[PATCH 1/2] rust: bitmap: add MAX_LEN and NO_ALLOC_MAX_LEN constants

Alice Ryhl
2025-10-20T13:33:56Z

To avoid hard-coding these values in drivers, define constants for them
that drivers can reference.

Signed-off-by: Alice Ryhl <aliceryhl@google.com>
---
rust/kernel/bitmap.rs | 16 +++++++++++-----
1 file changed, 11 insertions(+), 5 deletions(-)

diff --git a/rust/kernel/bitmap.rs b/rust/kernel/bitmap.rs
index aa8fc7bf06fc99865ae755d8694e4bec3dc8e7f0..15fa23b45054b9272415fcc000e3e3b52c74d7c1 100644
--- a/rust/kernel/bitmap.rs
+++ b/rust/kernel/bitmap.rs
@@ -149,14 +149,14 @@ macro_rules! bitmap_assert_return {
///
/// # Invariants
///
-/// * `nbits` is `<= i32::MAX` and never changes.
+/// * `nbits` is `<= MAX_LEN`.
/// * if `nbits <= bindings::BITS_PER_LONG`, then `repr` is a `usize`.
/// * otherwise, `repr` holds a non-null pointer to an initialized
/// array of `unsigned long` that is large enough to hold `nbits` bits.
pub struct BitmapVec {
/// Representation of bitmap.
repr: BitmapRepr,
- /// Length of this bitmap. Must be `<= i32::MAX`.
+ /// Length of this bitmap. Must be `<= MAX_LEN`.
nbits: usize,
}

@@ -226,10 +226,16 @@ fn drop(&mut self) {
}

impl BitmapVec {
+ /// The maximum possible length of a `BitmapVec`.
+ pub const MAX_LEN: usize = i32::MAX as usize;
+
+ /// The maximum length that avoids allocating.
+ pub const NO_ALLOC_MAX_LEN: usize = BITS_PER_LONG;
+
/// Constructs a new [`BitmapVec`].
///
/// Fails with [`AllocError`] when the [`BitmapVec`] could not be allocated. This
- /// includes the case when `nbits` is greater than `i32::MAX`.
+ /// includes the case when `nbits` is greater than `MAX_LEN`.
#[inline]
pub fn new(nbits: usize, flags: Flags) -> Result<Self, AllocError> {
if nbits <= BITS_PER_LONG {
@@ -238,11 +244,11 @@ pub fn new(nbits: usize, flags: Flags) -> Result<Self, AllocError> {
nbits,
});
}
- if nbits > i32::MAX.try_into().unwrap() {
+ if nbits > Self::MAX_LEN {
return Err(AllocError);
}
let nbits_u32 = u32::try_from(nbits).unwrap();
- // SAFETY: `BITS_PER_LONG < nbits` and `nbits <= i32::MAX`.
+ // SAFETY: `BITS_PER_LONG < nbits` and `nbits <= MAX_LEN`.
let ptr = unsafe { bindings::bitmap_zalloc(nbits_u32, flags.as_raw()) };
let ptr = NonNull::new(ptr).ok_or(AllocError)?;
// INVARIANT: `ptr` returned by C `bitmap_zalloc` and `nbits` checked.

--
2.51.0.915.g61a8936c21-goog