[PATCH v2] rust: impl_flags: add conversions for raw flag representations

[PATCH v2] rust: impl_flags: add conversions for raw flag representations

[Filipe Xavier]

Extend the impl_flags! macro to support conversions between generated flag
types and raw C/UAPI integers. Implement TryFrom<Repr> for individual flags
(exact variant match) and flag sets (rejecting unknown bits), along with an
unsafe from_raw() constructor for flag sets. Additionally, add BitOr and
BitOrAssign implementations between the raw representation and flag types.

Suggested-by: Daniel Almeida <daniel.almeida@collabora.com>
Suggested-by: Andreas Hindborg <a.hindborg@kernel.org>
Signed-off-by: Filipe Xavier <felipeaggger@gmail.com>
---
Changes in v2:
- New Error InvalidFlagValue for TryFrom implementations, mapping invalid flag values to EINVAL.
- Replace Tyr's local TryFrom implementation to use from the macro.
- Add raw BitOr<$flags> and BitOrAssign<$flags> support to complete operations.
- Link to v1: https://lore.kernel.org/r/20260912-add-from-raw-conversions-v1-1-0cee34684d24@gmail.com
---
 drivers/gpu/drm/tyr/vm.rs |  13 -----
 rust/kernel/error.rs      |  11 +++++
 rust/kernel/impl_flags.rs | 122 +++++++++++++++++++++++++++++++++++++++++-----
 3 files changed, 121 insertions(+), 25 deletions(-)

diff --git a/drivers/gpu/drm/tyr/vm.rs b/drivers/gpu/drm/tyr/vm.rs
index c5e307b1e2416837c85f890c074f62bc74289178..446672b4bc6bfedab789e20c97c4b754d5c019c0 100644
--- a/drivers/gpu/drm/tyr/vm.rs
+++ b/drivers/gpu/drm/tyr/vm.rs
@@ -141,19 +141,6 @@ fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
     }
 }
 
-impl TryFrom<u32> for VmMapFlags {
-    type Error = Error;
-
-    fn try_from(value: u32) -> Result<Self, Self::Error> {
-        let valid = VmFlag::Readonly as u32 | VmFlag::Noexec as u32 | VmFlag::Uncached as u32;
-
-        if value & !valid != 0 {
-            return Err(EINVAL);
-        }
-        Ok(Self(value))
-    }
-}
-
 /// Arguments for a virtual memory map operation.
 struct VmMapArgs<'drm> {
     /// Access permissions and caching behavior for the mapping.
diff --git a/rust/kernel/error.rs b/rust/kernel/error.rs
index e52793f771966f20258c7d021826f7b63f8cf35e..8d2c4300262743b504d82bcc041076e78636cda5 100644
--- a/rust/kernel/error.rs
+++ b/rust/kernel/error.rs
@@ -201,6 +201,10 @@ macro_rules! declare_err {
 #[derive(Clone, Copy, PartialEq, Eq)]
 pub struct Error(NonZeroI32);
 
+/// Represents an invalid value for a flag type.
+#[derive(Clone, Copy, PartialEq, Eq, Debug)]
+pub struct InvalidFlagValue;
+
 impl Error {
     /// Creates an [`Error`] from a kernel error code.
     ///
@@ -323,6 +327,13 @@ fn from(_: AllocError) -> Error {
     }
 }
 
+impl From<InvalidFlagValue> for Error {
+    #[inline]
+    fn from(_: InvalidFlagValue) -> Error {
+        code::EINVAL
+    }
+}
+
 impl From<TryFromIntError> for Error {
     #[inline]
     fn from(_: TryFromIntError) -> Error {
diff --git a/rust/kernel/impl_flags.rs b/rust/kernel/impl_flags.rs
index fdf44d5eea9cb907f6d8d209792a1d9b74b55be6..493cd7f51dcc5f4926e39c4e79c7d4ed7157233d 100644
--- a/rust/kernel/impl_flags.rs
+++ b/rust/kernel/impl_flags.rs
@@ -14,6 +14,8 @@
 /// - The struct and enum types with appropriate `#[repr]` attributes.
 /// - Implementations of common bitflag operators
 ///   ([`::core::ops::BitOr`], [`::core::ops::BitAnd`], etc.).
+/// - Conversions between the Rust-native types and their raw representation.
+/// - Validation when converting raw values back into Rust-native types.
 /// - Utility methods such as `.contains()` to check flags.
 ///
 /// # Examples
@@ -68,6 +70,25 @@
 /// let negated = !read_only;
 /// assert!(negated.contains(Permission::Write));
 /// assert!(!negated.contains(Permission::Read));
+///
+/// // Convert individual flags and flag sets to their raw representation.
+/// let raw: u32 = Permission::Read.into();
+/// assert_eq!(raw, 1);
+/// let raw: u32 = read_write.into();
+///
+/// // Raw values can be validated before entering the Rust-native API.
+/// assert_eq!(Permission::try_from(1), Ok(Permission::Read));
+/// assert!(Permission::try_from(3).is_err());
+/// assert!(Permissions::try_from(3).is_ok());
+///
+/// // Raw C/UAPI fields can be updated without an intermediate conversion.
+/// let mut raw = 0u32;
+/// raw |= Permission::Read;
+/// raw |= Permission::Write;
+/// assert_eq!(raw, 3);
+/// let read_write = Permission::Read | Permission::Write;
+/// raw |= read_write;
+/// assert_eq!(raw | read_write, 3);
 /// ```
 #[macro_export]
 macro_rules! impl_flags {
@@ -103,6 +124,13 @@ fn from(value: $flag) -> Self {
             }
         }
 
+        impl ::core::convert::From<$flag> for $ty {
+            #[inline]
+            fn from(value: $flag) -> Self {
+                value as $ty
+            }
+        }
+
         impl ::core::convert::From<$flags> for $ty {
             #[inline]
             fn from(value: $flags) -> Self {
@@ -110,32 +138,45 @@ fn from(value: $flags) -> Self {
             }
         }
 
-        impl ::core::ops::BitOr for $flags {
-            type Output = Self;
+        impl ::core::convert::TryFrom<$ty> for $flag {
+            type Error = ::kernel::error::InvalidFlagValue;
+
             #[inline]
-            fn bitor(self, rhs: Self) -> Self::Output {
-                Self(self.0 | rhs.0)
+            fn try_from(value: $ty) -> Result<Self, Self::Error> {
+                match value {
+                    $(
+                        v if v == ($value as $ty) => Ok($flag::$name),
+                    )+
+                    _ => Err(::kernel::error::InvalidFlagValue),
+                }
             }
         }
 
-        impl ::core::ops::BitOrAssign for $flags {
+        impl ::core::convert::TryFrom<$ty> for $flags {
+            type Error = ::kernel::error::InvalidFlagValue;
+
             #[inline]
-            fn bitor_assign(&mut self, rhs: Self) {
-                *self = *self | rhs;
+            fn try_from(value: $ty) -> Result<Self, Self::Error> {
+                if value & !Self::all_bits() != 0 {
+                    return Err(::kernel::error::InvalidFlagValue);
+                }
+
+                // SAFETY: All bits set in `value` are valid flag bits.
+                Ok(unsafe { Self::from_raw(value) })
             }
         }
 
-        impl ::core::ops::BitOr<$flag> for $flags {
+        impl ::core::ops::BitOr for $flags {
             type Output = Self;
             #[inline]
-            fn bitor(self, rhs: $flag) -> Self::Output {
-                self | Self::from(rhs)
+            fn bitor(self, rhs: Self) -> Self::Output {
+                Self(self.0 | rhs.0)
             }
         }
 
-        impl ::core::ops::BitOrAssign<$flag> for $flags {
+        impl ::core::ops::BitOrAssign for $flags {
             #[inline]
-            fn bitor_assign(&mut self, rhs: $flag) {
+            fn bitor_assign(&mut self, rhs: Self) {
                 *self = *self | rhs;
             }
         }
@@ -155,6 +196,21 @@ fn bitand_assign(&mut self, rhs: Self) {
             }
         }
 
+        impl ::core::ops::BitOr<$flag> for $flags {
+            type Output = Self;
+            #[inline]
+            fn bitor(self, rhs: $flag) -> Self::Output {
+                self | Self::from(rhs)
+            }
+        }
+
+        impl ::core::ops::BitOrAssign<$flag> for $flags {
+            #[inline]
+            fn bitor_assign(&mut self, rhs: $flag) {
+                *self = *self | rhs;
+            }
+        }
+
         impl ::core::ops::BitAnd<$flag> for $flags {
             type Output = Self;
             #[inline]
@@ -240,6 +296,38 @@ fn not(self) -> Self::Output {
             }
         }
 
+        impl ::core::ops::BitOr<$flag> for $ty {
+            type Output = Self;
+
+            #[inline]
+            fn bitor(self, rhs: $flag) -> Self::Output {
+                self | (rhs as $ty)
+            }
+        }
+
+        impl ::core::ops::BitOrAssign<$flag> for $ty {
+            #[inline]
+            fn bitor_assign(&mut self, rhs: $flag) {
+                *self |= rhs as $ty;
+            }
+        }
+
+        impl ::core::ops::BitOr<$flags> for $ty {
+            type Output = Self;
+
+            #[inline]
+            fn bitor(self, rhs: $flags) -> Self::Output {
+                self | rhs.0
+            }
+        }
+
+        impl ::core::ops::BitOrAssign<$flags> for $ty {
+            #[inline]
+            fn bitor_assign(&mut self, rhs: $flags) {
+                *self |= rhs.0;
+            }
+        }
+
         impl $flags {
             /// Returns an empty instance where no flags are set.
             #[inline]
@@ -253,6 +341,16 @@ pub const fn all_bits() -> $ty {
                 0 $( | $value )+
             }
 
+            /// Creates a flag set from its raw representation without validation.
+            ///
+            /// # Safety
+            ///
+            /// All bits set in `value` must correspond to valid flags.
+            #[inline]
+            pub const unsafe fn from_raw(value: $ty) -> Self {
+                Self(value)
+            }
+
             /// Checks if a specific flag is set.
             #[inline]
             pub fn contains(self, flag: $flag) -> bool {

---
base-commit: 08df884136f1c1197bab2a27814404fd329d9aac
change-id: 20260912-add-from-raw-conversions-4647d890bb77

Best regards,
-- 
Filipe Xavier <felipeaggger@gmail.com>

Re: [PATCH v2] rust: impl_flags: add conversions for raw flag representations

[Alexandre Courbot]

On Mon Sep 21, 2026 at 4:18 AM JST, Filipe Xavier wrote:
> Extend the impl_flags! macro to support conversions between generated flag
> types and raw C/UAPI integers. Implement TryFrom<Repr> for individual flags
> (exact variant match) and flag sets (rejecting unknown bits), along with an
> unsafe from_raw() constructor for flag sets. Additionally, add BitOr and
> BitOrAssign implementations between the raw representation and flag types.

This does 3 different things and would be much more comfortable to
review if split into 3 distinct patches.