Re: [PATCH v5 03/19] rust: faux: add Device type with AsBusDevice support

["Alexandre Courbot" <acourbot@nvidia.com>]

On Sun Jun 28, 2026 at 11:53 PM JST, Danilo Krummrich wrote:
> Add a faux::Device type that wraps struct faux_device and implements
> AsBusDevice, enabling faux devices to be used as parent devices for
> subsystems that require a bus device, such as DRM.
>
> Update Registration to return &faux::Device<Bound> via AsRef.
>
> Signed-off-by: Danilo Krummrich <dakr@kernel.org>

Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>

(one small comment below)

> ---
>  rust/kernel/drm/gem/shmem.rs     | 11 +++--
>  rust/kernel/faux.rs              | 69 +++++++++++++++++++++++++++-----
>  samples/rust/rust_driver_faux.rs |  3 +-
>  3 files changed, 68 insertions(+), 15 deletions(-)
>
> diff --git a/rust/kernel/drm/gem/shmem.rs b/rust/kernel/drm/gem/shmem.rs
> index 3ee19ef6264e..52de59b14dad 100644
> --- a/rust/kernel/drm/gem/shmem.rs
> +++ b/rust/kernel/drm/gem/shmem.rs
> @@ -692,10 +692,12 @@ impl drm::Driver for KunitDriver {
>      fn create_drm_dev() -> Result<(faux::Registration, UnregisteredDevice<KunitDriver>)> {
>          // Create a faux DRM device so we can test gem object creation.
>          let data = try_pin_init!(KunitData {});
> -        let dev = faux::Registration::new(c"Kunit", None)?;
> -        let drm = UnregisteredDevice::new(dev.as_ref(), data)?;
> +        let reg = faux::Registration::new(c"Kunit", None)?;
> +        let fdev = reg.as_ref();
> +        let dev = fdev.as_ref();
> +        let drm = UnregisteredDevice::new(dev, data)?;
>  
> -        Ok((dev, drm))
> +        Ok((reg, drm))
>      }
>  
>      #[test]
> @@ -755,7 +757,8 @@ fn vmap_io() -> Result {
>      #[test]
>      fn fail_sg_table_on_wrong_dev() -> Result {
>          let (_dev, drm) = create_drm_dev()?;
> -        let wrong_dev = faux::Registration::new(c"EvilKunit", None)?;
> +        let reg = faux::Registration::new(c"EvilKunit", None)?;
> +        let wrong_dev = reg.as_ref();
>  
>          let obj = Object::<KunitObject, _>::new(&drm, PAGE_SIZE, ObjectConfig::default(), ())?;
>  
> diff --git a/rust/kernel/faux.rs b/rust/kernel/faux.rs
> index 36c92ae2943c..cd0724af446b 100644
> --- a/rust/kernel/faux.rs
> +++ b/rust/kernel/faux.rs
> @@ -9,15 +9,63 @@
>  use crate::{
>      bindings,
>      device,
> -    prelude::*, //
> +    prelude::*,
> +    types::Opaque, //
>  };
> -use core::ptr::{
> -    addr_of_mut,
> -    null,
> -    null_mut,
> -    NonNull, //
> +use core::{
> +    marker::PhantomData,
> +    ptr::{
> +        null,
> +        null_mut,
> +        NonNull, //
> +    },
>  };
>  
> +/// A faux device.
> +///
> +/// A faux device is a virtual device backed by the faux bus, primarily used for scenarios where a
> +/// real hardware device is not available or for testing.
> +///
> +/// # Invariants
> +///
> +/// The underlying `struct faux_device` is valid and the embedded `struct device` is initialized.
> +#[repr(transparent)]
> +pub struct Device<Ctx: device::DeviceContext = device::Normal>(
> +    Opaque<bindings::faux_device>,
> +    PhantomData<Ctx>,
> +);
> +
> +impl<Ctx: device::DeviceContext> Device<Ctx> {
> +    #[inline]
> +    fn as_raw(&self) -> *mut bindings::faux_device {
> +        self.0.get()
> +    }
> +
> +    /// # Safety
> +    ///
> +    /// `ptr` must be a valid pointer to a `struct faux_device`.

The invariant for `Device` also says: "... and the embedded `struct
device` is valid". IIUC this requirement should also be mentioned here,
unless we consider that a valid `struct faux_device` also implies that
its embedded `struct device` is initialized, in which case that bit can
be removed from the invariant section of `Device`.