[PATCH v2 1/3] rust: sizes: add DeviceSize 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 with repeated
usize-to-u64 or usize-to-u32 conversion calls like
usize_as_u64(SZ_1M). This adds boilerplate with no safety benefit.

Add a DeviceSize trait with associated SZ_* constants, implemented for
u32 and u64. 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 both implementations are lossless. The
u32 impl has a const assert to catch any future constant that would
overflow; the u64 cast from usize is inherently lossless.

Replace the hand-written constant list with a define_sizes! macro that
generates the usize constants, the trait, and both trait impls from a
single list of names. Adding a new size or a new target type requires
changing only one place.

The trait also enables future generic APIs that accept T: DeviceSize,
so shared infrastructure can work with whatever address width the
driver declares.

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/
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
---
 rust/kernel/sizes.rs | 132 ++++++++++++++++++++++++++++---------------
 1 file changed, 88 insertions(+), 44 deletions(-)

diff --git a/rust/kernel/sizes.rs b/rust/kernel/sizes.rs
index 661e680d9330..6b11ec6d97b3 100644
--- a/rust/kernel/sizes.rs
+++ b/rust/kernel/sizes.rs
@@ -3,48 +3,92 @@
 //! 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 [`DeviceSize`] trait provides the same constants as associated constants
+//! on [`u32`] and [`u64`], 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.
+//!
+//! ```
+//! use kernel::sizes::{DeviceSize, SZ_1M};
+//!
+//! // usize constant (CPU-side)
+//! let pages: usize = SZ_1M / kernel::page::PAGE_SIZE;
+//!
+//! // Device-side constant via the trait
+//! let heap_size: u64 = 14 * u64::SZ_1M;
+//! let small: u32 = u32::SZ_4K;
+//! ```
+
+macro_rules! define_sizes {
+    ($($name:ident),* $(,)?) => {
+        // `usize` constants, from the C `SZ_*` defines in `include/linux/sizes.h`.
+        $(
+            #[doc = concat!("`", stringify!($name), "` as a [`usize`].")]
+            pub const $name: usize = bindings::$name as usize;
+        )*
+
+        /// Size constants for device address spaces.
+        ///
+        /// Implemented for [`u32`] and [`u64`] so drivers can choose the width
+        /// that matches their hardware. All `SZ_*` values fit in a [`u32`], so
+        /// both implementations are lossless.
+        ///
+        /// ```
+        /// use kernel::sizes::DeviceSize;
+        ///
+        /// let gpu_heap: u64 = 14 * u64::SZ_1M;
+        /// let mmio_window: u32 = u32::SZ_16M;
+        /// ```
+        pub trait DeviceSize {
+            $(
+                #[doc = concat!("`", stringify!($name), "` for this type.")]
+                const $name: Self;
+            )*
+        }
+
+        impl DeviceSize for u32 {
+            $(
+                const $name: Self = {
+                    assert!(self::$name <= u32::MAX as usize);
+                    self::$name as u32
+                };
+            )*
+        }
+
+        impl DeviceSize for u64 {
+            $(
+                const $name: Self = self::$name as u64;
+            )*
+        }
+    };
+}
 
-/// 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! {
+    SZ_1K,   // 0x0000_0400
+    SZ_2K,   // 0x0000_0800
+    SZ_4K,   // 0x0000_1000
+    SZ_8K,   // 0x0000_2000
+    SZ_16K,  // 0x0000_4000
+    SZ_32K,  // 0x0000_8000
+    SZ_64K,  // 0x0001_0000
+    SZ_128K, // 0x0002_0000
+    SZ_256K, // 0x0004_0000
+    SZ_512K, // 0x0008_0000
+    SZ_1M,   // 0x0010_0000
+    SZ_2M,   // 0x0020_0000
+    SZ_4M,   // 0x0040_0000
+    SZ_8M,   // 0x0080_0000
+    SZ_16M,  // 0x0100_0000
+    SZ_32M,  // 0x0200_0000
+    SZ_64M,  // 0x0400_0000
+    SZ_128M, // 0x0800_0000
+    SZ_256M, // 0x1000_0000
+    SZ_512M, // 0x2000_0000
+    SZ_1G,   // 0x4000_0000
+    SZ_2G,   // 0x8000_0000
+}
-- 
2.53.0