linux-kernel - Re: [PATCH v15 02/11] rust: add dma coherent allocator abstraction.

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Message-ID: <Z9mTSNj23fLlBbSF@google.com>
Date: Tue, 18 Mar 2025 15:37:44 +0000
From: Alice Ryhl <aliceryhl@...gle.com>
To: Andreas Hindborg <a.hindborg@...nel.org>
Cc: Abdiel Janulgue <abdiel.janulgue@...il.com>, rust-for-linux@...r.kernel.org, 
	daniel.almeida@...labora.com, dakr@...nel.org, robin.murphy@....com, 
	Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>, 
	Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>, 
	"Björn Roy Baron" <bjorn3_gh@...tonmail.com>, Benno Lossin <benno.lossin@...ton.me>, 
	Trevor Gross <tmgross@...ch.edu>, Valentin Obst <kernel@...entinobst.de>, linux-kernel@...r.kernel.org, 
	Christoph Hellwig <hch@....de>, Marek Szyprowski <m.szyprowski@...sung.com>, airlied@...hat.com, 
	iommu@...ts.linux.dev
Subject: Re: [PATCH v15 02/11] rust: add dma coherent allocator abstraction.

On Tue, Mar 18, 2025 at 02:07:40PM +0100, Andreas Hindborg wrote:
> "Abdiel Janulgue" <abdiel.janulgue@...il.com> writes:
> 
> [...]
> 
> > +/// Possible attributes associated with a DMA mapping.
> > +///
> > +/// They can be combined with the operators `|`, `&`, and `!`.
> > +///
> > +/// Values can be used from the [`attrs`] module.
> > +///
> > +/// # Examples
> > +///
> > +/// ```
> > +/// use kernel::device::Device;
> > +/// use kernel::dma::{attrs::*, CoherentAllocation};
> > +///
> > +/// # fn test(dev: &Device) -> Result {
> > +/// let attribs = DMA_ATTR_FORCE_CONTIGUOUS | DMA_ATTR_NO_WARN;
> > +/// let c: CoherentAllocation<u64> =
> > +///     CoherentAllocation::alloc_attrs(dev, 4, GFP_KERNEL, attribs)?;
> > +/// # Ok::<(), Error>(()) }
> > +/// ```
> > +#[derive(Clone, Copy, PartialEq)]
> > +#[repr(transparent)]
> > +pub struct Attrs(u32);
> > +
> > +impl Attrs {
> > +    /// Get the raw representation of this attribute.
> > +    pub(crate) fn as_raw(self) -> crate::ffi::c_ulong {
> > +        self.0 as _
> > +    }
> 
> OT: I wonder why we do not have `usize: From<u32>`? It seems like this
> should be fine, even on 32 bit systems?

usize only implements From<unn> when it would work on all concievable
platforms, so it's missing for u32 due to 16-bit platforms.

Alice