[PATCH v4 01/10] gpu: nova-core: introduce `bounded_enum` macro

[Alexandre Courbot]

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.

This allows us to generate the required conversion implementations for
using the kernel register macro and skip some tedious boilerplate.

Signed-off-by: Alexandre Courbot <acourbot@xxxxxxxxxx>
---
 drivers/gpu/nova-core/nova_core.rs |  1 +
 drivers/gpu/nova-core/num.rs       | 80 ++++++++++++++++++++++++++++++++++++++
 2 files changed, 81 insertions(+)

diff --git a/drivers/gpu/nova-core/nova_core.rs b/drivers/gpu/nova-core/nova_core.rs
index ccd14b757b49..98675c69d2b7 100644
--- a/drivers/gpu/nova-core/nova_core.rs
+++ b/drivers/gpu/nova-core/nova_core.rs
@@ -21,6 +21,7 @@
 mod gfw;
 mod gpu;
 mod gsp;
+#[macro_use]
 mod num;
 mod regs;
 mod sbuffer;
diff --git a/drivers/gpu/nova-core/num.rs b/drivers/gpu/nova-core/num.rs
index c952a834e662..6c824b8d7b97 100644
--- a/drivers/gpu/nova-core/num.rs
+++ b/drivers/gpu/nova-core/num.rs
@@ -215,3 +215,83 @@ pub(crate) const fn [<$from _into_ $into>]<const N: $from>() -> $into {
 impl_const_into!(u64 => { u8, u16, u32 });
 impl_const_into!(u32 => { u8, u16 });
 impl_const_into!(u16 => { u8 });
+
+/// Creates an enum type associated to a [`Bounded`](kernel::num::Bounded), with a [`From`]
+/// conversion to the associated `Bounded` and either a [`TryFrom`] or `From` conversion from the
+/// associated `Bounded`.
+// TODO[FPRI]: This is a temporary solution to be replaced with the corresponding derive macros
+// once they land.
+#[macro_export]
+macro_rules! bounded_enum {
+    (
+        $(#[$enum_meta:meta])*
+        $vis:vis enum $enum_type:ident with $from_impl:ident<Bounded<$width:ty, $length:literal>> {
+            $( $(#[doc = $variant_doc:expr])* $variant:ident = $value:expr),* $(,)*
+        }
+    ) => {
+        $(#[$enum_meta])*
+        $vis enum $enum_type {
+            $(
+                $(#[doc = $variant_doc])*
+                $variant = $value
+            ),*
+        }
+
+        impl core::convert::From<$enum_type> for kernel::num::Bounded<$width, $length> {
+            fn from(value: $enum_type) -> Self {
+                match value {
+                    $($enum_type::$variant =>
+                        kernel::num::Bounded::<$width, _>::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),* $(,)*
+    }) => {
+        impl core::convert::TryFrom<kernel::num::Bounded<$width, $length>> for $enum_type {
+            type Error = kernel::error::Error;
+
+            fn try_from(
+                value: kernel::num::Bounded<$width, $length>
+            ) -> kernel::error::Result<Self> {
+                match value.get() {
+                    $(
+                        $value => Ok($enum_type::$variant),
+                    )*
+                    _ => Err(kernel::error::code::EINVAL),
+                }
+            }
+        }
+    };
+
+    // `From` implementation from associated `Bounded` to enum type. Triggers a build-time 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),* $(,)*
+    }) => {
+        impl core::convert::From<kernel::num::Bounded<$width, $length>> for $enum_type {
+            fn from(value: kernel::num::Bounded<$width, $length>) -> Self {
+                const MAX: $width = 1 << $length;
+
+                // Makes the compiler optimizer aware of the possible range of values.
+                let value = value.get() & ((1 << $length) - 1);
+                match value {
+                    $(
+                        $value => $enum_type::$variant,
+                    )*
+                    // PANIC: we cannot reach this arm as all possible variants are handled by the
+                    // match arms above. It is here to make the compiler complain if `$enum_type`
+                    // does not cover all values of the `0..MAX` range.
+                    MAX.. => unreachable!(),
+                }
+            }
+        }
+    }
+}

-- 
2.53.0