Re: [PATCH v4 1/6] rust: bitmap: add MAX_LEN and NO_ALLOC_MAX_LEN constants

Yury Norov
2025-11-10T15:09:55Z

On Mon, Nov 10, 2025 at 10:01:32AM -0500, Yury Norov wrote:
> On Mon, Nov 10, 2025 at 02:20:17PM +0000, Alice Ryhl wrote:
> > On Mon, Nov 10, 2025 at 08:59:36AM -0500, Tamir Duberstein wrote:
> > > On Mon, Nov 10, 2025 at 8:06 AM Alice Ryhl <aliceryhl@google.com> wrote:
> > > >
> > > > 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`.
> > >
> > > Should this and other references to bindings::BITS_PER_LONG be
> > > `NO_ALLOC_MAX_LEN` instead?
> >
> > Ah yeah it probably makes sense to update this in a bunch of places.
>
> Yes, please.
>
> NO_ALLOC sounds a bit weird in exported API. Maybe NBITS_INPLACE
> or similar?

You add a 'new_inline()' constructor in this series, so this parameter
should be LEN_INLINE or NBITS_INLINE, like that.

> Also, at this point we're really close to:
>
> pub const NBITS_INPLACE: usize = CONFIG_NBITS_INPLACE;
>
> union BitmapRepr {
> bitmap: [usize, BITS_TO_LONGS(NBITS_INPLACE)]
> ptr: NonNull<usize>,
> }
>
> That would be a very useful addition for some particular scenarios,
> I believe. Even if you don't want to make it configurable, let's
> keep this option in mind?
>
> Thanks,
> Yury