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::CoherentInit, a type that encapsulates a dma::Coherent before its DMA address is exposed to the device. dma::CoherentInit can guarantee exclusive access to the inner dma::Coherent and implement Deref and DerefMut. Once the memory is properly initialized, dma::CoherentInit can be converted into a regular dma::Coherent. Signed-off-by: Danilo Krummrich <[email protected]> --- rust/kernel/dma.rs | 153 ++++++++++++++++++++++++++++++++++++++++++++- 1 file changed, 152 insertions(+), 1 deletion(-) diff --git a/rust/kernel/dma.rs b/rust/kernel/dma.rs index 291fdea3b52b..79dd8717ac47 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 +358,151 @@ fn from(direction: DataDirection) -> Self { } } +/// Initializer type for [`Coherent`]. +/// +/// A [`Coherent`] object can't provide access to its memory as (mutable) slice safely, since it +/// can't fulfill the requirements for creating a slice. For instance, it is not valid to have a +/// (mutable) slice to of the memory of a [`Coherent`] while the memory might be accessed by a +/// device. +/// +/// In contrast, this initializer type is able to fulfill the requirements to safely obtain a +/// (mutable) slice, as it neither provides access to the DMA address of the embedded [`Coherent`], +/// nor can it be used with the DMA projection accessors. +/// +/// Once initialized, this type can be converted to a regular [`Coherent`] object. +/// +/// # Examples +/// +/// `CoherentInit<T>`: +/// +/// ``` +/// # use kernel::device::{ +/// # Bound, +/// # Device, +/// # }; +/// use kernel::dma::{attrs::*, +/// Coherent, +/// CoherentInit, +/// }; +/// +/// # fn test(dev: &Device<Bound>) -> Result { +/// let mut dmem: CoherentInit<u64> = +/// CoherentInit::zeroed_with_attrs(dev, GFP_KERNEL, DMA_ATTR_NO_WARN)?; +/// *dmem = 42; +/// let dmem: Coherent<u64> = dmem.into(); +/// # Ok::<(), Error>(()) } +/// ``` +/// +/// `CoherentInit<[T]>`: +/// +/// +/// ``` +/// # use kernel::device::{ +/// # Bound, +/// # Device, +/// # }; +/// use kernel::dma::{attrs::*, +/// Coherent, +/// CoherentInit, +/// }; +/// +/// # fn test(dev: &Device<Bound>) -> Result { +/// let mut dmem: CoherentInit<[u64]> = +/// CoherentInit::zeroed_slice_with_attrs(dev, 4, GFP_KERNEL, DMA_ATTR_NO_WARN)?; +/// dmem.fill(42); +/// let dmem: Coherent<[u64]> = dmem.into(); +/// # Ok::<(), Error>(()) } +/// ``` +pub struct CoherentInit<T: AsBytes + FromBytes + KnownSize + ?Sized>(Coherent<T>); + +impl<T: AsBytes + FromBytes> CoherentInit<[T]> { + /// Initializer variant of [`Coherent::zeroed_slice_with_attrs`]. + 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 [CoherentInit::zeroed_slice_with_attrs], but with `dma::Attrs(0)`. + 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. + pub fn init_at<E>(&mut self, i: usize, init: impl Init<T, E>) -> Result + where + Error: From<E>, + { + if i >= self.0.len() { + return Err(EINVAL); + } + + let ptr = core::ptr::from_mut(&mut self[i]); + + // SAFETY: + // - `ptr` is valid, properly aligned, and within this allocation. + // - `T: AsBytes + FromBytes` guarantees all bit patterns are valid, so partial writes on + // error cannot leave the element in an invalid state. + // - The DMA address has not been exposed yet, so there is no concurrent device access. + unsafe { init.__init(ptr)? }; + + Ok(()) + } +} + +impl<T: AsBytes + FromBytes> CoherentInit<T> { + /// Same as [`CoherentInit::zeroed_slice_with_attrs`], but for a single element. + pub fn zeroed_with_attrs( + dev: &device::Device<Bound>, + gfp_flags: kernel::alloc::Flags, + dma_attrs: Attrs, + ) -> Result<Self> { + Coherent::zeroed_with_attrs(dev, gfp_flags, dma_attrs).map(Self) + } + + /// Same as [`CoherentInit::zeroed_slice`], but for a single element. + pub fn zeroed(dev: &device::Device<Bound>, gfp_flags: kernel::alloc::Flags) -> Result<Self> { + Self::zeroed_with_attrs(dev, gfp_flags, Attrs(0)) + } +} + +impl<T: AsBytes + FromBytes + KnownSize + ?Sized> Deref for CoherentInit<T> { + type Target = T; + + fn deref(&self) -> &Self::Target { + // SAFETY: + // - We have not exposed the DMA address yet, so there can't be any concurrent access by a + // device. + // - We have exclusive access to `self.0`. + unsafe { self.0.as_ref() } + } +} + +impl<T: AsBytes + FromBytes + KnownSize + ?Sized> DerefMut for CoherentInit<T> { + fn deref_mut(&mut self) -> &mut Self::Target { + // SAFETY: + // - We have not exposed the DMA address yet, so there can't be any concurrent access by a + // device. + // - We have exclusive access to `self.0`. + unsafe { self.0.as_mut() } + } +} + +impl<T: AsBytes + FromBytes + KnownSize + ?Sized> From<CoherentInit<T>> for Coherent<T> { + fn from(value: CoherentInit<T>) -> Self { + value.0 + } +} + /// An abstraction of the `dma_alloc_coherent` API. /// /// This is an abstraction around the `dma_alloc_coherent` API which is used to allocate and map -- 2.53.0
