Re: [PATCH v2 07/10] gpu: nova-core: falcon: introduce `bounded_enum` macro

["Gary Guo" <gary@garyguo.net>]

On Fri Mar 20, 2026 at 12:19 PM GMT, Alexandre Courbot wrote:
> Introduce a powered-up version of our ad-hoc `impl_from_enum_to_u8`
> macro that allows the definition of an enum type associated to a
> `Bounded` of a given width, and provides the `From` and `TryFrom`
> implementations required to use that enum as a register field member.
>
> The next patch will make use of it to convert all falcon registers to
> the kernel register macro.
>
> The macro is unused in this patch: it is introduced ahead-of-time to
> avoid diff mingling in the next patch that would make it difficult to
> review.
>
> Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
> ---
>  drivers/gpu/nova-core/falcon.rs | 82 +++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 82 insertions(+)
>
> diff --git a/drivers/gpu/nova-core/falcon.rs b/drivers/gpu/nova-core/falcon.rs
> index 5a4f7fc85160..5221e4476f90 100644
> --- a/drivers/gpu/nova-core/falcon.rs
> +++ b/drivers/gpu/nova-core/falcon.rs
> @@ -54,6 +54,88 @@ fn from(value: $enum_type) -> Self {
>      };
>  }
>  
> +/// Creates an enum type associated to a `Bounded`, with a `From` conversion to the associated
> +/// `Bounded` and either a `TryFrom` or `From` converting from the associated `Bounded`.
> +// TODO[FPRI]: This is a temporary solution to be replaced with the corresponding derive macros
> +// once they land.
> +#[expect(unused)]
> +macro_rules! bounded_enum {
> +    (
> +        $(#[doc = $enum_doc:expr])*
> +        enum $enum_type:ident with $from_impl:ident<Bounded<$width:ty, $length:literal>> {
> +            $( $(#[doc = $variant_doc:expr])* $variant:ident = $value:expr),* $(,)*
> +        }
> +    ) => {
> +        $(#[doc = $enum_doc])*
> +        #[derive(Debug, Copy, Clone, PartialEq, Eq, PartialOrd, Ord)]
> +        pub(crate) enum $enum_type {
> +            $(
> +                $(#[doc = $variant_doc])*
> +                $variant = $value
> +            ),*
> +        }
> +
> +        impl From<$enum_type> for Bounded<$width, $length> {
> +            fn from(value: $enum_type) -> Self {
> +                match value {
> +                    $($enum_type::$variant => Bounded::<$width, _>::new::<$value>()),*

Given that you're having `$value:expr` instead of `$value:literal`, this
probably should be `::new::<{$value}>`

> +                }
> +            }
> +        }
> +
> +        bounded_enum!(@impl_from $enum_type with $from_impl<Bounded<$width, $length>> {
> +            $($variant = $value),*
> +        });
> +    };
> +
> +    // `TryFrom` implementation from associated `Bounded` to enum type.
> +    (@impl_from $enum_type:ident with TryFrom<Bounded<$width:ty, $length:literal>> {
> +        $($variant:ident = $value:expr),* $(,)*
> +    }) => {
> +        impl TryFrom<Bounded<$width, $length>> for $enum_type {
> +            type Error = Error;
> +
> +            fn try_from(value: Bounded<$width, $length>) -> Result<Self> {
> +                match value.get() {
> +                    $(
> +                        $value => Ok($enum_type::$variant),
> +                    )*
> +                    _ => Err(EINVAL),
> +                }
> +            }
> +        }
> +    };
> +
> +    // `From` implementation from associated `Bounded` to enum type. Triggers a `build_error` if
> +    // all possible values of the `Bounded` are not covered by the enum type.
> +    (@impl_from $enum_type:ident with From<Bounded<$width:ty, $length:literal>> {
> +        $($variant:ident = $value:expr),* $(,)*
> +    }) => {
> +        impl From<Bounded<$width, $length>> for $enum_type {
> +            fn from(value: Bounded<$width, $length>) -> Self {
> +                $(
> +                    // Ensure all enum values fit into the `Bounded` type.
> +                    const { assert!(
> +                        $value < (1 << $length),
> +                        "Enum variant doesn't fit into assigned `Bounded` type."
> +                    ); }
> +                )*
> +
> +                // Makes the compiler optimizer aware of the possible range of values.
> +                let value = value.get() & ((1 << $length) - 1);
> +                match value {
> +                    $(
> +                        $value => $enum_type::$variant,
> +                    )*
> +                    // We land here if the match didn't cover all possible values for the
> +                    // `Bounded`.
> +                    _ => build_error!("Enum doesn't cover all values of the `Bounded` type."),

Given that the length is actually fixed, you should be able to use the
exhaustive check instead of `build_error!`:

    const MAX: $width = 1 << $length;

    match value {
        ...
        MAX.. => ...,
    }

This way compiler will complain about non-exhaustive patterns.

Best,
Gary

> +                }
> +            }
> +        }
> +    }
> +}
> +
>  /// Revision number of a falcon core, used in the [`crate::regs::NV_PFALCON_FALCON_HWCFG1`]
>  /// register.
>  #[repr(u8)]