  config SAMPLE_RUST_PRINT
> >  	tristate "Printing macros"
> >  	help
> > diff --git a/samples/rust/rust_misc_device.rs b/samples/rust/rust_misc_device.rs
> > index 9bf1a0f64e6e..175638d6d341 100644
> > --- a/samples/rust/rust_misc_device.rs
> > +++ b/samples/rust/rust_misc_device.rs
> > @@ -167,6 +167,9 @@
> >      uaccess::{UserSlice, UserSliceReader, UserSliceWriter},
> >  };
> >  
> > +#[cfg(CONFIG_SAMPLE_RUST_MISC_DEVICE_WITH_PARENT)]
> > +use kernel::faux;
> > +
> >  const RUST_MISC_DEV_HELLO: u32 = _IO('|' as u32, 0x80);
> >  const RUST_MISC_DEV_GET_VALUE: u32 = _IOR::<i32>('|' as u32, 0x81);
> >  const RUST_MISC_DEV_SET_VALUE: u32 = _IOW::<i32>('|' as u32, 0x82);
> > @@ -181,19 +184,33 @@
> >      license: "GPL",
> >  }
> >  
> > +#[cfg(not(CONFIG_SAMPLE_RUST_MISC_DEVICE_WITH_PARENT))]
> >  #[pin_data]
> >  struct RustMiscDeviceModule {
> >      #[pin]
> >      _miscdev: MiscDeviceRegistration<RustMiscDevice>,
> >  }
> >  
> > -impl kernel::InPlaceModule for RustMiscDeviceModule {
> > -    fn init(_module: &'static ThisModule) -> impl PinInit<Self, Error> {
> > +#[cfg(CONFIG_SAMPLE_RUST_MISC_DEVICE_WITH_PARENT)]
> > +struct RustMiscDeviceModule {
> > +    _faux: faux::Registration,
> > +    _miscdev: Pin<KBox<MiscDeviceRegistration<RustMiscDevice>>>,
> > +}
> > +
> > +impl RustMiscDeviceModule {
> > +    fn init() -> MiscDeviceOptions {
> >          pr_info!("Initializing Rust Misc Device Sample\n");
> >  
> > -        let options = MiscDeviceOptions {
> > +        MiscDeviceOptions {
> >              name: c_str!("rust-misc-device"),
> > -        };
> > +        }
> > +    }
> > +}
> > +
> > +#[cfg(not(CONFIG_SAMPLE_RUST_MISC_DEVICE_WITH_PARENT))]
> > +impl kernel::InPlaceModule for RustMiscDeviceModule {
> > +    fn init(_module: &'static ThisModule) -> impl PinInit<Self, Error> {
> > +        let options = Self::init();
> >  
> >          try_pin_init!(Self {
> >              _miscdev <- MiscDeviceRegistration::register(
> > @@ -205,6 +222,31 @@ fn init(_module: &'static ThisModule) -> impl PinInit<Self, Error> {
> >      }
> >  }
> >  
> > +#[cfg(CONFIG_SAMPLE_RUST_MISC_DEVICE_WITH_PARENT)]
> > +impl kernel::Module for RustMiscDeviceModule {
> > +    fn init(_module: &'static ThisModule) -> Result<Self> {
> > +        let options = Self::init();
> > +        let faux = faux::Registration::new(c_str!("rust-misc-device-sample"), None)?;
> > +
> > +        // For every other bus, this would be called from Driver::probe(), which would return a
> > +        // `Result<Pin<KBox<T>>>`, but faux always binds to a "dummy" driver, hence probe() is
> 
> Not clear what `T` is supposed to be, do you mean `Self`?

From the perspective of the type implementing the corresponding Driver trait it
would indeed be `Self`. But I found it ambiguous to write `Self`, since I do *not*
mean `RustMiscDeviceModule` with `Self`.

> > +        // not required.
> > +        let misc = KBox::pin_init(
> > +            MiscDeviceRegistration::register(
> > +                options,
> > +                Arc::pin_init(new_mutex!(Inner { value: 0_i32 }), GFP_KERNEL),
> > +                Some(faux.as_ref()),
> > +            ),
> > +            GFP_KERNEL,
> > +        )?;
> 
> You could also initialize this module variation in-place. (this would
> also require the pin-init change to reference initialized fields)

Yes, I also thought about that. But this way is a bit closer to what things
would look like within a probe() callback.