gpu: nova-core: add functions and traits for lossless integer conversions

The core library's `From` implementations do not cover conversions
that are not portable or future-proof. For instance, even though it is
safe today, `From<usize>` is not implemented for `u64` because of the
possibility to support larger-than-64bit architectures in the future.

However, the kernel supports a narrower set of architectures, and in the
case of Nova we only support 64-bit. This makes it helpful and desirable
to provide more infallible conversions, lest we need to rely on the `as`
keyword and carry the risk of silently losing data.

Thus, introduce a new module `num` that provides safe const functions
performing more conversions allowed by the build target, as well as
`FromSafeCast` and `IntoSafeCast` traits that are just extensions of
`From` and `Into` to conversions that are known to be lossless.

Suggested-by: Danilo Krummrich <dakr@kernel.org>
Link: https://lore.kernel.org/rust-for-linux/DDK4KADWJHMG.1FUPL3SDR26XF@kernel.org/
Acked-by: Danilo Krummrich <dakr@kernel.org>
[acourbot@nvidia.com: fix merge conflicts after rebase.]
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Message-ID: <20251029-nova-as-v3-4-6a30c7333ad9@nvidia.com>

drivers/gpu/nova-core/nova_core.rs

@@ -13,6 +13,7 @@ mod firmware;
 mod gfw;
 mod gpu;
 mod gsp;
+mod num;
 mod regs;
 mod vbios;
 

drivers/gpu/nova-core/num.rs

@@ -0,0 +1,167 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Numerical helpers functions and traits.
+//!
+//! This is essentially a staging module for code to mature until it can be moved to the `kernel`
+//! crate.
+
+use kernel::{
+    macros::paste,
+    prelude::*, //
+};
+
+/// Implements safe `as` conversion functions from a given type into a series of target types.
+///
+/// These functions can be used in place of `as`, with the guarantee that they will be lossless.
+macro_rules! impl_safe_as {
+    ($from:ty as { $($into:ty),* }) => {
+        $(
+        paste! {
+            #[doc = ::core::concat!(
+                "Losslessly converts a [`",
+                ::core::stringify!($from),
+                "`] into a [`",
+                ::core::stringify!($into),
+                "`].")]
+            ///
+            /// This conversion is allowed as it is always lossless. Prefer this over the `as`
+            /// keyword to ensure no lossy casts are performed.
+            ///
+            /// This is for use from a `const` context. For non `const` use, prefer the
+            /// [`FromSafeCast`] and [`IntoSafeCast`] traits.
+            ///
+            /// # Examples
+            ///
+            /// ```
+            /// use crate::num;
+            ///
+            #[doc = ::core::concat!(
+                "assert_eq!(num::",
+                ::core::stringify!($from),
+                "_as_",
+                ::core::stringify!($into),
+                "(1",
+                ::core::stringify!($from),
+                "), 1",
+                ::core::stringify!($into),
+                ");")]
+            /// ```
+            #[allow(unused)]
+            #[inline(always)]
+            pub(crate) const fn [<$from _as_ $into>](value: $from) -> $into {
+                kernel::static_assert!(size_of::<$into>() >= size_of::<$from>());
+
+                value as $into
+            }
+        }
+        )*
+    };
+}
+
+impl_safe_as!(u8 as { u16, u32, u64, usize });
+impl_safe_as!(u16 as { u32, u64, usize });
+impl_safe_as!(u32 as { u64, usize } );
+// `u64` and `usize` have the same size on 64-bit platforms.
+#[cfg(CONFIG_64BIT)]
+impl_safe_as!(u64 as { usize } );
+
+// A `usize` fits into a `u64` on 32 and 64-bit platforms.
+#[cfg(any(CONFIG_32BIT, CONFIG_64BIT))]
+impl_safe_as!(usize as { u64 });
+
+// A `usize` fits into a `u32` on 32-bit platforms.
+#[cfg(CONFIG_32BIT)]
+impl_safe_as!(usize as { u32 });
+
+/// Extension trait providing guaranteed lossless cast to `Self` from `T`.
+///
+/// The standard library's `From` implementations do not cover conversions that are not portable or
+/// future-proof. For instance, even though it is safe today, `From<usize>` is not implemented for
+/// [`u64`] because of the possibility to support larger-than-64bit architectures in the future.
+///
+/// The workaround is to either deal with the error handling of [`TryFrom`] for an operation that
+/// technically cannot fail, or to use the `as` keyword, which can silently strip data if the
+/// destination type is smaller than the source.
+///
+/// Both options are hardly acceptable for the kernel. It is also a much more architecture
+/// dependent environment, supporting only 32 and 64 bit architectures, with some modules
+/// explicitly depending on a specific bus width that could greatly benefit from infallible
+/// conversion operations.
+///
+/// Thus this extension trait that provides, for the architecture the kernel is built for, safe
+/// conversion between types for which such cast is lossless.
+///
+/// In other words, this trait is implemented if, for the current build target and with `t: T`, the
+/// `t as Self` operation is completely lossless.
+///
+/// Prefer this over the `as` keyword to ensure no lossy casts are performed.
+///
+/// If you need to perform a conversion in `const` context, use [`u64_as_usize`], [`u32_as_usize`],
+/// [`usize_as_u64`], etc.
+///
+/// # Examples
+///
+/// ```
+/// use crate::num::FromSafeCast;
+///
+/// assert_eq!(usize::from_safe_cast(0xf00u32), 0xf00u32 as usize);
+/// ```
+#[expect(unused)]
+pub(crate) trait FromSafeCast<T> {
+    /// Create a `Self` from `value`. This operation is guaranteed to be lossless.
+    fn from_safe_cast(value: T) -> Self;
+}
+
+impl FromSafeCast<usize> for u64 {
+    fn from_safe_cast(value: usize) -> Self {
+        usize_as_u64(value)
+    }
+}
+
+#[cfg(CONFIG_32BIT)]
+impl FromSafeCast<usize> for u32 {
+    fn from_safe_cast(value: usize) -> Self {
+        usize_as_u32(value)
+    }
+}
+
+impl FromSafeCast<u32> for usize {
+    fn from_safe_cast(value: u32) -> Self {
+        u32_as_usize(value)
+    }
+}
+
+#[cfg(CONFIG_64BIT)]
+impl FromSafeCast<u64> for usize {
+    fn from_safe_cast(value: u64) -> Self {
+        u64_as_usize(value)
+    }
+}
+
+/// Counterpart to the [`FromSafeCast`] trait, i.e. this trait is to [`FromSafeCast`] what [`Into`]
+/// is to [`From`].
+///
+/// See the documentation of [`FromSafeCast`] for the motivation.
+///
+/// # Examples
+///
+/// ```
+/// use crate::num::IntoSafeCast;
+///
+/// assert_eq!(0xf00u32.into_safe_cast(), 0xf00u32 as usize);
+/// ```
+#[expect(unused)]
+pub(crate) trait IntoSafeCast<T> {
+    /// Convert `self` into a `T`. This operation is guaranteed to be lossless.
+    fn into_safe_cast(self) -> T;
+}
+
+/// Reverse operation for types implementing [`FromSafeCast`].
+impl<S, T> IntoSafeCast<T> for S
+where
+    T: FromSafeCast<S>,
+{
+    fn into_safe_cast(self) -> T {
+        T::from_safe_cast(self)
+    }
+}