[PATCH 17/36] rust: sync: atomic: Add i8/i16 load and store support

[Boqun Feng <boqun.feng@gmail.com>]

From: FUJITA Tomonori <fujita.tomonori@gmail.com>

Add atomic operation support for i8 and i16 types using volatile
read/write and smp_load_acquire/smp_store_release helpers.

[boqun: Adjust [1] to avoid introduction of
impl_atomic_only_load_and_store_ops!() in the middle]

Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Joel Fernandes <joelagnelf@nvidia.com>
Link: https://lore.kernel.org/all/20251228120546.1602275-1-fujita.tomonori@gmail.com/ [1]
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Link: https://patch.msgid.link/20251211113826.1299077-4-fujita.tomonori@gmail.com
---
 rust/kernel/sync/atomic/internal.rs  | 25 +++++++++++++++++++------
 rust/kernel/sync/atomic/predefine.rs | 14 +++++++++++++-
 2 files changed, 32 insertions(+), 7 deletions(-)

diff --git a/rust/kernel/sync/atomic/internal.rs b/rust/kernel/sync/atomic/internal.rs
index 41b4ce2935e3..1b2a7933bc14 100644
--- a/rust/kernel/sync/atomic/internal.rs
+++ b/rust/kernel/sync/atomic/internal.rs
@@ -13,17 +13,22 @@ mod private {
     pub trait Sealed {}
 }
 
-// `i32` and `i64` are only supported atomic implementations.
+// The C side supports atomic primitives only for `i32` and `i64` (`atomic_t` and `atomic64_t`),
+// while the Rust side also layers provides atomic support for `i8` and `i16`
+// on top of lower-level C primitives.
+impl private::Sealed for i8 {}
+impl private::Sealed for i16 {}
 impl private::Sealed for i32 {}
 impl private::Sealed for i64 {}
 
 /// A marker trait for types that implement atomic operations with C side primitives.
 ///
-/// This trait is sealed, and only types that have directly mapping to the C side atomics should
-/// impl this:
+/// This trait is sealed, and only types that map directly to the C side atomics
+/// or can be implemented with lower-level C primitives are allowed to implement this:
 ///
-/// - `i32` maps to `atomic_t`.
-/// - `i64` maps to `atomic64_t`.
+/// - `i8` and `i16` are implemented with lower-level C primitives.
+/// - `i32` map to `atomic_t`
+/// - `i64` map to `atomic64_t`
 pub trait AtomicImpl: Sized + Send + Copy + private::Sealed {
     /// The type of the delta in arithmetic or logical operations.
     ///
@@ -32,6 +37,14 @@ pub trait AtomicImpl: Sized + Send + Copy + private::Sealed {
     type Delta;
 }
 
+impl AtomicImpl for i8 {
+    type Delta = Self;
+}
+
+impl AtomicImpl for i16 {
+    type Delta = Self;
+}
+
 // `atomic_t` implements atomic operations on `i32`.
 impl AtomicImpl for i32 {
     type Delta = Self;
@@ -243,7 +256,7 @@ macro_rules! declare_and_impl_atomic_methods {
 }
 
 declare_and_impl_atomic_methods!(
-    [ i32 => atomic, i64 => atomic64 ]
+    [ i8 => atomic_i8, i16 => atomic_i16, i32 => atomic, i64 => atomic64 ]
     /// Basic atomic operations
     pub trait AtomicBasicOps {
         /// Atomic read (load).
diff --git a/rust/kernel/sync/atomic/predefine.rs b/rust/kernel/sync/atomic/predefine.rs
index 45a17985cda4..09b357be59b8 100644
--- a/rust/kernel/sync/atomic/predefine.rs
+++ b/rust/kernel/sync/atomic/predefine.rs
@@ -5,6 +5,18 @@
 use crate::static_assert;
 use core::mem::{align_of, size_of};
 
+// SAFETY: `i8` has the same size and alignment with itself, and is round-trip transmutable to
+// itself.
+unsafe impl super::AtomicType for i8 {
+    type Repr = i8;
+}
+
+// SAFETY: `i16` has the same size and alignment with itself, and is round-trip transmutable to
+// itself.
+unsafe impl super::AtomicType for i16 {
+    type Repr = i16;
+}
+
 // SAFETY: `i32` has the same size and alignment with itself, and is round-trip transmutable to
 // itself.
 unsafe impl super::AtomicType for i32 {
@@ -118,7 +130,7 @@ macro_rules! for_each_type {
 
     #[test]
     fn atomic_basic_tests() {
-        for_each_type!(42 in [i32, i64, u32, u64, isize, usize] |v| {
+        for_each_type!(42 in [i8, i16, i32, i64, u32, u64, isize, usize] |v| {
             let x = Atomic::new(v);
 
             assert_eq!(v, x.load(Relaxed));
-- 
2.51.0