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

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Message-ID: <87friihloe.fsf@kernel.org>
Date: Tue, 08 Apr 2025 14:39:29 +0200
From: Andreas Hindborg <a.hindborg@...nel.org>
To: "Abdiel Janulgue" <abdiel.janulgue@...il.com>
Cc: <ojeda@...nel.org>,  "Danilo Krummrich" <dakr@...nel.org>,  "Daniel
 Almeida" <daniel.almeida@...labora.com>,  "Robin Murphy"
 <robin.murphy@....com>,  "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>,  "Alice Ryhl" <aliceryhl@...gle.com>,  "Trevor
 Gross" <tmgross@...ch.edu>,  <rust-for-linux@...r.kernel.org>,  "Marek
 Szyprowski" <m.szyprowski@...sung.com>,  <iommu@...ts.linux.dev>,
  <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/3] rust: dma: add as_slice/write functions for
 CoherentAllocation

"Abdiel Janulgue" <abdiel.janulgue@...il.com> writes:

> 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 | 87 ++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 87 insertions(+)
>
> diff --git a/rust/kernel/dma.rs b/rust/kernel/dma.rs
> index 24a6f10370c4..24025ec602ff 100644
> --- a/rust/kernel/dma.rs
> +++ b/rust/kernel/dma.rs
> @@ -218,6 +218,93 @@ 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.
> +    ///
> +    /// Due to the safety requirements of slice, the caller should consider that the region could
> +    /// be modified by the device at anytime.

The user does not need to consider this, because the safety requirements
make sure this is not a problem. The user only needs to consider the
safety requirements.

>> For ringbuffer type of r/w access or use-cases where
> +    /// the pointer to the live data is needed, `start_ptr()` or `start_ptr_mut()` could be
> +    /// used instead.
> +    ///
> +    /// # Safety
> +    ///
> +    /// * Callers must ensure that no hardware operations that involve the buffer are currently
> +    ///   taking place 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);
> +        }
> +        // SAFETY:
> +        // - The pointer is valid due to type invariant on `CoherentAllocation`,
> +        // we've just checked that the range and index is within bounds. The immutability of the
> +        // of data is also guaranteed by the safety requirements of the function.
> +        // - `offset` can't overflow since it is smaller than `self.count` and we've checked
> +        // that `self.count` won't overflow early in the constructor.
> +        Ok(unsafe { core::slice::from_raw_parts(self.cpu_addr.add(offset), count) })
> +    }
> +
> +    /// Performs the same functionality as [`CoherentAllocation::as_slice`], except that a mutable
> +    /// slice is returned.
> +    ///
> +    /// # Safety
> +    ///
> +    /// * Callers must ensure that no hardware operations that involve the buffer are currently
> +    ///   taking place while the returned slice is live.
> +    /// * Callers must ensure that this call does not race with a read or write to the same region
> +    ///   while the returned slice is live.
> +    pub unsafe fn as_slice_mut(&self, offset: usize, count: usize) -> Result<&mut [T]> {
> +        let end = offset.checked_add(count).ok_or(EOVERFLOW)?;
> +        if end >= self.count {
> +            return Err(EINVAL);
> +        }
> +        // SAFETY:
> +        // - The pointer is valid due to type invariant on `CoherentAllocation`,
> +        // we've just checked that the range and index is within bounds. The immutability of the
> +        // of data is also guaranteed by the safety requirements of the function.

Formatting nit: could you indent the paragraph under the bullet:

  - The pointer is valid due to type invariant on `CoherentAllocation`,
    we've just checked that the range and index is within bounds. The immutability of the
    of data is also guaranteed by the safety requirements of the function.



Best regards,
Andreas Hindborg