Re: [PATCH v3] rust: add new macro for common bitmap operations

[Lyude Paul]

I think there's still a few more changes needed but so far this looks
wonderful! Some comments down below:

On Fri, 2025-04-11 at 09:05 -0300, Filipe Xavier wrote:
> We have seen a proliferation of mod_whatever::foo::Flags
> being defined with essentially the same implementation
> for BitAnd, BitOr, contains and etc.
> 
> This macro aims to bring a solution for this,
> allowing to generate these methods for user-defined structs.
> With some use cases in KMS and upcoming GPU drivers.
> 
> Link: https://rust-for-linux.zulipchat.com/#narrow/channel/288089-General/topic/We.20really.20need.20a.20common.20.60Flags.60.20type
> Signed-off-by: Filipe Xavier <felipeaggger@xxxxxxxxx>
> Suggested-by: Daniel Almeida <daniel.almeida@xxxxxxxxxxxxx>
> Suggested-by: Lyude Paul <lyude@xxxxxxxxxx>
> ---
> Changes in v3:
> - New Feat: added support to declare flags inside macro use.
> - Minor fixes: used absolute paths to refer to items, fix rustdoc and fix example cases.
> - Link to v2: https://lore.kernel.org/r/20250325-feat-add-bitmask-macro-v2-1-d3beabdad90f@xxxxxxxxx
> 
> Changes in v2:
> - rename: change macro and file name to impl_flags.
> - negation sign: change char for negation to `!`. 
> - transpose docs: add support to transpose user provided docs.
> - visibility: add support to use user defined visibility.
> - operations: add new operations for flag, 
> to support use between bit and bitmap, eg: flag & flags.
> - code style: small fixes to remove warnings.
> - Link to v1: https://lore.kernel.org/r/20250304-feat-add-bitmask-macro-v1-1-1c2d2bcb476b@xxxxxxxxx
> ---
>  rust/kernel/impl_flags.rs | 231 ++++++++++++++++++++++++++++++++++++++++++++++
>  rust/kernel/lib.rs        |   1 +
>  rust/kernel/prelude.rs    |   1 +
>  3 files changed, 233 insertions(+)
> 
> diff --git a/rust/kernel/impl_flags.rs b/rust/kernel/impl_flags.rs
> new file mode 100644
> index 0000000000000000000000000000000000000000..992988d26bc82e8461987206cc3ae9335f9387c8
> --- /dev/null
> +++ b/rust/kernel/impl_flags.rs
> @@ -0,0 +1,231 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +//! impl_flags utilities for working with flags.
> +
> +/// Declares a impl_flags type with its corresponding flag type.
> +///
> +/// This macro generates:
> +/// - Implementations of common bitmap op. ([`::core::ops::BitOr`], [`::core::ops::BitAnd`], etc.).
> +/// - Utility methods such as `.contains()` to check flags.
> +///
> +/// # Examples
> +///
> +/// Defining and using impl_flags:
> +///
> +/// ```
> +/// impl_flags!(
> +///     /// Represents multiple permissions.
> +///     #[derive(Debug, Clone, Default, Copy, PartialEq, Eq)]
> +///     pub Permissions,
> +///     /// Represents a single permission.
> +///     #[derive(Debug, Clone, Copy, PartialEq, Eq)]
> +///     pub Permission {
> +///         const READ = 1 << 0,
> +///         const WRITE = 1 << 1,
> +///         const EXECUTE = 1 << 2,
> +///         },
> +///     u32
> +/// );
> +///
> +///
> +/// // Combine multiple permissions using operation OR (`|`).
> +/// let read_write: Permissions = Permission::READ | Permission::WRITE;
> +///
> +/// assert!(read_write.contains(Permission::READ));
> +/// assert!(read_write.contains(Permission::WRITE));
> +/// assert!(!read_write.contains(Permission::EXECUTE));
> +///
> +/// // Removing a permission with operation AND (`&`).
> +/// let read_only: Permissions = read_write & Permission::READ;
> +/// assert!(read_only.contains(Permission::READ));
> +/// assert!(!read_only.contains(Permission::WRITE));
> +///
> +/// // Toggling permissions with XOR (`^`).
> +/// let toggled: Permissions = read_only ^ Permission::READ;
> +/// assert!(!toggled.contains(Permission::READ));
> +///
> +/// // Inverting permissions with negation (`!`).
> +/// let negated = !read_only;
> +/// assert!(negated.contains(Permission::WRITE));
> +/// ```
> +#[macro_export]
> +macro_rules! impl_flags {
> +    (
> +        $(#[$outer_flags:meta])*
> +        $vis_flags:vis $flags:ident,
> +
> +        $(#[$outer_flag:meta])*
> +        $vis_flag:vis $flag:ident {
> +            $(
> +                $(#[$inner_flag:meta])*
> +                $kw:ident $name:ident = $value:expr
> +            ),* $(,)?
> +        },
> +        $ty:ty
> +    ) => {
> +        $(#[$outer_flags])*
> +        #[repr(transparent)]
> +        $vis_flags struct $flags($ty);
> +
> +        $(#[$outer_flag])*
> +        $vis_flag struct $flag($ty);
> +

So - I think that we would actually want $flag to be an enum, and manually add
a #[repr(TYPE)] (where TYPE is like u32, u64, etc.) so that we can just use
the discriminant instead of having to define a whole struct (this also should
allow for better behavior when using flag types in matches, since rust will
know that each enum member is a possible flag).

This would change the syntax a bit more for the actual Flag type (Flags would
stay the same), probably to something like:

  $(#[$outer_flag:meta])*
  $vis_flag:vis $flag:ident {
    $(
        $(#[$inner_Flag:meta])*
        $name:ident = $value:expr
    ),+
  }

And then I assume we could implement it with something like this:

  $(#[$outer_flag])*
  #[repr($ty)]
  $vis_flag enum $flag {
    $(
        $(#[$inner_flag:meta])*
        #[repr($ty:ty)]
        $name:ident = $value:expr
    ),+
  }

And instead of using .0 to convert from the Flag type to it's actual numeric
value, we would simply cast using as. E.g. CoolFlag as u32.

I think(?) it's probably fine if we just leave out the case of defining a Flag
type without explicit members, I don't think most users will be doing this and
we can add another form to macro_rules! in the future if it does end up being
important.

Does that make sense?

> +        impl ::core::convert::From<$flag> for $flags {
> +            #[inline]
> +            fn from(value: $flag) -> Self {
> +                Self(value.0)
> +            }
> +        }
> +
> +        impl ::core::convert::From<$flags> for $ty {
> +            #[inline]
> +            fn from(value: $flags) -> Self {
> +                value.0
> +            }
> +        }
> +
> +        impl ::core::ops::BitOr for $flags {
> +            type Output = Self;
> +            #[inline]
> +            fn bitor(self, rhs: Self) -> Self::Output {
> +                Self(self.0 | rhs.0)
> +            }
> +        }
> +
> +        impl ::core::ops::BitOrAssign for $flags {
> +            #[inline]
> +            fn bitor_assign(&mut self, rhs: Self) {
> +                *self = *self | rhs;
> +            }
> +        }
> +
> +        impl ::core::ops::BitAnd for $flags {
> +            type Output = Self;
> +            #[inline]
> +            fn bitand(self, rhs: Self) -> Self::Output {
> +                Self(self.0 & rhs.0)
> +            }
> +        }
> +
> +        impl ::core::ops::BitAndAssign for $flags {
> +            #[inline]
> +            fn bitand_assign(&mut self, rhs: Self) {
> +                *self = *self & rhs;
> +            }
> +        }
> +
> +        impl ::core::ops::BitOr<$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]
> +            fn bitand(self, rhs: $flag) -> Self::Output {
> +                self & Self::from(rhs)
> +            }
> +        }
> +
> +        impl ::core::ops::BitAndAssign<$flag> for $flags {
> +            #[inline]
> +            fn bitand_assign(&mut self, rhs: $flag) {
> +                *self = *self & rhs;
> +            }
> +        }
> +
> +        impl ::core::ops::BitXor for $flags {
> +            type Output = Self;
> +            #[inline]
> +            fn bitxor(self, rhs: Self) -> Self::Output {
> +                Self(self.0 ^ rhs.0)
> +            }
> +        }
> +
> +        impl ::core::ops::BitXorAssign for $flags {
> +            #[inline]
> +            fn bitxor_assign(&mut self, rhs: Self) {
> +                *self = *self ^ rhs;
> +            }
> +        }
> +
> +        impl ::core::ops::Not for $flags {
> +            type Output = Self;
> +            #[inline]
> +            fn not(self) -> Self::Output {
> +                Self(!self.0)
> +            }
> +        }
> +
> +        impl ::core::ops::BitOr for $flag {
> +            type Output = $flags;
> +            #[inline]
> +            fn bitor(self, rhs: Self) -> Self::Output {
> +                $flags(self.0 | rhs.0)
> +            }
> +        }
> +
> +        impl ::core::ops::BitAnd for $flag {
> +            type Output = $flags;
> +            #[inline]
> +            fn bitand(self, rhs: Self) -> Self::Output {
> +                $flags(self.0 & rhs.0)
> +            }
> +        }
> +
> +        impl ::core::ops::BitXor for $flag {
> +            type Output = $flags;
> +            #[inline]
> +            fn bitxor(self, rhs: Self) -> Self::Output {
> +                $flags(self.0 ^ rhs.0)
> +            }
> +        }
> +
> +        impl ::core::ops::Not for $flag {
> +            type Output = $flags;
> +            #[inline]
> +            fn not(self) -> Self::Output {
> +                $flags(!self.0)
> +            }
> +        }
> +
> +        impl ::core::ops::BitXor<$flag> for $flags {
> +            type Output = Self;
> +            #[inline]
> +            fn bitxor(self, rhs: $flag) -> Self::Output {
> +                self ^ Self::from(rhs)
> +            }
> +        }
> +
> +        impl $flag {
> +            $(
> +                $(#[$inner_flag])*
> +                pub $kw $name: Self = Self($value);
> +            )*
> +        }
> +
> +        impl $flags {
> +            /// Returns an empty instance of `type` where no flags are set.
> +            #[inline]
> +            pub const fn empty() -> Self {
> +                Self(0)
> +            }
> +
> +            /// Checks if a specific flag is set.
> +            #[inline]
> +            pub fn contains(self, flag: $flag) -> bool {
> +                (self.0 & flag.0) == flag.0
> +            }
> +        }
> +    };
> +}
> diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
> index 496ed32b0911a9fdbce5d26738b9cf7ef910b269..7653485a456ae5aa51becbf04153ea54a7067d9e 100644
> --- a/rust/kernel/lib.rs
> +++ b/rust/kernel/lib.rs
> @@ -49,6 +49,7 @@
>  #[cfg(CONFIG_RUST_FW_LOADER_ABSTRACTIONS)]
>  pub mod firmware;
>  pub mod fs;
> +pub mod impl_flags;
>  pub mod init;
>  pub mod io;
>  pub mod ioctl;
> diff --git a/rust/kernel/prelude.rs b/rust/kernel/prelude.rs
> index dde2e0649790ca24e6c347b29465ea0a1c3e503b..0f691dd2df71d821265fae01555ba50e6a76f372 100644
> --- a/rust/kernel/prelude.rs
> +++ b/rust/kernel/prelude.rs
> @@ -25,6 +25,7 @@
>  #[doc(no_inline)]
>  pub use super::dbg;
>  pub use super::fmt;
> +pub use super::impl_flags;
>  pub use super::{dev_alert, dev_crit, dev_dbg, dev_emerg, dev_err, dev_info, dev_notice, dev_warn};
>  pub use super::{pr_alert, pr_crit, pr_debug, pr_emerg, pr_err, pr_info, pr_notice, pr_warn};
>  
> 
> ---
> base-commit: beeb78d46249cab8b2b8359a2ce8fa5376b5ad2d
> change-id: 20250304-feat-add-bitmask-macro-6424b1c317e2
> 
> Best regards,

-- 
Cheers,
 Lyude Paul (she/her)
 Software Engineer at Red Hat

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