linux-kernel - Re: [PATCH 2/5] rust: dma: add DMA addressing capabilities

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Message-ID: <CAH5fLggL+F-B=9YX7jGJux10i+W2t-4_QRx=ze9WHYasx-vRww@mail.gmail.com>
Date: Wed, 16 Jul 2025 12:20:13 +0200
From: Alice Ryhl <aliceryhl@...gle.com>
To: Greg KH <gregkh@...uxfoundation.org>
Cc: Danilo Krummrich <dakr@...nel.org>, abdiel.janulgue@...il.com, 
	daniel.almeida@...labora.com, robin.murphy@....com, a.hindborg@...nel.org, 
	ojeda@...nel.org, alex.gaynor@...il.com, boqun.feng@...il.com, 
	gary@...yguo.net, bjorn3_gh@...tonmail.com, lossin@...nel.org, 
	tmgross@...ch.edu, bhelgaas@...gle.com, kwilczynski@...nel.org, 
	rafael@...nel.org, rust-for-linux@...r.kernel.org, linux-pci@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/5] rust: dma: add DMA addressing capabilities

On Wed, Jul 16, 2025 at 11:15 AM Greg KH <gregkh@...uxfoundation.org> wrote:
>
> On Thu, Jul 10, 2025 at 09:45:44PM +0200, Danilo Krummrich wrote:
> > +/// Returns a bitmask with the lowest `n` bits set to `1`.
> > +///
> > +/// For `n` in `0..=64`, returns a mask with the lowest `n` bits set.
> > +/// For `n > 64`, returns `u64::MAX` (all bits set).
> > +///
> > +/// # Examples
> > +///
> > +/// ```
> > +/// use kernel::dma::dma_bit_mask;
> > +///
> > +/// assert_eq!(dma_bit_mask(0), 0);
> > +/// assert_eq!(dma_bit_mask(1), 0b1);
> > +/// assert_eq!(dma_bit_mask(64), u64::MAX);
> > +/// assert_eq!(dma_bit_mask(100), u64::MAX); // Saturates at all bits set.
> > +/// ```
> > +pub const fn dma_bit_mask(n: usize) -> u64 {
> > +    match n {
> > +        0 => 0,
> > +        1..=64 => u64::MAX >> (64 - n),
> > +        _ => u64::MAX,
> > +    }
> > +}
>
> This is just the C macro DMA_BIT_MASK(), right?  If so, can that be said
> here somewhere?  Or, how about turning DMA_BIT_MASK() into an inline
> function which could then be just called by the rust code directly
> instead?

Converting to an inline method would not make a difference for calling
it from Rust. We would need a helper in rust/helpers/ either way, and
it's also possible to define a helper for a macro.

Alice