[PATCH v5 1/3] rust: sizes: add SizeConstants trait for device address space constants

[John Hubbard <jhubbard@nvidia.com>]

The SZ_* constants are usize, matching the CPU pointer width. But
device address spaces have their own widths (32-bit MMIO windows,
64-bit GPU framebuffers, etc.), so drivers end up casting these
constants with SZ_1M as u64 or helper functions. This adds
boilerplate with no safety benefit.

Add a SizeConstants trait with associated SZ_* constants, implemented
for u32, u64, and usize. With the trait in scope, callers write
u64::SZ_1M or u32::SZ_4K to get the constant in their device's
native width. All SZ_* values fit in a u32, so every implementation
is lossless. Each impl has a const assert to catch any future
constant that would overflow.

A define_sizes! macro generates everything from a single internal
list of names. The macro takes the target types as arguments, so
adding a new target type requires changing only the call site.

Suggested-by: Danilo Krummrich <dakr@kernel.org>
Link: https://lore.kernel.org/all/DGB9G697GSWO.3VBFGU5MKFPMR@kernel.org/
Link: https://lore.kernel.org/all/DGHI8WRKBQS9.38910L6FIIZTE@kernel.org/
Reviewed-by: Eliot Courtney <ecourtney@nvidia.com>
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Acked-by: Gary Guo <gary@garyguo.net>
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
---
 rust/kernel/sizes.rs | 167 +++++++++++++++++++++++++++++++------------
 1 file changed, 123 insertions(+), 44 deletions(-)

diff --git a/rust/kernel/sizes.rs b/rust/kernel/sizes.rs
index 661e680d9330..837404a2042b 100644
--- a/rust/kernel/sizes.rs
+++ b/rust/kernel/sizes.rs
@@ -3,48 +3,127 @@
 //! Commonly used sizes.
 //!
 //! C headers: [`include/linux/sizes.h`](srctree/include/linux/sizes.h).
+//!
+//! The top-level `SZ_*` constants are [`usize`]-typed, for use in kernel page
+//! arithmetic and similar CPU-side work.
+//!
+//! The [`SizeConstants`] trait provides the same constants as associated constants
+//! on [`u32`], [`u64`], and [`usize`], for use in device address spaces where
+//! the address width depends on the hardware. Device drivers frequently need
+//! these constants as [`u64`] (or [`u32`]) rather than [`usize`], because
+//! device address spaces are sized independently of the CPU pointer width.
+//!
+//! # Examples
+//!
+//! ```
+//! use kernel::page::PAGE_SIZE;
+//! use kernel::sizes::{SizeConstants, SZ_1M};
+//!
+//! // Module-level constants continue to work without a type qualifier.
+//! let num_pages_in_1m = SZ_1M / PAGE_SIZE;
+//!
+//! // Trait associated constants require a type qualifier.
+//! let heap_size = 14 * u64::SZ_1M;
+//! let small = u32::SZ_4K;
+//! ```
+
+macro_rules! define_sizes {
+    ($($type:ty),* $(,)?) => {
+        define_sizes!(@internal [$($type),*]
+            /// `0x0000_0400`.
+            SZ_1K,
+            /// `0x0000_0800`.
+            SZ_2K,
+            /// `0x0000_1000`.
+            SZ_4K,
+            /// `0x0000_2000`.
+            SZ_8K,
+            /// `0x0000_4000`.
+            SZ_16K,
+            /// `0x0000_8000`.
+            SZ_32K,
+            /// `0x0001_0000`.
+            SZ_64K,
+            /// `0x0002_0000`.
+            SZ_128K,
+            /// `0x0004_0000`.
+            SZ_256K,
+            /// `0x0008_0000`.
+            SZ_512K,
+            /// `0x0010_0000`.
+            SZ_1M,
+            /// `0x0020_0000`.
+            SZ_2M,
+            /// `0x0040_0000`.
+            SZ_4M,
+            /// `0x0080_0000`.
+            SZ_8M,
+            /// `0x0100_0000`.
+            SZ_16M,
+            /// `0x0200_0000`.
+            SZ_32M,
+            /// `0x0400_0000`.
+            SZ_64M,
+            /// `0x0800_0000`.
+            SZ_128M,
+            /// `0x1000_0000`.
+            SZ_256M,
+            /// `0x2000_0000`.
+            SZ_512M,
+            /// `0x4000_0000`.
+            SZ_1G,
+            /// `0x8000_0000`.
+            SZ_2G,
+        );
+    };
+
+    (@internal [$($type:ty),*] $($names_and_metas:tt)*) => {
+        define_sizes!(@consts_and_trait $($names_and_metas)*);
+        define_sizes!(@impls [$($type),*] $($names_and_metas)*);
+    };
+
+    (@consts_and_trait $($(#[$meta:meta])* $name:ident,)*) => {
+        $(
+            $(#[$meta])*
+            pub const $name: usize = bindings::$name as usize;
+        )*
+
+        /// Size constants for device address spaces.
+        ///
+        /// Implemented for [`u32`], [`u64`], and [`usize`] so drivers can
+        /// choose the width that matches their hardware. All `SZ_*` values fit
+        /// in a [`u32`], so all implementations are lossless.
+        ///
+        /// # Examples
+        ///
+        /// ```
+        /// use kernel::sizes::SizeConstants;
+        ///
+        /// let gpu_heap = 14 * u64::SZ_1M;
+        /// let mmio_window = u32::SZ_16M;
+        /// ```
+        pub trait SizeConstants {
+            $(
+                $(#[$meta])*
+                const $name: Self;
+            )*
+        }
+    };
+
+    (@impls [] $($(#[$meta:meta])* $name:ident,)*) => {};
+
+    (@impls [$first:ty $(, $rest:ty)*] $($(#[$meta:meta])* $name:ident,)*) => {
+        impl SizeConstants for $first {
+            $(
+                const $name: Self = {
+                    assert!((self::$name as u128) <= (<$first>::MAX as u128));
+                    self::$name as $first
+                };
+            )*
+        }
+
+        define_sizes!(@impls [$($rest),*] $($(#[$meta])* $name,)*);
+    };
+}
 
-/// 0x00000400
-pub const SZ_1K: usize = bindings::SZ_1K as usize;
-/// 0x00000800
-pub const SZ_2K: usize = bindings::SZ_2K as usize;
-/// 0x00001000
-pub const SZ_4K: usize = bindings::SZ_4K as usize;
-/// 0x00002000
-pub const SZ_8K: usize = bindings::SZ_8K as usize;
-/// 0x00004000
-pub const SZ_16K: usize = bindings::SZ_16K as usize;
-/// 0x00008000
-pub const SZ_32K: usize = bindings::SZ_32K as usize;
-/// 0x00010000
-pub const SZ_64K: usize = bindings::SZ_64K as usize;
-/// 0x00020000
-pub const SZ_128K: usize = bindings::SZ_128K as usize;
-/// 0x00040000
-pub const SZ_256K: usize = bindings::SZ_256K as usize;
-/// 0x00080000
-pub const SZ_512K: usize = bindings::SZ_512K as usize;
-/// 0x00100000
-pub const SZ_1M: usize = bindings::SZ_1M as usize;
-/// 0x00200000
-pub const SZ_2M: usize = bindings::SZ_2M as usize;
-/// 0x00400000
-pub const SZ_4M: usize = bindings::SZ_4M as usize;
-/// 0x00800000
-pub const SZ_8M: usize = bindings::SZ_8M as usize;
-/// 0x01000000
-pub const SZ_16M: usize = bindings::SZ_16M as usize;
-/// 0x02000000
-pub const SZ_32M: usize = bindings::SZ_32M as usize;
-/// 0x04000000
-pub const SZ_64M: usize = bindings::SZ_64M as usize;
-/// 0x08000000
-pub const SZ_128M: usize = bindings::SZ_128M as usize;
-/// 0x10000000
-pub const SZ_256M: usize = bindings::SZ_256M as usize;
-/// 0x20000000
-pub const SZ_512M: usize = bindings::SZ_512M as usize;
-/// 0x40000000
-pub const SZ_1G: usize = bindings::SZ_1G as usize;
-/// 0x80000000
-pub const SZ_2G: usize = bindings::SZ_2G as usize;
+define_sizes!(u32, u64, usize);
-- 
2.53.0