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

[Gladyshev Ilya <foxido@foxido.dev>]

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, $table_name);
     };
 }
+
+/// An ACPI object.
+///
+/// This structure represents the Rust abstraction for a C [`struct acpi_object`].
+/// You probably want to convert it into actual object type (e.g [`AcpiBuffer`]).
+///
+/// # Example
+/// ```
+/// # use kernel::prelude::*;
+/// use kernel::acpi::{AcpiObject, AcpiBuffer};
+///
+/// fn read_first_acpi_byte(obj: &AcpiObject) -> Result<u8> {
+///     let buf: &AcpiBuffer = obj.try_into()?;
+///
+///     Ok(buf[0])
+/// }
+/// ```
+///
+/// [`struct acpi_object`]: srctree/include/acpi/actypes.h
+#[repr(transparent)]
+pub struct AcpiObject<'a> {
+    inner: bindings::acpi_object,
+    _p: PhantomData<&'a bindings::acpi_object>,
+}
+
+impl AcpiObject<'_> {
+    /// Returns object type id (see [`actypes.h`](srctree/include/acpi/actypes.h)).
+    pub fn type_id(&self) -> u32 {
+        // SAFETY: `type` field is valid in all union variants.
+        unsafe { self.inner.type_ }
+    }
+}
+
+/// Generate wrapper type for AcpiObject subtype.
+///
+/// For given subtype implements
+/// - `#[repr(transparent)]` type wrapper,
+/// - `TryFrom<&AcpiObject> for &SubType` trait,
+/// - unsafe from_unchecked() for 'trusted' conversion.
+macro_rules! acpi_object_subtype {
+    ($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
+    <- (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.
+        unsafe { ::core::slice::from_raw_parts(self.0.pointer, self.0.length as usize) }
+    }
+}
-- 
2.52.0