rust/kernel/dma.rs

// SPDX-License-Identifier: GPL-2.0

//! Direct memory access (DMA).
//!
//! C header: [`include/linux/dma-mapping.h`](srctree/include/linux/dma-mapping.h)

use crate::{
    bindings, build_assert, device,
    device::{Bound, Core},
    error::{to_result, Result},
    prelude::*,
    transmute::{AsBytes, FromBytes},
    types::ARef,
};

/// Trait to be implemented by DMA capable bus devices.
///
/// The [`dma::Device`](Device) trait should be implemented by bus specific device representations,
/// where the underlying bus is DMA capable, such as [`pci::Device`](::kernel::pci::Device) or
/// [`platform::Device`](::kernel::platform::Device).
pub trait Device: AsRef<device::Device<Core>> {
    /// Set up the device's DMA streaming addressing capabilities.
    ///
    /// This method is usually called once from `probe()` as soon as the device capabilities are
    /// known.
    ///
    /// # Safety
    ///
    /// This method must not be called concurrently with any DMA allocation or mapping primitives,
    /// such as [`CoherentAllocation::alloc_attrs`].
    unsafe fn dma_set_mask(&self, mask: DmaMask) -> Result {
        // SAFETY:
        // - By the type invariant of `device::Device`, `self.as_ref().as_raw()` is valid.
        // - The safety requirement of this function guarantees that there are no concurrent calls
        //   to DMA allocation and mapping primitives using this mask.
        to_result(unsafe { bindings::dma_set_mask(self.as_ref().as_raw(), mask.value()) })
    }

    /// Set up the device's DMA coherent addressing capabilities.
    ///
    /// This method is usually called once from `probe()` as soon as the device capabilities are
    /// known.
    ///
    /// # Safety
    ///
    /// This method must not be called concurrently with any DMA allocation or mapping primitives,
    /// such as [`CoherentAllocation::alloc_attrs`].
    unsafe fn dma_set_coherent_mask(&self, mask: DmaMask) -> Result {
        // SAFETY:
        // - By the type invariant of `device::Device`, `self.as_ref().as_raw()` is valid.
        // - The safety requirement of this function guarantees that there are no concurrent calls
        //   to DMA allocation and mapping primitives using this mask.
        to_result(unsafe { bindings::dma_set_coherent_mask(self.as_ref().as_raw(), mask.value()) })
    }

    /// Set up the device's DMA addressing capabilities.
    ///
    /// This is a combination of [`Device::dma_set_mask`] and [`Device::dma_set_coherent_mask`].
    ///
    /// This method is usually called once from `probe()` as soon as the device capabilities are
    /// known.
    ///
    /// # Safety
    ///
    /// This method must not be called concurrently with any DMA allocation or mapping primitives,
    /// such as [`CoherentAllocation::alloc_attrs`].
    unsafe fn dma_set_mask_and_coherent(&self, mask: DmaMask) -> Result {
        // SAFETY:
        // - By the type invariant of `device::Device`, `self.as_ref().as_raw()` is valid.
        // - The safety requirement of this function guarantees that there are no concurrent calls
        //   to DMA allocation and mapping primitives using this mask.
        to_result(unsafe {
            bindings::dma_set_mask_and_coherent(self.as_ref().as_raw(), mask.value())
        })
    }
}

/// A DMA mask that holds a bitmask with the lowest `n` bits set.
///
/// Use [`DmaMask::new`] or [`DmaMask::try_new`] to construct a value. Values
/// are guaranteed to never exceed the bit width of `u64`.
///
/// This is the Rust equivalent of the C macro `DMA_BIT_MASK()`.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub struct DmaMask(u64);

impl DmaMask {
    /// Constructs a `DmaMask` with the lowest `n` bits set to `1`.
    ///
    /// For `n <= 64`, sets exactly the lowest `n` bits.
    /// For `n > 64`, results in a build error.
    ///
    /// # Examples
    ///
    /// ```
    /// use kernel::dma::DmaMask;
    ///
    /// let mask0 = DmaMask::new::<0>();
    /// assert_eq!(mask0.value(), 0);
    ///
    /// let mask1 = DmaMask::new::<1>();
    /// assert_eq!(mask1.value(), 0b1);
    ///
    /// let mask64 = DmaMask::new::<64>();
    /// assert_eq!(mask64.value(), u64::MAX);
    ///
    /// // Build failure.
    /// // let mask_overflow = DmaMask::new::<100>();
    /// ```
    #[inline]
    pub const fn new<const N: u32>() -> Self {
        let Ok(mask) = Self::try_new(N) else {
            build_error!("Invalid DMA Mask.");
        };

        mask
    }

    /// Constructs a `DmaMask` with the lowest `n` bits set to `1`.
    ///
    /// For `n <= 64`, sets exactly the lowest `n` bits.
    /// For `n > 64`, returns [`EINVAL`].
    ///
    /// # Examples
    ///
    /// ```
    /// use kernel::dma::DmaMask;
    ///
    /// let mask0 = DmaMask::try_new(0)?;
    /// assert_eq!(mask0.value(), 0);
    ///
    /// let mask1 = DmaMask::try_new(1)?;
    /// assert_eq!(mask1.value(), 0b1);
    ///
    /// let mask64 = DmaMask::try_new(64)?;
    /// assert_eq!(mask64.value(), u64::MAX);
    ///
    /// let mask_overflow = DmaMask::try_new(100);
    /// assert!(mask_overflow.is_err());
    /// # Ok::<(), Error>(())
    /// ```
    #[inline]
    pub const fn try_new(n: u32) -> Result<Self> {
        Ok(Self(match n {
            0 => 0,
            1..=64 => u64::MAX >> (64 - n),
            _ => return Err(EINVAL),
        }))
    }

    /// Returns the underlying `u64` bitmask value.
    #[inline]
    pub const fn value(&self) -> u64 {
        self.0
    }
}

/// 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::{Bound, Device};
/// use kernel::dma::{attrs::*, CoherentAllocation};
///
/// # fn test(dev: &Device<Bound>) -> 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 crate::ffi::c_ulong
    }

    /// Check whether `flags` is contained in `self`.
    pub fn contains(self, flags: Attrs) -> bool {
        (self & flags) == flags
    }
}

impl core::ops::BitOr for Attrs {
    type Output = Self;
    fn bitor(self, rhs: Self) -> Self::Output {
        Self(self.0 | rhs.0)
    }
}

impl core::ops::BitAnd for Attrs {
    type Output = Self;
    fn bitand(self, rhs: Self) -> Self::Output {
        Self(self.0 & rhs.0)
    }
}

impl core::ops::Not for Attrs {
    type Output = Self;
    fn not(self) -> Self::Output {
        Self(!self.0)
    }
}

/// DMA mapping attributes.
pub mod attrs {
    use super::Attrs;

    /// Specifies that reads and writes to the mapping may be weakly ordered, that is that reads
    /// and writes may pass each other.
    pub const DMA_ATTR_WEAK_ORDERING: Attrs = Attrs(bindings::DMA_ATTR_WEAK_ORDERING);

    /// Specifies that writes to the mapping may be buffered to improve performance.
    pub const DMA_ATTR_WRITE_COMBINE: Attrs = Attrs(bindings::DMA_ATTR_WRITE_COMBINE);

    /// Lets the platform to avoid creating a kernel virtual mapping for the allocated buffer.
    pub const DMA_ATTR_NO_KERNEL_MAPPING: Attrs = Attrs(bindings::DMA_ATTR_NO_KERNEL_MAPPING);

    /// Allows platform code to skip synchronization of the CPU cache for the given buffer assuming
    /// that it has been already transferred to 'device' domain.
    pub const DMA_ATTR_SKIP_CPU_SYNC: Attrs = Attrs(bindings::DMA_ATTR_SKIP_CPU_SYNC);

    /// Forces contiguous allocation of the buffer in physical memory.
    pub const DMA_ATTR_FORCE_CONTIGUOUS: Attrs = Attrs(bindings::DMA_ATTR_FORCE_CONTIGUOUS);

    /// Hints DMA-mapping subsystem that it's probably not worth the time to try
    /// to allocate memory to in a way that gives better TLB efficiency.
    pub const DMA_ATTR_ALLOC_SINGLE_PAGES: Attrs = Attrs(bindings::DMA_ATTR_ALLOC_SINGLE_PAGES);

    /// This tells the DMA-mapping subsystem to suppress allocation failure reports (similarly to
    /// `__GFP_NOWARN`).
    pub const DMA_ATTR_NO_WARN: Attrs = Attrs(bindings::DMA_ATTR_NO_WARN);

    /// Indicates that the buffer is fully accessible at an elevated privilege level (and
    /// ideally inaccessible or at least read-only at lesser-privileged levels).
    pub const DMA_ATTR_PRIVILEGED: Attrs = Attrs(bindings::DMA_ATTR_PRIVILEGED);
}

/// 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
/// large coherent DMA regions.
///
/// A [`CoherentAllocation`] instance contains a pointer to the allocated region (in the
/// processor's virtual address space) and the device address which can be given to the device
/// as the DMA address base of the region. The region is released once [`CoherentAllocation`]
/// is dropped.
///
/// # Invariants
///
/// - For the lifetime of an instance of [`CoherentAllocation`], the `cpu_addr` is a valid pointer
///   to an allocated region of coherent memory and `dma_handle` is the DMA address base of the
///   region.
/// - The size in bytes of the allocation is equal to `size_of::<T> * count`.
/// - `size_of::<T> * count` fits into a `usize`.
// TODO
//
// DMA allocations potentially carry device resources (e.g.IOMMU mappings), hence for soundness
// reasons DMA allocation would need to be embedded in a `Devres` container, in order to ensure
// that device resources can never survive device unbind.
//
// However, it is neither desirable nor necessary to protect the allocated memory of the DMA
// allocation from surviving device unbind; it would require RCU read side critical sections to
// access the memory, which may require subsequent unnecessary copies.
//
// Hence, find a way to revoke the device resources of a `CoherentAllocation`, but not the
// entire `CoherentAllocation` including the allocated memory itself.
pub struct CoherentAllocation<T: AsBytes + FromBytes> {
    dev: ARef<device::Device>,
    dma_handle: bindings::dma_addr_t,
    count: usize,
    cpu_addr: *mut T,
    dma_attrs: Attrs,
}

impl<T: AsBytes + FromBytes> CoherentAllocation<T> {
    /// Allocates a region of `size_of::<T> * count` of coherent memory.
    ///
    /// # Examples
    ///
    /// ```
    /// # use kernel::device::{Bound, Device};
    /// use kernel::dma::{attrs::*, CoherentAllocation};
    ///
    /// # fn test(dev: &Device<Bound>) -> Result {
    /// let c: CoherentAllocation<u64> =
    ///     CoherentAllocation::alloc_attrs(dev, 4, GFP_KERNEL, DMA_ATTR_NO_WARN)?;
    /// # Ok::<(), Error>(()) }
    /// ```
    pub fn alloc_attrs(
        dev: &device::Device<Bound>,
        count: usize,
        gfp_flags: kernel::alloc::Flags,
        dma_attrs: Attrs,
    ) -> Result<CoherentAllocation<T>> {
        build_assert!(
            core::mem::size_of::<T>() > 0,
            "It doesn't make sense for the allocated type to be a ZST"
        );

        let size = count
            .checked_mul(core::mem::size_of::<T>())
            .ok_or(EOVERFLOW)?;
        let mut dma_handle = 0;
        // SAFETY: Device pointer is guaranteed as valid by the type invariant on `Device`.
        let ret = unsafe {
            bindings::dma_alloc_attrs(
                dev.as_raw(),
                size,
                &mut dma_handle,
                gfp_flags.as_raw(),
                dma_attrs.as_raw(),
            )
        };
        if ret.is_null() {
            return Err(ENOMEM);
        }
        // INVARIANT:
        // - We just successfully allocated a coherent region which is accessible for
        //   `count` elements, hence the cpu address is valid. We also hold a refcounted reference
        //   to the device.
        // - The allocated `size` is equal to `size_of::<T> * count`.
        // - The allocated `size` fits into a `usize`.
        Ok(Self {
            dev: dev.into(),
            dma_handle,
            count,
            cpu_addr: ret.cast::<T>(),
            dma_attrs,
        })
    }

    /// Performs the same functionality as [`CoherentAllocation::alloc_attrs`], except the
    /// `dma_attrs` is 0 by default.
    pub fn alloc_coherent(
        dev: &device::Device<Bound>,
        count: usize,
        gfp_flags: kernel::alloc::Flags,
    ) -> Result<CoherentAllocation<T>> {
        CoherentAllocation::alloc_attrs(dev, count, gfp_flags, Attrs(0))
    }

    /// Returns the number of elements `T` in this allocation.
    ///
    /// Note that this is not the size of the allocation in bytes, which is provided by
    /// [`Self::size`].
    pub fn count(&self) -> usize {
        self.count
    }

    /// Returns the size in bytes of this allocation.
    pub fn size(&self) -> usize {
        // INVARIANT: The type invariant of `Self` guarantees that `size_of::<T> * count` fits into
        // a `usize`.
        self.count * core::mem::size_of::<T>()
    }

    /// Returns the base address to the allocated region in the CPU's virtual address space.
    pub fn start_ptr(&self) -> *const T {
        self.cpu_addr
    }

    /// Returns the base address to the allocated region in the CPU's virtual address space as
    /// a mutable pointer.
    pub fn start_ptr_mut(&mut self) -> *mut T {
        self.cpu_addr
    }

    /// Returns a DMA handle which may be given to the device as the DMA address base of
    /// the region.
    pub fn dma_handle(&self) -> bindings::dma_addr_t {
        self.dma_handle
    }

    /// Returns a DMA handle starting at `offset` (in units of `T`) which may be given to the
    /// device as the DMA address base of the region.
    ///
    /// Returns `EINVAL` if `offset` is not within the bounds of the allocation.
    pub fn dma_handle_with_offset(&self, offset: usize) -> Result<bindings::dma_addr_t> {
        if offset >= self.count {
            Err(EINVAL)
        } else {
            // INVARIANT: The type invariant of `Self` guarantees that `size_of::<T> * count` fits
            // into a `usize`, and `offset` is inferior to `count`.
            Ok(self.dma_handle + (offset * core::mem::size_of::<T>()) as bindings::dma_addr_t)
        }
    }

    /// Common helper to validate a range applied from the allocated region in the CPU's virtual
    /// address space.
    fn validate_range(&self, offset: usize, count: usize) -> Result {
        if offset.checked_add(count).ok_or(EOVERFLOW)? > self.count {
            return Err(EINVAL);
        }
        Ok(())
    }

    /// 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
    ///   the returned slice is live.
    pub unsafe fn as_slice(&self, offset: usize, count: usize) -> Result<&[T]> {
        self.validate_range(offset, count)?;
        // 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
        //   data is also guaranteed by the safety requirements of the function.
        // - `offset + count` 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 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 read or write to the same region
    ///   while the returned slice is live.
    pub unsafe fn as_slice_mut(&mut self, offset: usize, count: usize) -> Result<&mut [T]> {
        self.validate_range(offset, count)?;
        // 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
        //   data is also guaranteed by the safety requirements of the function.
        // - `offset + count` 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_mut(self.cpu_addr.add(offset), count) })
    }

    /// Writes data to the region starting from `offset`. `offset` is in units of `T`, not the
    /// number of bytes.
    ///
    /// # 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 read or write to the same region
    ///   that overlaps with this write.
    ///
    /// # Examples
    ///
    /// ```
    /// # fn test(alloc: &mut kernel::dma::CoherentAllocation<u8>) -> Result {
    /// let somedata: [u8; 4] = [0xf; 4];
    /// let buf: &[u8] = &somedata;
    /// // SAFETY: There is no concurrent HW operation on the device and no other R/W access to the
    /// // region.
    /// unsafe { alloc.write(buf, 0)?; }
    /// # Ok::<(), Error>(()) }
    /// ```
    pub unsafe fn write(&mut self, src: &[T], offset: usize) -> Result {
        self.validate_range(offset, src.len())?;
        // SAFETY:
        // - The pointer is valid due to type invariant on `CoherentAllocation`
        //   and we've just checked that the range and index is within bounds.
        // - `offset + count` can't overflow since it is smaller than `self.count` and we've checked
        //   that `self.count` won't overflow early in the constructor.
        unsafe {
            core::ptr::copy_nonoverlapping(src.as_ptr(), self.cpu_addr.add(offset), src.len())
        };
        Ok(())
    }

    /// Returns a pointer to an element from the region with bounds checking. `offset` is in
    /// units of `T`, not the number of bytes.
    ///
    /// Public but hidden since it should only be used from [`dma_read`] and [`dma_write`] macros.
    #[doc(hidden)]
    pub fn item_from_index(&self, offset: usize) -> Result<*mut T> {
        if offset >= self.count {
            return Err(EINVAL);
        }
        // SAFETY:
        // - The pointer is valid due to type invariant on `CoherentAllocation`
        // and we've just checked that the range and index is within bounds.
        // - `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 { self.cpu_addr.add(offset) })
    }

    /// Reads the value of `field` and ensures that its type is [`FromBytes`].
    ///
    /// # Safety
    ///
    /// This must be called from the [`dma_read`] macro which ensures that the `field` pointer is
    /// validated beforehand.
    ///
    /// Public but hidden since it should only be used from [`dma_read`] macro.
    #[doc(hidden)]
    pub unsafe fn field_read<F: FromBytes>(&self, field: *const F) -> F {
        // SAFETY:
        // - By the safety requirements field is valid.
        // - Using read_volatile() here is not sound as per the usual rules, the usage here is
        // a special exception with the following notes in place. When dealing with a potential
        // race from a hardware or code outside kernel (e.g. user-space program), we need that
        // read on a valid memory is not UB. Currently read_volatile() is used for this, and the
        // rationale behind is that it should generate the same code as READ_ONCE() which the
        // kernel already relies on to avoid UB on data races. Note that the usage of
        // read_volatile() is limited to this particular case, it cannot be used to prevent
        // the UB caused by racing between two kernel functions nor do they provide atomicity.
        unsafe { field.read_volatile() }
    }

    /// Writes a value to `field` and ensures that its type is [`AsBytes`].
    ///
    /// # Safety
    ///
    /// This must be called from the [`dma_write`] macro which ensures that the `field` pointer is
    /// validated beforehand.
    ///
    /// Public but hidden since it should only be used from [`dma_write`] macro.
    #[doc(hidden)]
    pub unsafe fn field_write<F: AsBytes>(&self, field: *mut F, val: F) {
        // SAFETY:
        // - By the safety requirements field is valid.
        // - Using write_volatile() here is not sound as per the usual rules, the usage here is
        // a special exception with the following notes in place. When dealing with a potential
        // race from a hardware or code outside kernel (e.g. user-space program), we need that
        // write on a valid memory is not UB. Currently write_volatile() is used for this, and the
        // rationale behind is that it should generate the same code as WRITE_ONCE() which the
        // kernel already relies on to avoid UB on data races. Note that the usage of
        // write_volatile() is limited to this particular case, it cannot be used to prevent
        // the UB caused by racing between two kernel functions nor do they provide atomicity.
        unsafe { field.write_volatile(val) }
    }
}

/// Note that the device configured to do DMA must be halted before this object is dropped.
impl<T: AsBytes + FromBytes> Drop for CoherentAllocation<T> {
    fn drop(&mut self) {
        let size = self.count * core::mem::size_of::<T>();
        // SAFETY: Device pointer is guaranteed as valid by the type invariant on `Device`.
        // The cpu address, and the dma handle are valid due to the type invariants on
        // `CoherentAllocation`.
        unsafe {
            bindings::dma_free_attrs(
                self.dev.as_raw(),
                size,
                self.cpu_addr.cast(),
                self.dma_handle,
                self.dma_attrs.as_raw(),
            )
        }
    }
}

// SAFETY: It is safe to send a `CoherentAllocation` to another thread if `T`
// can be sent to another thread.
unsafe impl<T: AsBytes + FromBytes + Send> Send for CoherentAllocation<T> {}

/// Reads a field of an item from an allocated region of structs.
///
/// # Examples
///
/// ```
/// use kernel::device::Device;
/// use kernel::dma::{attrs::*, CoherentAllocation};
///
/// struct MyStruct { field: u32, }
///
/// // SAFETY: All bit patterns are acceptable values for `MyStruct`.
/// unsafe impl kernel::transmute::FromBytes for MyStruct{};
/// // SAFETY: Instances of `MyStruct` have no uninitialized portions.
/// unsafe impl kernel::transmute::AsBytes for MyStruct{};
///
/// # fn test(alloc: &kernel::dma::CoherentAllocation<MyStruct>) -> Result {
/// let whole = kernel::dma_read!(alloc[2]);
/// let field = kernel::dma_read!(alloc[1].field);
/// # Ok::<(), Error>(()) }
/// ```
#[macro_export]
macro_rules! dma_read {
    ($dma:expr, $idx: expr, $($field:tt)*) => {{
        (|| -> ::core::result::Result<_, $crate::error::Error> {
            let item = $crate::dma::CoherentAllocation::item_from_index(&$dma, $idx)?;
            // SAFETY: `item_from_index` ensures that `item` is always a valid pointer and can be
            // dereferenced. The compiler also further validates the expression on whether `field`
            // is a member of `item` when expanded by the macro.
            unsafe {
                let ptr_field = ::core::ptr::addr_of!((*item) $($field)*);
                ::core::result::Result::Ok(
                    $crate::dma::CoherentAllocation::field_read(&$dma, ptr_field)
                )
            }
        })()
    }};
    ($dma:ident [ $idx:expr ] $($field:tt)* ) => {
        $crate::dma_read!($dma, $idx, $($field)*)
    };
    ($($dma:ident).* [ $idx:expr ] $($field:tt)* ) => {
        $crate::dma_read!($($dma).*, $idx, $($field)*)
    };
}

/// Writes to a field of an item from an allocated region of structs.
///
/// # Examples
///
/// ```
/// use kernel::device::Device;
/// use kernel::dma::{attrs::*, CoherentAllocation};
///
/// struct MyStruct { member: u32, }
///
/// // SAFETY: All bit patterns are acceptable values for `MyStruct`.
/// unsafe impl kernel::transmute::FromBytes for MyStruct{};
/// // SAFETY: Instances of `MyStruct` have no uninitialized portions.
/// unsafe impl kernel::transmute::AsBytes for MyStruct{};
///
/// # fn test(alloc: &kernel::dma::CoherentAllocation<MyStruct>) -> Result {
/// kernel::dma_write!(alloc[2].member = 0xf);
/// kernel::dma_write!(alloc[1] = MyStruct { member: 0xf });
/// # Ok::<(), Error>(()) }
/// ```
#[macro_export]
macro_rules! dma_write {
    ($dma:ident [ $idx:expr ] $($field:tt)*) => {{
        $crate::dma_write!($dma, $idx, $($field)*)
    }};
    ($($dma:ident).* [ $idx:expr ] $($field:tt)* ) => {{
        $crate::dma_write!($($dma).*, $idx, $($field)*)
    }};
    ($dma:expr, $idx: expr, = $val:expr) => {
        (|| -> ::core::result::Result<_, $crate::error::Error> {
            let item = $crate::dma::CoherentAllocation::item_from_index(&$dma, $idx)?;
            // SAFETY: `item_from_index` ensures that `item` is always a valid item.
            unsafe { $crate::dma::CoherentAllocation::field_write(&$dma, item, $val) }
            ::core::result::Result::Ok(())
        })()
    };
    ($dma:expr, $idx: expr, $(.$field:ident)* = $val:expr) => {
        (|| -> ::core::result::Result<_, $crate::error::Error> {
            let item = $crate::dma::CoherentAllocation::item_from_index(&$dma, $idx)?;
            // SAFETY: `item_from_index` ensures that `item` is always a valid pointer and can be
            // dereferenced. The compiler also further validates the expression on whether `field`
            // is a member of `item` when expanded by the macro.
            unsafe {
                let ptr_field = ::core::ptr::addr_of_mut!((*item) $(.$field)*);
                $crate::dma::CoherentAllocation::field_write(&$dma, ptr_field, $val)
            }
            ::core::result::Result::Ok(())
        })()
    };
}