[PATCH v2 1/5] rust: bitmap: add MAX_LEN and NO_ALLOC_MAX_LEN constants

[Alice Ryhl <aliceryhl@google.com>]

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

Acked-by: Danilo Krummrich <dakr@kernel.org>
Reviewed-by: Burak Emir <bqe@google.com>
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.

-- 
2.51.0.869.ge66316f041-goog