[PATCH 1/2] rust: io: document const-offset requirement for infallible accessors

[PATCH 1/2] rust: io: document const-offset requirement for infallible accessors

[Gary Guo]

From: Gary Guo <gary@garyguo.net>

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>
---
 rust/kernel/io.rs | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

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 @@ fn try_write64(self, value: u64, offset: usize) -> Result
     }
 
     /// 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 @@ fn read8(self, offset: usize) -> u8
     }
 
     /// 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 @@ fn read16(self, offset: usize) -> u16
     }
 
     /// 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 @@ fn read32(self, offset: usize) -> u32
     }
 
     /// 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 @@ fn read64(self, offset: usize) -> u64
     }
 
     /// 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 @@ fn write8(self, value: u8, offset: usize)
     }
 
     /// 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 @@ fn write16(self, value: u16, offset: usize)
     }
 
     /// 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 @@ fn write32(self, value: u32, offset: usize)
     }
 
     /// Infallible 64-bit write with compile-time bounds check.
+    ///
+    /// `offset` should be constant.
     #[inline(always)]
     fn write64(self, value: u64, offset: usize)
     where

base-commit: b8809969e1d7a591e0f49dd464a5d04b3cf02ab1
-- 
2.54.0

[PATCH 2/2] rust: drm: fix non-const `read8` in unit test

[Gary Guo]

From: Gary Guo <gary@garyguo.net>

With CONFIG_CC_OPTIMIZE_FOR_SIZE, the address validity check in non-const
`read8` invocaction is not optimized away, leading to build failure.

Signed-off-by: Gary Guo <gary@garyguo.net>
---
 rust/kernel/drm/gem/shmem.rs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/rust/kernel/drm/gem/shmem.rs b/rust/kernel/drm/gem/shmem.rs
index 580e0808d6c0..a687d46d170d 100644
--- a/rust/kernel/drm/gem/shmem.rs
+++ b/rust/kernel/drm/gem/shmem.rs
@@ -688,7 +688,7 @@ fn vmap_io() -> Result {
         // Ensure the ordering in memory is correct
         let expected = 0xFEDCBA98_u32.to_ne_bytes().into_iter();
         for (offset, expected) in (0x20..=0x23).zip(expected) {
-            assert_eq!(vmap.read8(offset), expected);
+            assert_eq!(vmap.try_read8(offset).unwrap(), expected);
         }
 
         Ok(())
-- 
2.54.0

Re: [PATCH 1/2] rust: io: document const-offset requirement for infallible accessors

[Alexandre Courbot]

On Thu Jul 16, 2026 at 7:25 AM PDT, Gary Guo wrote:
> From: Gary Guo <gary@garyguo.net>
>
> 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>

Do we want to say something similar for read/write?

Re: [PATCH 2/2] rust: drm: fix non-const `read8` in unit test

[Alexandre Courbot]

On Thu Jul 16, 2026 at 7:25 AM PDT, Gary Guo wrote:
> From: Gary Guo <gary@garyguo.net>
>
> With CONFIG_CC_OPTIMIZE_FOR_SIZE, the address validity check in non-const
> `read8` invocaction is not optimized away, leading to build failure.
>
> Signed-off-by: Gary Guo <gary@garyguo.net>

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

Re: [PATCH 2/2] rust: drm: fix non-const `read8` in unit test

[Danilo Krummrich]

On Thu Jul 16, 2026 at 4:25 PM CEST, Gary Guo wrote:
> From: Gary Guo <gary@garyguo.net>
>
> With CONFIG_CC_OPTIMIZE_FOR_SIZE, the address validity check in non-const
> `read8` invocaction is not optimized away, leading to build failure.
>
> Signed-off-by: Gary Guo <gary@garyguo.net>

Fixes: d055768429b3 ("rust: drm: gem: shmem: Add vmap functions")

> ---
>  rust/kernel/drm/gem/shmem.rs | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/rust/kernel/drm/gem/shmem.rs b/rust/kernel/drm/gem/shmem.rs
> index 580e0808d6c0..a687d46d170d 100644
> --- a/rust/kernel/drm/gem/shmem.rs
> +++ b/rust/kernel/drm/gem/shmem.rs
> @@ -688,7 +688,7 @@ fn vmap_io() -> Result {
>          // Ensure the ordering in memory is correct
>          let expected = 0xFEDCBA98_u32.to_ne_bytes().into_iter();
>          for (offset, expected) in (0x20..=0x23).zip(expected) {
> -            assert_eq!(vmap.read8(offset), expected);
> +            assert_eq!(vmap.try_read8(offset).unwrap(), expected);
>          }
>  
>          Ok(())
> -- 
> 2.54.0