rust: io: document const-offset requirement for infallible accessors

Due to the usage of `build_assert!` for address validity checking, these
accessors want constant offsets. Non-constant offsets can work but it
depend on compiler optimization levels, so it should be avoided.

Signed-off-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Link: https://patch.msgid.link/20260716142545.3622278-1-gary@kernel.org
Signed-off-by: Danilo Krummrich <dakr@kernel.org>

1 files changed, 16 insertions, 0 deletions

diff --git a/rust/kernel/io.rs b/rust/kernel/io.rs
index 95f46bb75f9e..a38c20ba3d23 100644
--- a/rust/kernel/io.rs
+++ b/rust/kernel/io.rs
@@ -721,6 +721,8 @@ pub trait Io<'a>: IoBase<'a> {
     }
 
     /// Infallible 8-bit read with compile-time bounds check.
+    ///
+    /// `offset` should be constant.
     #[inline(always)]
     fn read8(self, offset: usize) -> u8
     where
@@ -731,6 +733,8 @@ pub trait Io<'a>: IoBase<'a> {
     }
 
     /// Infallible 16-bit read with compile-time bounds check.
+    ///
+    /// `offset` should be constant.
     #[inline(always)]
     fn read16(self, offset: usize) -> u16
     where
@@ -741,6 +745,8 @@ pub trait Io<'a>: IoBase<'a> {
     }
 
     /// Infallible 32-bit read with compile-time bounds check.
+    ///
+    /// `offset` should be constant.
     #[inline(always)]
     fn read32(self, offset: usize) -> u32
     where
@@ -751,6 +757,8 @@ pub trait Io<'a>: IoBase<'a> {
     }
 
     /// Infallible 64-bit read with compile-time bounds check.
+    ///
+    /// `offset` should be constant.
     #[inline(always)]
     fn read64(self, offset: usize) -> u64
     where
@@ -761,6 +769,8 @@ pub trait Io<'a>: IoBase<'a> {
     }
 
     /// Infallible 8-bit write with compile-time bounds check.
+    ///
+    /// `offset` should be constant.
     #[inline(always)]
     fn write8(self, value: u8, offset: usize)
     where
@@ -771,6 +781,8 @@ pub trait Io<'a>: IoBase<'a> {
     }
 
     /// Infallible 16-bit write with compile-time bounds check.
+    ///
+    /// `offset` should be constant.
     #[inline(always)]
     fn write16(self, value: u16, offset: usize)
     where
@@ -781,6 +793,8 @@ pub trait Io<'a>: IoBase<'a> {
     }
 
     /// Infallible 32-bit write with compile-time bounds check.
+    ///
+    /// `offset` should be constant.
     #[inline(always)]
     fn write32(self, value: u32, offset: usize)
     where
@@ -791,6 +805,8 @@ pub trait Io<'a>: IoBase<'a> {
     }
 
     /// Infallible 64-bit write with compile-time bounds check.
+    ///
+    /// `offset` should be constant.
     #[inline(always)]
     fn write64(self, value: u64, offset: usize)
     where