[PATCH v8 11/12] rust: id_pool: do not round capacity up to BitmapVec::MAX_INLINE_LEN

[Eliot Courtney <ecourtney@nvidia.com>]

Current code in IdPool::with_capacity rounds the capacity up to
BitmapVec::MAX_INLINE_LEN, but BitmapVec::new works fine with values
smaller than this and still uses an inline representation. Remove this
behaviour.

This allows specifying a real capacity of 0, which was not previously
possible. This breaks `grow_request` in this case, so change it to grow
to at least `BitmapVec::MAX_INLINE_LEN`, mirroring the capacity floor in
`shrink_request`.

Signed-off-by: Eliot Courtney <ecourtney@nvidia.com>
---
 rust/kernel/id_pool.rs | 38 ++++++++++++++++++++++++++++++++------
 1 file changed, 32 insertions(+), 6 deletions(-)

diff --git a/rust/kernel/id_pool.rs b/rust/kernel/id_pool.rs
index 06a4c71c4c6c..4f329249df9d 100644
--- a/rust/kernel/id_pool.rs
+++ b/rust/kernel/id_pool.rs
@@ -112,13 +112,8 @@ pub fn new() -> Self {
     }
 
     /// Constructs a new [`IdPool`] with space for a specific number of bits.
-    ///
-    /// A capacity below [`MAX_INLINE_LEN`] is adjusted to [`MAX_INLINE_LEN`].
-    ///
-    /// [`MAX_INLINE_LEN`]: BitmapVec::MAX_INLINE_LEN
     #[inline]
     pub fn with_capacity(num_ids: usize, flags: Flags) -> Result<Self, AllocError> {
-        let num_ids = usize::max(num_ids, BitmapVec::MAX_INLINE_LEN);
         let map = BitmapVec::new(num_ids, flags)?;
         Ok(Self { map })
     }
@@ -152,6 +147,13 @@ pub fn capacity(&self) -> usize {
     /// let resizer = alloc_request.realloc(GFP_KERNEL)?;
     /// pool.shrink(resizer);
     /// assert_eq!(pool.capacity(), BitmapVec::MAX_INLINE_LEN);
+    ///
+    /// // A pool at the `MAX_INLINE_LEN` floor cannot shrink further.
+    /// assert!(pool.shrink_request().is_none());
+    ///
+    /// // Neither can a pool with a capacity below `MAX_INLINE_LEN`.
+    /// let small = IdPool::with_capacity(8, GFP_KERNEL)?;
+    /// assert!(small.shrink_request().is_none());
     /// # Ok::<(), AllocError>(())
     /// ```
     #[inline]
@@ -198,12 +200,36 @@ pub fn shrink(&mut self, mut resizer: PoolResizer) {
 
     /// Returns a [`ReallocRequest`] for growing this [`IdPool`], if possible.
     ///
+    /// Grows to at least [`MAX_INLINE_LEN`].
     /// The capacity of an [`IdPool`] cannot be grown above [`MAX_LEN`].
     ///
+    /// [`MAX_INLINE_LEN`]: BitmapVec::MAX_INLINE_LEN
     /// [`MAX_LEN`]: BitmapVec::MAX_LEN
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use kernel::{
+    ///     alloc::AllocError,
+    ///     bitmap::BitmapVec,
+    ///     id_pool::IdPool, //
+    /// };
+    ///
+    /// // Grow goes to at least BitmapVec::MAX_INLINE_LEN.
+    /// let mut pool = IdPool::with_capacity(0, GFP_KERNEL)?;
+    /// let resizer = pool.grow_request().ok_or(AllocError)?.realloc(GFP_KERNEL)?;
+    /// pool.grow(resizer);
+    /// assert_eq!(pool.capacity(), BitmapVec::MAX_INLINE_LEN);
+    ///
+    /// // Grow doubles if at least BitmapVec::MAX_INLINE_LEN.
+    /// let resizer = pool.grow_request().ok_or(AllocError)?.realloc(GFP_KERNEL)?;
+    /// pool.grow(resizer);
+    /// assert_eq!(pool.capacity(), 2 * BitmapVec::MAX_INLINE_LEN);
+    /// # Ok::<(), AllocError>(())
+    /// ```
     #[inline]
     pub fn grow_request(&self) -> Option<ReallocRequest> {
-        let num_ids = self.capacity() * 2;
+        let num_ids = usize::max(BitmapVec::MAX_INLINE_LEN, self.capacity() * 2);
         if num_ids > BitmapVec::MAX_LEN {
             return None;
         }

-- 
2.55.0