linux-kernel - Re: [PATCH v2 3/3] rust: dma: add as_slice/write functions for CoherentAllocation

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Message-Id: <D92VBJWRTE2N.1O4IA5X7I1YFS@nvidia.com>
Date: Thu, 10 Apr 2025 18:57:35 +0900
From: "Alexandre Courbot" <acourbot@...dia.com>
To: "Abdiel Janulgue" <abdiel.janulgue@...il.com>, <a.hindborg@...nel.org>,
 <benno.lossin@...ton.me>, <rust-for-linux@...r.kernel.org>
Cc: "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>, "Alice Ryhl" <aliceryhl@...gle.com>, "Trevor
 Gross" <tmgross@...ch.edu>, "Danilo Krummrich" <dakr@...nel.org>, "Valentin
 Obst" <kernel@...entinobst.de>, "open list" <linux-kernel@...r.kernel.org>,
 "Christoph Hellwig" <hch@....de>, "Marek Szyprowski"
 <m.szyprowski@...sung.com>, "Robin Murphy" <robin.murphy@....com>,
 <airlied@...hat.com>, "open list:DMA MAPPING HELPERS"
 <iommu@...ts.linux.dev>
Subject: Re: [PATCH v2 3/3] rust: dma: add as_slice/write functions for
 CoherentAllocation

Hi Abdiel,

On Thu Apr 10, 2025 at 5:58 PM JST, Abdiel Janulgue wrote:
> Add unsafe accessors for the region for reading or writing large
> blocks of data.
>
> Reviewed-by: Andreas Hindborg <a.hindborg@...nel.org>
> Signed-off-by: Abdiel Janulgue <abdiel.janulgue@...il.com>
> ---
>  rust/kernel/dma.rs | 85 ++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 85 insertions(+)
>
> diff --git a/rust/kernel/dma.rs b/rust/kernel/dma.rs
> index a61da5eeb017..880f6f04ba86 100644
> --- a/rust/kernel/dma.rs
> +++ b/rust/kernel/dma.rs
> @@ -218,6 +218,91 @@ pub fn dma_handle(&self) -> bindings::dma_addr_t {
>          self.dma_handle
>      }
>  
> +    /// Returns the data from the region starting from `offset` as a slice.
> +    /// `offset` and `count` are in units of `T`, not the number of bytes.
> +    ///
> +    /// For ringbuffer type of r/w access or use-cases where the pointer to the live data is needed,
> +    /// [`CoherentAllocation::start_ptr`] or [`CoherentAllocation::start_ptr_mut`] could be used instead.
> +    ///
> +    /// # Safety
> +    ///
> +    /// * Callers must ensure that the device does not read/write to/from memory while the returned
> +    ///   slice is live.
> +    /// * Callers must ensure that this call does not race with a write to the same region while
> +    ///   while the returned slice is live.
> +    pub unsafe fn as_slice(&self, offset: usize, count: usize) -> Result<&[T]> {
> +        let end = offset.checked_add(count).ok_or(EOVERFLOW)?;
> +        if end >= self.count {
> +            return Err(EINVAL);
> +        }

Not sure if you have overlooked my comment on the previous iteration or
if I completely missed the mark, but my understanding if that the bound
check should be `if end > self.count`. Also applies to the other methods
of this patch.