rust: bitmap: add MAX_LEN and MAX_INLINE_LEN constants

To avoid hard-coding these values in drivers, define constants for them
that drivers can reference. Also, update all instances in bitmap.rs and
id_pool.rs that use these values to use the new constants.

Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Burak Emir <bqe@google.com>
Signed-off-by: Yury Norov (NVIDIA) <yury.norov@gmail.com>

2 files changed, 41 insertions, 31 deletions

diff --git a/rust/kernel/bitmap.rs b/rust/kernel/bitmap.rs
index aa8fc7bf06fc..0705646c6251 100644
--- a/rust/kernel/bitmap.rs
+++ b/rust/kernel/bitmap.rs
@@ -12,8 +12,6 @@ use crate::bindings;
 use crate::pr_err;
 use core::ptr::NonNull;
 
-const BITS_PER_LONG: usize = bindings::BITS_PER_LONG as usize;
-
 /// Represents a C bitmap. Wraps underlying C bitmap API.
 ///
 /// # Invariants
@@ -149,14 +147,14 @@ macro_rules! bitmap_assert_return {
 ///
 /// # Invariants
 ///
-/// * `nbits` is `<= i32::MAX` and never changes.
-/// * if `nbits <= bindings::BITS_PER_LONG`, then `repr` is a `usize`.
+/// * `nbits` is `<= MAX_LEN`.
+/// * if `nbits <= MAX_INLINE_LEN`, 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,
 }
 
@@ -164,7 +162,7 @@ impl core::ops::Deref for BitmapVec {
     type Target = Bitmap;
 
     fn deref(&self) -> &Bitmap {
-        let ptr = if self.nbits <= BITS_PER_LONG {
+        let ptr = if self.nbits <= BitmapVec::MAX_INLINE_LEN {
             // SAFETY: Bitmap is represented inline.
             #[allow(unused_unsafe, reason = "Safe since Rust 1.92.0")]
             unsafe {
@@ -183,7 +181,7 @@ impl core::ops::Deref for BitmapVec {
 
 impl core::ops::DerefMut for BitmapVec {
     fn deref_mut(&mut self) -> &mut Bitmap {
-        let ptr = if self.nbits <= BITS_PER_LONG {
+        let ptr = if self.nbits <= BitmapVec::MAX_INLINE_LEN {
             // SAFETY: Bitmap is represented inline.
             #[allow(unused_unsafe, reason = "Safe since Rust 1.92.0")]
             unsafe {
@@ -213,7 +211,7 @@ unsafe impl Sync for BitmapVec {}
 
 impl Drop for BitmapVec {
     fn drop(&mut self) {
-        if self.nbits <= BITS_PER_LONG {
+        if self.nbits <= BitmapVec::MAX_INLINE_LEN {
             return;
         }
         // SAFETY: `self.ptr` was returned by the C `bitmap_zalloc`.
@@ -226,23 +224,29 @@ impl Drop for BitmapVec {
 }
 
 impl BitmapVec {
+    /// The maximum possible length of a `BitmapVec`.
+    pub const MAX_LEN: usize = i32::MAX as usize;
+
+    /// The maximum length that uses the inline representation.
+    pub const MAX_INLINE_LEN: usize = usize::BITS as usize;
+
     /// 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 {
+        if nbits <= BitmapVec::MAX_INLINE_LEN {
             return Ok(BitmapVec {
                 repr: BitmapRepr { bitmap: 0 },
                 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: `MAX_INLINE_LEN < 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.
@@ -495,9 +499,10 @@ mod tests {
     #[test]
     fn bitmap_borrow() {
         let fake_bitmap: [usize; 2] = [0, 0];
+        let fake_bitmap_len = 2 * usize::BITS as usize;
         // SAFETY: `fake_c_bitmap` is an array of expected length.
-        let b = unsafe { Bitmap::from_raw(fake_bitmap.as_ptr(), 2 * BITS_PER_LONG) };
-        assert_eq!(2 * BITS_PER_LONG, b.len());
+        let b = unsafe { Bitmap::from_raw(fake_bitmap.as_ptr(), fake_bitmap_len) };
+        assert_eq!(fake_bitmap_len, b.len());
         assert_eq!(None, b.next_bit(0));
     }
 
diff --git a/rust/kernel/id_pool.rs b/rust/kernel/id_pool.rs
index a41a3404213c..0b1f720a1f7d 100644
--- a/rust/kernel/id_pool.rs
+++ b/rust/kernel/id_pool.rs
@@ -7,8 +7,6 @@
 use crate::alloc::{AllocError, Flags};
 use crate::bitmap::BitmapVec;
 
-const BITS_PER_LONG: usize = bindings::BITS_PER_LONG as usize;
-
 /// Represents a dynamic ID pool backed by a [`BitmapVec`].
 ///
 /// Clients acquire and release IDs from unset bits in a bitmap.
@@ -97,13 +95,12 @@ impl ReallocRequest {
 impl IdPool {
     /// Constructs a new [`IdPool`].
     ///
-    /// A capacity below [`BITS_PER_LONG`] is adjusted to
-    /// [`BITS_PER_LONG`].
+    /// A capacity below [`MAX_INLINE_LEN`] is adjusted to [`MAX_INLINE_LEN`].
     ///
-    /// [`BITS_PER_LONG`]: srctree/include/asm-generic/bitsperlong.h
+    /// [`MAX_INLINE_LEN`]: BitmapVec::MAX_INLINE_LEN
     #[inline]
     pub fn new(num_ids: usize, flags: Flags) -> Result<Self, AllocError> {
-        let num_ids = core::cmp::max(num_ids, BITS_PER_LONG);
+        let num_ids = usize::max(num_ids, BitmapVec::MAX_INLINE_LEN);
         let map = BitmapVec::new(num_ids, flags)?;
         Ok(Self { map })
     }
@@ -116,28 +113,34 @@ impl IdPool {
 
     /// Returns a [`ReallocRequest`] if the [`IdPool`] can be shrunk, [`None`] otherwise.
     ///
-    /// The capacity of an [`IdPool`] cannot be shrunk below [`BITS_PER_LONG`].
+    /// The capacity of an [`IdPool`] cannot be shrunk below [`MAX_INLINE_LEN`].
     ///
-    /// [`BITS_PER_LONG`]: srctree/include/asm-generic/bitsperlong.h
+    /// [`MAX_INLINE_LEN`]: BitmapVec::MAX_INLINE_LEN
     ///
     /// # Examples
     ///
     /// ```
-    /// use kernel::alloc::{AllocError, flags::GFP_KERNEL};
-    /// use kernel::id_pool::{ReallocRequest, IdPool};
+    /// use kernel::{
+    ///     alloc::AllocError,
+    ///     bitmap::BitmapVec,
+    ///     id_pool::{
+    ///         IdPool,
+    ///         ReallocRequest,
+    ///     },
+    /// };
     ///
     /// let mut pool = IdPool::new(1024, GFP_KERNEL)?;
     /// let alloc_request = pool.shrink_request().ok_or(AllocError)?;
     /// let resizer = alloc_request.realloc(GFP_KERNEL)?;
     /// pool.shrink(resizer);
-    /// assert_eq!(pool.capacity(), kernel::bindings::BITS_PER_LONG as usize);
+    /// assert_eq!(pool.capacity(), BitmapVec::MAX_INLINE_LEN);
     /// # Ok::<(), AllocError>(())
     /// ```
     #[inline]
     pub fn shrink_request(&self) -> Option<ReallocRequest> {
         let cap = self.capacity();
-        // Shrinking below [`BITS_PER_LONG`] is never possible.
-        if cap <= BITS_PER_LONG {
+        // Shrinking below `MAX_INLINE_LEN` is never possible.
+        if cap <= BitmapVec::MAX_INLINE_LEN {
             return None;
         }
         // Determine if the bitmap can shrink based on the position of
@@ -146,13 +149,13 @@ impl IdPool {
         // bitmap should shrink to half its current size.
         let Some(bit) = self.map.last_bit() else {
             return Some(ReallocRequest {
-                num_ids: BITS_PER_LONG,
+                num_ids: BitmapVec::MAX_INLINE_LEN,
             });
         };
         if bit >= (cap / 4) {
             return None;
         }
-        let num_ids = usize::max(BITS_PER_LONG, cap / 2);
+        let num_ids = usize::max(BitmapVec::MAX_INLINE_LEN, cap / 2);
         Some(ReallocRequest { num_ids })
     }
 
@@ -177,11 +180,13 @@ impl IdPool {
 
     /// Returns a [`ReallocRequest`] for growing this [`IdPool`], if possible.
     ///
-    /// The capacity of an [`IdPool`] cannot be grown above [`i32::MAX`].
+    /// The capacity of an [`IdPool`] cannot be grown above [`MAX_LEN`].
+    ///
+    /// [`MAX_LEN`]: BitmapVec::MAX_LEN
     #[inline]
     pub fn grow_request(&self) -> Option<ReallocRequest> {
         let num_ids = self.capacity() * 2;
-        if num_ids > i32::MAX.try_into().unwrap() {
+        if num_ids > BitmapVec::MAX_LEN {
             return None;
         }
         Some(ReallocRequest { num_ids })