Re: [PATCH v2 4/8] rust: dma: introduce dma::CoherentBox for memory initialization

[Andreas Hindborg <a.hindborg@kernel.org>]

"Danilo Krummrich" <dakr@kernel.org> writes:

> Currently, dma::Coherent cannot safely provide (mutable) access to its
> underlying memory because the memory might be concurrently accessed by a
> DMA device. This makes it difficult to safely initialize the memory
> before handing it over to the hardware.
>
> Introduce dma::CoherentBox, a type that encapsulates a dma::Coherent
> before its DMA address is exposed to the device. dma::CoherentBox can
> guarantee exclusive access to the inner dma::Coherent and implement
> Deref and DerefMut.
>
> Once the memory is properly initialized, dma::CoherentBox can be
> converted into a regular dma::Coherent.
>
> Reviewed-by: Alice Ryhl <aliceryhl@google.com>
> Signed-off-by: Danilo Krummrich <dakr@kernel.org>
> ---
>  rust/kernel/dma.rs | 154 ++++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 153 insertions(+), 1 deletion(-)
>
> diff --git a/rust/kernel/dma.rs b/rust/kernel/dma.rs
> index db645b01bdd0..cefb54f0424a 100644
> --- a/rust/kernel/dma.rs
> +++ b/rust/kernel/dma.rs
> @@ -20,7 +20,13 @@
>          FromBytes, //
>      }, //
>  };
> -use core::ptr::NonNull;
> +use core::{
> +    ops::{
> +        Deref,
> +        DerefMut, //
> +    },
> +    ptr::NonNull, //
> +};
>
>  /// DMA address type.
>  ///
> @@ -352,6 +358,152 @@ fn from(direction: DataDirection) -> Self {
>      }
>  }
>
> +/// CPU-owned DMA allocation that can be converted into a device-shared [`Coherent`] object.
> +///
> +/// Unlike [`Coherent`], a [`CoherentBox`] is guaranteed to be fully owned by the CPU -- its DMA
> +/// address is not exposed and it cannot be accessed by a device. This means it can safely be used
> +/// like a normal boxed allocation (e.g. direct reads, writes, and mutable slices are all safe).
> +///
> +/// A typical use is to allocate a [`CoherentBox`], populate it with normal CPU access, and then
> +/// convert it into a [`Coherent`] object to share it with the device.
> +///
> +/// # Examples
> +///
> +/// `CoherentBox<T>`:
> +///
> +/// ```
> +/// # use kernel::device::{
> +/// #     Bound,
> +/// #     Device,
> +/// # };
> +/// use kernel::dma::{attrs::*,
> +///     Coherent,
> +///     CoherentBox,
> +/// };
> +///
> +/// # fn test(dev: &Device<Bound>) -> Result {
> +/// let mut dmem: CoherentBox<u64> = CoherentBox::zeroed(dev, GFP_KERNEL)?;
> +/// *dmem = 42;
> +/// let dmem: Coherent<u64> = dmem.into();
> +/// # Ok::<(), Error>(()) }
> +/// ```
> +///
> +/// `CoherentBox<[T]>`:
> +///
> +///
> +/// ```
> +/// # use kernel::device::{
> +/// #     Bound,
> +/// #     Device,
> +/// # };
> +/// use kernel::dma::{attrs::*,
> +///     Coherent,
> +///     CoherentBox,
> +/// };
> +///
> +/// # fn test(dev: &Device<Bound>) -> Result {
> +/// let mut dmem: CoherentBox<[u64]> = CoherentBox::zeroed_slice(dev, 4, GFP_KERNEL)?;
> +/// dmem.fill(42);
> +/// let dmem: Coherent<[u64]> = dmem.into();
> +/// # Ok::<(), Error>(()) }
> +/// ```
> +pub struct CoherentBox<T: AsBytes + FromBytes + KnownSize + ?Sized>(Coherent<T>);
> +
> +impl<T: AsBytes + FromBytes> CoherentBox<[T]> {
> +    /// [`CoherentBox`] variant of [`Coherent::zeroed_slice_with_attrs`].
> +    #[inline]
> +    pub fn zeroed_slice_with_attrs(
> +        dev: &device::Device<Bound>,
> +        count: usize,
> +        gfp_flags: kernel::alloc::Flags,
> +        dma_attrs: Attrs,
> +    ) -> Result<Self> {
> +        Coherent::zeroed_slice_with_attrs(dev, count, gfp_flags, dma_attrs).map(Self)
> +    }
> +
> +    /// Same as [CoherentBox::zeroed_slice_with_attrs], but with `dma::Attrs(0)`.
> +    #[inline]
> +    pub fn zeroed_slice(
> +        dev: &device::Device<Bound>,
> +        count: usize,
> +        gfp_flags: kernel::alloc::Flags,
> +    ) -> Result<Self> {
> +        Self::zeroed_slice_with_attrs(dev, count, gfp_flags, Attrs(0))
> +    }
> +
> +    /// Initializes the element at `i` using the given initializer.
> +    ///
> +    /// Returns `EINVAL` if `i` is out of bounds.

Could you add an example for this function?


Reviewed-by: Andreas Hindborg <a.hindborg@kernel.org>


Best regards,
Andreas Hindborg