Re: [PATCH v2 1/2] rust: implement wrapper for acpi_object

["Gary Guo" <gary@garyguo.net>]

On Sun Jan 11, 2026 at 6:31 PM GMT, Gladyshev Ilya wrote:
> ACPI Object is represented via union on C-side. On Rust side, this union
> is transparently wrapped for each ACPI Type, with individual methods and
> Defer implementation to represented type (integer, string, buffer, etc).
>
> Signed-off-by: Gladyshev Ilya <foxido@foxido.dev>
> ---
>  rust/kernel/acpi.rs | 97 +++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 97 insertions(+)
>
> diff --git a/rust/kernel/acpi.rs b/rust/kernel/acpi.rs
> index 9b8efa623130..c730c12d3979 100644
> --- a/rust/kernel/acpi.rs
> +++ b/rust/kernel/acpi.rs
> @@ -2,6 +2,11 @@
>  
>  //! Advanced Configuration and Power Interface abstractions.
>  
> +use core::{
> +    marker::PhantomData,
> +    ops::Deref, //
> +};
> +
>  use crate::{
>      bindings,
>      device_id::{RawDeviceId, RawDeviceIdIndex},
> @@ -63,3 +68,95 @@ macro_rules! acpi_device_table {
>          $crate::module_device_table!("acpi", $module_table_name, $table_name);
>      };
>  }
> +
> +/// An ACPI object.
> +///
> +/// This structure represents the Rust abstraction for a C [`struct acpi_object`].
> +/// You probably want to convert it into actual object type (e.g [`AcpiBuffer`]).
> +///
> +/// # Example
> +/// ```
> +/// # use kernel::prelude::*;
> +/// use kernel::acpi::{AcpiObject, AcpiBuffer};
> +///
> +/// fn read_first_acpi_byte(obj: &AcpiObject) -> Result<u8> {
> +///     let buf: &AcpiBuffer = obj.try_into()?;
> +///
> +///     Ok(buf[0])
> +/// }
> +/// ```
> +///
> +/// [`struct acpi_object`]: srctree/include/acpi/actypes.h
> +#[repr(transparent)]
> +pub struct AcpiObject<'a> {
> +    inner: bindings::acpi_object,
> +    _p: PhantomData<&'a bindings::acpi_object>,

This can just be `&'a ()`.

> +}
> +
> +impl AcpiObject<'_> {
> +    /// Returns object type id (see [`actypes.h`](srctree/include/acpi/actypes.h)).
> +    pub fn type_id(&self) -> u32 {
> +        // SAFETY: `type` field is valid in all union variants.
> +        unsafe { self.inner.type_ }
> +    }
> +}
> +
> +/// Generate wrapper type for AcpiObject subtype.
> +///
> +/// For given subtype implements
> +/// - `#[repr(transparent)]` type wrapper,
> +/// - `TryFrom<&AcpiObject> for &SubType` trait,
> +/// - unsafe from_unchecked() for 'trusted' conversion.
> +macro_rules! acpi_object_subtype {
> +    ($subtype_name:ident <- ($acpi_type:ident, $field_name:ident, $union_type:ty)) => {
> +        /// Wraps `acpi_object` subtype.
> +        #[repr(transparent)]
> +        pub struct $subtype_name($union_type);
> +
> +        impl<'a> TryFrom<&'a AcpiObject<'a>> for &'a $subtype_name {
> +            type Error = Error;
> +
> +            fn try_from(value: &'a AcpiObject<'a>) -> core::result::Result<Self, Self::Error> {
> +                if (value.type_id() != $subtype_name::ACPI_TYPE) {
> +                    return Err(EINVAL);
> +                }
> +
> +                // SAFETY: Requested cast is valid because we validated type_id
> +                Ok(unsafe { $subtype_name::from_unchecked(&value) })
> +            }
> +        }
> +
> +        impl $subtype_name {
> +            /// Int value, representing this ACPI type (see [`acpitypes.h`]).
> +            ///
> +            /// [`acpitypes.h`]: srctree/include/linux/acpitypes.h
> +            pub const ACPI_TYPE: u32 = bindings::$acpi_type;
> +
> +            /// Converts opaque AcpiObject reference into exact ACPI type reference.
> +            ///
> +            /// # Safety
> +            ///
> +            /// - Requested cast should be valid (value.type_id() is `Self::ACPI_TYPE`).
> +            pub unsafe fn from_unchecked<'a>(value: &'a AcpiObject<'a>) -> &'a Self {
> +                // SAFETY:
> +                // - $field_name is currently active union's field due to external safety contract,
> +                // - Transmuting to `repr(transparent)` wrapper is safe.
> +                unsafe {
> +                    ::core::mem::transmute::<&$union_type, &$subtype_name>(&value.inner.$field_name)
> +                }
> +            }
> +        }
> +    };
> +}
> +
> +acpi_object_subtype!(AcpiBuffer

The AcpiBuffer type needs a lifetime too.

> +    <- (ACPI_TYPE_BUFFER, buffer, bindings::acpi_object__bindgen_ty_3));
> +
> +impl Deref for AcpiBuffer {
> +    type Target = [u8];
> +
> +    fn deref(&self) -> &Self::Target {
> +        // SAFETY: (pointer, length) indeed represents byte slice.

The safety comment needs to mention on the lifetime.

To make this explict you could do

    fn as_slice(&self) -> &'a [u8] { ... }

and then just call it inside Deref.

> +        unsafe { ::core::slice::from_raw_parts(self.0.pointer, self.0.length as usize) }
> +    }
> +}


Best,
Gary