path: root/rust/kernel/io.rs

// SPDX-License-Identifier: GPL-2.0

//! Memory-mapped IO.
//!
//! C header: [`include/asm-generic/io.h`](srctree/include/asm-generic/io.h)

use core::{
    marker::PhantomData, //
};

use crate::{
    bindings,
    prelude::*,
    ptr::{
        Alignment,
        KnownSize, //
    }, //
};

pub mod mem;
pub mod poll;
pub mod register;
pub mod resource;

pub use crate::register;
pub use resource::Resource;

use register::LocatedRegister;

/// Physical address type.
///
/// This is a type alias to either `u32` or `u64` depending on the config option
/// `CONFIG_PHYS_ADDR_T_64BIT`, and it can be a u64 even on 32-bit architectures.
pub type PhysAddr = bindings::phys_addr_t;

/// Resource Size type.
///
/// This is a type alias to either `u32` or `u64` depending on the config option
/// `CONFIG_PHYS_ADDR_T_64BIT`, and it can be a u64 even on 32-bit architectures.
pub type ResourceSize = bindings::resource_size_t;

/// Untyped I/O region.
///
/// This type can be used when an I/O region without known type information has a compile-time known
/// minimum size (and a runtime known actual size).
///
/// # Invariants
///
/// - Size of the region is at least as large as the `SIZE` generic parameter.
/// - Size of the region is multiple of 4.
#[repr(C, align(4))]
pub struct Region<const SIZE: usize = 0> {
    inner: [u8],
}

impl<const SIZE: usize> Region<SIZE> {
    /// Create a raw mutable pointer from given base address and size.
    ///
    /// `size` should be at least as large as the minimum size `SIZE`, and `base` and `size` should
    /// be 4-byte aligned to uphold the type invariant.
    ///
    /// Just like other methods on raw pointers, it is not unsafe to create a raw pointer
    /// that does not uphold the type invariants. However such pointers are not valid.
    #[inline]
    pub fn ptr_from_raw_parts_mut(base: *mut u8, size: usize) -> *mut Self {
        core::ptr::slice_from_raw_parts_mut(base, size) as *mut Region<SIZE>
    }

    /// Create a raw mutable pointer from given base address and size.
    ///
    /// The alignment of `base` is checked, and `size` is checked against the minimum size specified
    /// via const generics.
    #[inline]
    pub fn ptr_try_from_raw_parts_mut(base: *mut u8, size: usize) -> Result<*mut Self> {
        if size < SIZE || base.align_offset(4) != 0 || !size.is_multiple_of(4) {
            return Err(EINVAL);
        }

        Ok(Self::ptr_from_raw_parts_mut(base, size))
    }
}

impl<const SIZE: usize> KnownSize for Region<SIZE> {
    const MIN_SIZE: usize = SIZE;
    // Alignment of 4 is the most common; different base types can be added once required.
    const MIN_ALIGN: Alignment = Alignment::new::<4>();

    #[inline(always)]
    fn size(p: *const Self) -> usize {
        (p as *const [u8]).len()
    }
}

/// Raw representation of an MMIO region.
///
/// `MmioRaw<T>` is equivalent to `T __iomem *` in C.
///
/// By itself, the existence of an instance of this structure does not provide any guarantees that
/// the represented MMIO region does exist or is properly mapped.
///
/// Instead, the bus specific MMIO implementation must convert this raw representation into an
/// `Mmio` instance providing the actual memory accessors. Only by the conversion into an `Mmio`
/// structure any guarantees are given.
pub struct MmioRaw<T: ?Sized> {
    /// Pointer is in I/O address space.
    ///
    /// The provenance does not matter, only the address and metadata do.
    ptr: *mut T,
}

impl<T: ?Sized> Copy for MmioRaw<T> {}
impl<T: ?Sized> Clone for MmioRaw<T> {
    #[inline]
    fn clone(&self) -> Self {
        *self
    }
}

// SAFETY: `MmioRaw` is just an address, so is thread-safe.
unsafe impl<T: ?Sized> Send for MmioRaw<T> {}
// SAFETY: `MmioRaw` is just an address, so is thread-safe.
unsafe impl<T: ?Sized> Sync for MmioRaw<T> {}

impl<T> MmioRaw<T> {
    /// Create a `MmioRaw` from address.
    #[inline]
    pub fn new(addr: usize) -> Self {
        Self {
            ptr: core::ptr::without_provenance_mut(addr),
        }
    }
}

impl<const SIZE: usize> MmioRaw<Region<SIZE>> {
    /// Create a `MmioRaw` representing a I/O region with given size.
    ///
    /// The size is checked against the minimum size specified via const generics.
    #[inline]
    pub fn new_region(addr: usize, size: usize) -> Result<Self> {
        Ok(Self {
            ptr: Region::ptr_try_from_raw_parts_mut(core::ptr::without_provenance_mut(addr), size)?,
        })
    }
}

impl<T: ?Sized + KnownSize> MmioRaw<T> {
    /// Returns the base address of the MMIO region.
    #[inline]
    pub fn addr(&self) -> usize {
        self.ptr.addr()
    }

    /// Returns the size of the MMIO region.
    #[inline]
    pub fn size(&self) -> usize {
        KnownSize::size(self.ptr)
    }
}

/// Checks whether an access of type `U` at the given `base` and the given `offset`
/// is valid within this region.
///
/// The `base` is used for alignment checking only. This can be set to 0 to skip the check.
#[inline]
const fn offset_valid<U>(base: usize, offset: usize, size: usize) -> bool {
    if let Some(end) = offset.checked_add(size_of::<U>()) {
        end <= size && (base.wrapping_add(offset) % align_of::<U>() == 0)
    } else {
        false
    }
}

/// Returns a view for a given `offset`, performing compile-time bound checks.
// Always inline to optimize out error path of `build_assert`.
#[inline(always)]
fn io_view_assert<'a, IO: Io<'a>, U>(
    this: IO,
    offset: usize,
) -> <IO::Backend as IoBackend>::View<'a, U> {
    // We cannot check alignment with `offset_valid` using `ptr.addr()`. So set 0 for it and
    // ensure alignment by checking that the alignment of `U` is smaller or equal to the
    // alignment of `IO::Target`.
    const_assert!(Alignment::of::<U>().as_usize() <= IO::Target::MIN_ALIGN.as_usize());
    build_assert!(offset_valid::<U>(0, offset, IO::Target::MIN_SIZE));

    let view = this.as_view();
    let ptr = IO::Backend::as_ptr(view);
    let projected_ptr = ptr.cast::<U>().wrapping_byte_add(offset);
    // SAFETY: `offset_valid` checks for size and alignment and therefore `projected_ptr` is a
    // valid projection.
    unsafe { IO::Backend::project_view(view, projected_ptr) }
}

/// Returns a view for a given `offset`, performing runtime bound checks.
#[inline]
fn io_view<'a, IO: Io<'a>, U>(
    this: IO,
    offset: usize,
) -> Result<<IO::Backend as IoBackend>::View<'a, U>> {
    let view = this.as_view();
    let ptr = IO::Backend::as_ptr(view);

    if !offset_valid::<U>(ptr.addr(), offset, KnownSize::size(ptr)) {
        return Err(EINVAL);
    }

    let projected_ptr = ptr.cast::<U>().wrapping_byte_add(offset);
    // SAFETY: `offset_valid` checks for size and alignment and therefore `projected_ptr` is a
    // valid projection.
    Ok(unsafe { IO::Backend::project_view(view, projected_ptr) })
}

/// I/O backends.
///
/// This is an abstract representation to be implemented by arbitrary I/O
/// backends (e.g. MMIO, PCI config space, etc.).
///
/// The base trait only defines the projection operations; which I/O methods are available depends
/// on which [`IoCapable<T>`] traits are implemented for the type. For example, for MMIO regions,
/// all widths (u8, u16, u32, and u64 on 64-bit systems) are typically supported. For PCI
/// configuration space, u8, u16, and u32 are supported but u64 is not.
///
/// This trait is separate from the `Io` trait as multiple different I/O types may share the same
/// operation.
pub trait IoBackend {
    /// View type for this I/O backend.
    type View<'a, T: ?Sized + KnownSize>: Io<'a, Backend = Self, Target = T>;

    /// Convert a `view` to a raw pointer for projection.
    ///
    /// The returned pointer is private implementation detail of the backend; it is likely not
    /// valid. It should not be dereferenced.
    fn as_ptr<'a, T: ?Sized + KnownSize>(view: Self::View<'a, T>) -> *mut T;

    /// Project `view` to its subregion indicated by `ptr`.
    ///
    /// If input `view` is valid, returned view must also be valid.
    ///
    /// # Safety
    ///
    /// `ptr` must be a projection of `Self::as_ptr(view)`.
    unsafe fn project_view<'a, T: ?Sized + KnownSize, U: ?Sized + KnownSize>(
        view: Self::View<'a, T>,
        ptr: *mut U,
    ) -> Self::View<'a, U>;
}

/// Trait indicating that an I/O backend supports operations of a certain type and providing an
/// implementation for these operations.
///
/// Different I/O backends can implement this trait to expose only the operations they support.
///
/// For example, a PCI configuration space may implement `IoCapable<u8>`, `IoCapable<u16>`,
/// and `IoCapable<u32>`, but not `IoCapable<u64>`, while an MMIO region on a 64-bit
/// system might implement all four.
pub trait IoCapable<T>: IoBackend {
    /// Performs an I/O read of type `T` at `view` and returns the result.
    fn io_read<'a>(view: Self::View<'a, T>) -> T;

    /// Performs an I/O write of `value` at `view`.
    fn io_write<'a>(view: Self::View<'a, T>, value: T);
}

/// Describes a given I/O location: its offset, width, and type to convert the raw value from and
/// into.
///
/// This trait is the key abstraction allowing [`Io::read`], [`Io::write`], and [`Io::update`] (and
/// their fallible [`try_read`](Io::try_read), [`try_write`](Io::try_write) and
/// [`try_update`](Io::try_update) counterparts) to work uniformly with both raw [`usize`] offsets
/// (for primitive types like [`u32`]) and typed ones (like those generated by the [`register!`]
/// macro).
///
/// An `IoLoc<Base, T>` carries the following pieces of information:
///
/// - The valid `Base` to operate on. For most registers, this should be [`Region`].
/// - The offset to access (returned by [`IoLoc::offset`]),
/// - The width of the access (determined by [`IoLoc::IoType`]),
/// - The type `T` in which the raw data is returned or provided.
///
/// `T` and `IoLoc::IoType` may differ: for instance, a typed register has `T` = the register type
/// with its bitfields, and `IoType` = its backing primitive (e.g. `u32`).
pub trait IoLoc<Base: ?Sized, T> {
    /// Size ([`u8`], [`u16`], etc) of the I/O performed on the returned [`offset`](IoLoc::offset).
    type IoType: Into<T> + From<T>;

    /// Consumes `self` and returns the offset of this location.
    fn offset(self) -> usize;
}

/// Implements [`IoLoc<Region<SIZE>, $ty>`] for [`usize`], allowing [`usize`] to be used as a
/// parameter of [`Io::read`] and [`Io::write`].
macro_rules! impl_usize_ioloc {
    ($($ty:ty),*) => {
        $(
            impl<const SIZE: usize> IoLoc<Region<SIZE>, $ty> for usize {
                type IoType = $ty;

                #[inline(always)]
                fn offset(self) -> usize {
                    self
                }
            }
        )*
    }
}

// Provide the ability to read any primitive type from a [`usize`].
impl_usize_ioloc!(u8, u16, u32, u64);

/// Types implementing this trait (e.g. MMIO BARs or PCI config regions)
/// can perform I/O operations on regions of memory.
///
/// The [`Io`] trait provides:
/// - Method to convert into [`IoBackend::View`].
/// - Helper methods for offset validation and address calculation
/// - Fallible (runtime checked) accessors for different data widths
///
/// Which I/O methods are available depends on the associated [`IoBackend`] implementation.
///
/// This should be implemented on cheaply copyable handles, such as references or view types.
pub trait Io<'a>: Copy {
    /// Type that defines all I/O operations.
    type Backend: IoBackend;

    /// Type of this I/O region. For untyped regions, [`Region`] can be used.
    type Target: ?Sized + KnownSize;

    /// Return a view that covers the full region.
    fn as_view(self) -> <Self::Backend as IoBackend>::View<'a, Self::Target>;

    /// Fallible 8-bit read with runtime bounds check.
    #[inline(always)]
    fn try_read8(self, offset: usize) -> Result<u8>
    where
        usize: IoLoc<Self::Target, u8, IoType = u8>,
        Self::Backend: IoCapable<u8>,
    {
        self.try_read(offset)
    }

    /// Fallible 16-bit read with runtime bounds check.
    #[inline(always)]
    fn try_read16(self, offset: usize) -> Result<u16>
    where
        usize: IoLoc<Self::Target, u16, IoType = u16>,
        Self::Backend: IoCapable<u16>,
    {
        self.try_read(offset)
    }

    /// Fallible 32-bit read with runtime bounds check.
    #[inline(always)]
    fn try_read32(self, offset: usize) -> Result<u32>
    where
        usize: IoLoc<Self::Target, u32, IoType = u32>,
        Self::Backend: IoCapable<u32>,
    {
        self.try_read(offset)
    }

    /// Fallible 64-bit read with runtime bounds check.
    #[inline(always)]
    fn try_read64(self, offset: usize) -> Result<u64>
    where
        usize: IoLoc<Self::Target, u64, IoType = u64>,
        Self::Backend: IoCapable<u64>,
    {
        self.try_read(offset)
    }

    /// Fallible 8-bit write with runtime bounds check.
    #[inline(always)]
    fn try_write8(self, value: u8, offset: usize) -> Result
    where
        usize: IoLoc<Self::Target, u8, IoType = u8>,
        Self::Backend: IoCapable<u8>,
    {
        self.try_write(offset, value)
    }

    /// Fallible 16-bit write with runtime bounds check.
    #[inline(always)]
    fn try_write16(self, value: u16, offset: usize) -> Result
    where
        usize: IoLoc<Self::Target, u16, IoType = u16>,
        Self::Backend: IoCapable<u16>,
    {
        self.try_write(offset, value)
    }

    /// Fallible 32-bit write with runtime bounds check.
    #[inline(always)]
    fn try_write32(self, value: u32, offset: usize) -> Result
    where
        usize: IoLoc<Self::Target, u32, IoType = u32>,
        Self::Backend: IoCapable<u32>,
    {
        self.try_write(offset, value)
    }

    /// Fallible 64-bit write with runtime bounds check.
    #[inline(always)]
    fn try_write64(self, value: u64, offset: usize) -> Result
    where
        usize: IoLoc<Self::Target, u64, IoType = u64>,
        Self::Backend: IoCapable<u64>,
    {
        self.try_write(offset, value)
    }

    /// Infallible 8-bit read with compile-time bounds check.
    #[inline(always)]
    fn read8(self, offset: usize) -> u8
    where
        usize: IoLoc<Self::Target, u8, IoType = u8>,
        Self::Backend: IoCapable<u8>,
    {
        self.read(offset)
    }

    /// Infallible 16-bit read with compile-time bounds check.
    #[inline(always)]
    fn read16(self, offset: usize) -> u16
    where
        usize: IoLoc<Self::Target, u16, IoType = u16>,
        Self::Backend: IoCapable<u16>,
    {
        self.read(offset)
    }

    /// Infallible 32-bit read with compile-time bounds check.
    #[inline(always)]
    fn read32(self, offset: usize) -> u32
    where
        usize: IoLoc<Self::Target, u32, IoType = u32>,
        Self::Backend: IoCapable<u32>,
    {
        self.read(offset)
    }

    /// Infallible 64-bit read with compile-time bounds check.
    #[inline(always)]
    fn read64(self, offset: usize) -> u64
    where
        usize: IoLoc<Self::Target, u64, IoType = u64>,
        Self::Backend: IoCapable<u64>,
    {
        self.read(offset)
    }

    /// Infallible 8-bit write with compile-time bounds check.
    #[inline(always)]
    fn write8(self, value: u8, offset: usize)
    where
        usize: IoLoc<Self::Target, u8, IoType = u8>,
        Self::Backend: IoCapable<u8>,
    {
        self.write(offset, value)
    }

    /// Infallible 16-bit write with compile-time bounds check.
    #[inline(always)]
    fn write16(self, value: u16, offset: usize)
    where
        usize: IoLoc<Self::Target, u16, IoType = u16>,
        Self::Backend: IoCapable<u16>,
    {
        self.write(offset, value)
    }

    /// Infallible 32-bit write with compile-time bounds check.
    #[inline(always)]
    fn write32(self, value: u32, offset: usize)
    where
        usize: IoLoc<Self::Target, u32, IoType = u32>,
        Self::Backend: IoCapable<u32>,
    {
        self.write(offset, value)
    }

    /// Infallible 64-bit write with compile-time bounds check.
    #[inline(always)]
    fn write64(self, value: u64, offset: usize)
    where
        usize: IoLoc<Self::Target, u64, IoType = u64>,
        Self::Backend: IoCapable<u64>,
    {
        self.write(offset, value)
    }

    /// Generic fallible read with runtime bounds check.
    ///
    /// # Examples
    ///
    /// Read a primitive type from an I/O address:
    ///
    /// ```no_run
    /// use kernel::io::{
    ///     Io,
    ///     Mmio,
    ///     Region,
    /// };
    ///
    /// fn do_reads(io: Mmio<'_, Region>) -> Result {
    ///     // 32-bit read from address `0x10`.
    ///     let v: u32 = io.try_read(0x10)?;
    ///
    ///     // 8-bit read from address `0xfff`.
    ///     let v: u8 = io.try_read(0xfff)?;
    ///
    ///     Ok(())
    /// }
    /// ```
    #[inline(always)]
    fn try_read<T, L>(self, location: L) -> Result<T>
    where
        L: IoLoc<Self::Target, T>,
        Self::Backend: IoCapable<L::IoType>,
    {
        let view = io_view::<Self, L::IoType>(self, location.offset())?;
        Ok(Self::Backend::io_read(view).into())
    }

    /// Generic fallible write with runtime bounds check.
    ///
    /// # Examples
    ///
    /// Write a primitive type to an I/O address:
    ///
    /// ```no_run
    /// use kernel::io::{
    ///     Io,
    ///     Mmio,
    ///     Region,
    /// };
    ///
    /// fn do_writes(io: Mmio<'_, Region>) -> Result {
    ///     // 32-bit write of value `1` at address `0x10`.
    ///     io.try_write(0x10, 1u32)?;
    ///
    ///     // 8-bit write of value `0xff` at address `0xfff`.
    ///     io.try_write(0xfff, 0xffu8)?;
    ///
    ///     Ok(())
    /// }
    /// ```
    #[inline(always)]
    fn try_write<T, L>(self, location: L, value: T) -> Result
    where
        L: IoLoc<Self::Target, T>,
        Self::Backend: IoCapable<L::IoType>,
    {
        let view = io_view::<Self, L::IoType>(self, location.offset())?;
        let io_value = value.into();
        Self::Backend::io_write(view, io_value);
        Ok(())
    }

    /// Generic fallible write of a fully-located register value.
    ///
    /// # Examples
    ///
    /// Tuples carrying a location and a value can be used with this method:
    ///
    /// ```no_run
    /// use kernel::io::{
    ///     register,
    ///     Io,
    ///     Mmio,
    ///     Region,
    /// };
    ///
    /// register! {
    ///     VERSION(u32) @ 0x100 {
    ///         15:8 major;
    ///         7:0  minor;
    ///     }
    /// }
    ///
    /// impl VERSION {
    ///     fn new(major: u8, minor: u8) -> Self {
    ///         VERSION::zeroed().with_major(major).with_minor(minor)
    ///     }
    /// }
    ///
    /// fn do_write_reg(io: Mmio<'_, Region>) -> Result {
    ///
    ///     io.try_write_reg(VERSION::new(1, 0))
    /// }
    /// ```
    #[inline(always)]
    fn try_write_reg<T, L, V>(self, value: V) -> Result
    where
        L: IoLoc<Self::Target, T>,
        V: LocatedRegister<Self::Target, Location = L, Value = T>,
        Self::Backend: IoCapable<L::IoType>,
    {
        let (location, value) = value.into_io_op();

        self.try_write(location, value)
    }

    /// Generic fallible update with runtime bounds check.
    ///
    /// Note: this does not perform any synchronization. The caller is responsible for ensuring
    /// exclusive access if required.
    ///
    /// # Examples
    ///
    /// Read the u32 value at address `0x10`, increment it, and store the updated value back:
    ///
    /// ```no_run
    /// use kernel::io::{
    ///     Io,
    ///     Mmio,
    ///     Region,
    /// };
    ///
    /// fn do_update(io: Mmio<'_, Region<0x1000>>) -> Result {
    ///     io.try_update(0x10, |v: u32| {
    ///         v + 1
    ///     })
    /// }
    /// ```
    #[inline(always)]
    fn try_update<T, L, F>(self, location: L, f: F) -> Result
    where
        L: IoLoc<Self::Target, T>,
        Self::Backend: IoCapable<L::IoType>,
        F: FnOnce(T) -> T,
    {
        let view = io_view::<Self, L::IoType>(self, location.offset())?;

        let value: T = Self::Backend::io_read(view).into();
        let io_value = f(value).into();
        Self::Backend::io_write(view, io_value);

        Ok(())
    }

    /// Generic infallible read with compile-time bounds check.
    ///
    /// # Examples
    ///
    /// Read a primitive type from an I/O address:
    ///
    /// ```no_run
    /// use kernel::io::{
    ///     Io,
    ///     Mmio,
    ///     Region,
    /// };
    ///
    /// fn do_reads(io: Mmio<'_, Region<0x1000>>) {
    ///     // 32-bit read from address `0x10`.
    ///     let v: u32 = io.read(0x10);
    ///
    ///     // 8-bit read from the top of the I/O space.
    ///     let v: u8 = io.read(0xfff);
    /// }
    /// ```
    #[inline(always)]
    fn read<T, L>(self, location: L) -> T
    where
        L: IoLoc<Self::Target, T>,
        Self::Backend: IoCapable<L::IoType>,
    {
        let view = io_view_assert::<Self, L::IoType>(self, location.offset());
        Self::Backend::io_read(view).into()
    }

    /// Generic infallible write with compile-time bounds check.
    ///
    /// # Examples
    ///
    /// Write a primitive type to an I/O address:
    ///
    /// ```no_run
    /// use kernel::io::{
    ///     Io,
    ///     Mmio,
    ///     Region,
    /// };
    ///
    /// fn do_writes(io: Mmio<'_, Region<0x1000>>) {
    ///     // 32-bit write of value `1` at address `0x10`.
    ///     io.write(0x10, 1u32);
    ///
    ///     // 8-bit write of value `0xff` at the top of the I/O space.
    ///     io.write(0xfff, 0xffu8);
    /// }
    /// ```
    #[inline(always)]
    fn write<T, L>(self, location: L, value: T)
    where
        L: IoLoc<Self::Target, T>,
        Self::Backend: IoCapable<L::IoType>,
    {
        let view = io_view_assert::<Self, L::IoType>(self, location.offset());
        let io_value = value.into();
        Self::Backend::io_write(view, io_value);
    }

    /// Generic infallible write of a fully-located register value.
    ///
    /// # Examples
    ///
    /// Tuples carrying a location and a value can be used with this method:
    ///
    /// ```no_run
    /// use kernel::io::{
    ///     register,
    ///     Io,
    ///     Mmio,
    ///     Region,
    /// };
    ///
    /// register! {
    ///     VERSION(u32) @ 0x100 {
    ///         15:8 major;
    ///         7:0  minor;
    ///     }
    /// }
    ///
    /// impl VERSION {
    ///     fn new(major: u8, minor: u8) -> Self {
    ///         VERSION::zeroed().with_major(major).with_minor(minor)
    ///     }
    /// }
    ///
    /// fn do_write_reg(io: Mmio<'_, Region<0x1000>>) {
    ///     io.write_reg(VERSION::new(1, 0));
    /// }
    /// ```
    #[inline(always)]
    fn write_reg<T, L, V>(self, value: V)
    where
        L: IoLoc<Self::Target, T>,
        V: LocatedRegister<Self::Target, Location = L, Value = T>,
        Self::Backend: IoCapable<L::IoType>,
    {
        let (location, value) = value.into_io_op();

        self.write(location, value)
    }

    /// Generic infallible update with compile-time bounds check.
    ///
    /// Note: this does not perform any synchronization. The caller is responsible for ensuring
    /// exclusive access if required.
    ///
    /// # Examples
    ///
    /// Read the u32 value at address `0x10`, increment it, and store the updated value back:
    ///
    /// ```no_run
    /// use kernel::io::{
    ///     Io,
    ///     Mmio,
    ///     Region,
    /// };
    ///
    /// fn do_update(io: Mmio<'_, Region<0x1000>>) {
    ///     io.update(0x10, |v: u32| {
    ///         v + 1
    ///     })
    /// }
    /// ```
    #[inline(always)]
    fn update<T, L, F>(self, location: L, f: F)
    where
        L: IoLoc<Self::Target, T>,
        Self::Backend: IoCapable<L::IoType>,
        F: FnOnce(T) -> T,
    {
        let view = io_view_assert::<Self, L::IoType>(self, location.offset());
        let value: T = Self::Backend::io_read(view).into();
        let io_value = f(value).into();
        Self::Backend::io_write(view, io_value);
    }
}

/// A view of memory-mapped I/O region.
///
/// # Invariant
///
/// `ptr` points to a valid and aligned memory-mapped I/O region for the duration lifetime `'a`.
pub struct Mmio<'a, T: ?Sized> {
    ptr: *mut T,
    phantom: PhantomData<&'a ()>,
}

impl<T: ?Sized> Copy for Mmio<'_, T> {}
impl<T: ?Sized> Clone for Mmio<'_, T> {
    #[inline]
    fn clone(&self) -> Self {
        *self
    }
}

impl<'a, T: ?Sized> Mmio<'a, T> {
    /// Create a `Mmio`, providing the accessors to the MMIO mapping.
    ///
    /// # Safety
    ///
    /// `raw` represents a valid and aligned memory-mapped I/O region while `'a` is alive.
    #[inline]
    pub unsafe fn from_raw(raw: MmioRaw<T>) -> Self {
        // INVARIANT: Per safety requirement.
        Self {
            ptr: raw.ptr,
            phantom: PhantomData,
        }
    }
}

// SAFETY: `Mmio<'_, T>` is conceptually `&T` but in I/O memory.
unsafe impl<T: ?Sized + Sync> Send for Mmio<'_, T> {}

// SAFETY: `Mmio<'_, T>` is conceptually `&T` but in I/O memory.
unsafe impl<T: ?Sized + Sync> Sync for Mmio<'_, T> {}

impl<'a, T: ?Sized + KnownSize> Io<'a> for Mmio<'a, T> {
    type Backend = MmioBackend;
    type Target = T;

    #[inline]
    fn as_view(self) -> Mmio<'a, T> {
        self
    }
}

/// I/O Backend for memory-mapped I/O.
pub struct MmioBackend;

impl IoBackend for MmioBackend {
    type View<'a, T: ?Sized + KnownSize> = Mmio<'a, T>;

    #[inline]
    fn as_ptr<'a, T: ?Sized + KnownSize>(view: Self::View<'a, T>) -> *mut T {
        view.ptr
    }

    #[inline]
    unsafe fn project_view<'a, T: ?Sized + KnownSize, U: ?Sized + KnownSize>(
        _view: Self::View<'a, T>,
        ptr: *mut U,
    ) -> Self::View<'a, U> {
        // INVARIANT: Per safety requirement, `ptr` is projection from `view`, so it is also a valid
        // memory-mapped I/O region.
        Mmio {
            ptr,
            phantom: PhantomData,
        }
    }
}

/// Implements [`IoCapable`] on `$backend` for `$ty` using `$read_fn` and `$write_fn`.
macro_rules! impl_mmio_io_capable {
    ($backend: ident, $ty:ty, $read_fn:ident, $write_fn:ident) => {
        impl IoCapable<$ty> for $backend {
            #[inline]
            fn io_read(view: <$backend as IoBackend>::View<'_, $ty>) -> $ty {
                // SAFETY: `$backend::as_ptr(view)` is a valid pointer for MMIO operations for both
                // `MmioBackend` and `RelaxedMmioBackend`.
                unsafe { bindings::$read_fn($backend::as_ptr(view).cast_const().cast()) }
            }

            #[inline]
            fn io_write(view: <$backend as IoBackend>::View<'_, $ty>, value: $ty) {
                // SAFETY: `$backend::as_ptr(view)` is a valid pointer for MMIO operations for both
                // `MmioBackend` and `RelaxedMmioBackend`.
                unsafe { bindings::$write_fn(value, $backend::as_ptr(view).cast()) }
            }
        }
    };
}

// MMIO regions support 8, 16, and 32-bit accesses.
impl_mmio_io_capable!(MmioBackend, u8, readb, writeb);
impl_mmio_io_capable!(MmioBackend, u16, readw, writew);
impl_mmio_io_capable!(MmioBackend, u32, readl, writel);
// MMIO regions on 64-bit systems also support 64-bit accesses.
#[cfg(CONFIG_64BIT)]
impl_mmio_io_capable!(MmioBackend, u64, readq, writeq);

/// [`Mmio`] but using relaxed accessors.
///
/// This type provides an implementation of [`Io`] that uses relaxed I/O MMIO operands instead of
/// the regular ones.
///
/// See [`Mmio::relaxed`] for a usage example.
pub struct RelaxedMmio<'a, T: ?Sized>(Mmio<'a, T>);

impl<T: ?Sized> Copy for RelaxedMmio<'_, T> {}
impl<T: ?Sized> Clone for RelaxedMmio<'_, T> {
    #[inline]
    fn clone(&self) -> Self {
        *self
    }
}

/// I/O Backend for memory-mapped I/O, with relaxed access semantics.
pub struct RelaxedMmioBackend;

impl IoBackend for RelaxedMmioBackend {
    type View<'a, T: ?Sized + KnownSize> = RelaxedMmio<'a, T>;

    #[inline]
    fn as_ptr<'a, T: ?Sized + KnownSize>(view: Self::View<'a, T>) -> *mut T {
        MmioBackend::as_ptr(view.0)
    }

    #[inline]
    unsafe fn project_view<'a, T: ?Sized + KnownSize, U: ?Sized + KnownSize>(
        view: Self::View<'a, T>,
        ptr: *mut U,
    ) -> Self::View<'a, U> {
        // SAFETY: Per safety requirement.
        RelaxedMmio(unsafe { MmioBackend::project_view(view.0, ptr) })
    }
}

impl<'a, T: ?Sized + KnownSize> Io<'a> for RelaxedMmio<'a, T> {
    type Backend = RelaxedMmioBackend;
    type Target = T;

    #[inline]
    fn as_view(self) -> RelaxedMmio<'a, T> {
        self
    }
}

impl<'a, T: ?Sized> Mmio<'a, T> {
    /// Returns a [`RelaxedMmio`] that performs relaxed I/O operations.
    ///
    /// Relaxed accessors do not provide ordering guarantees with respect to DMA or memory accesses
    /// and can be used when such ordering is not required.
    ///
    /// # Examples
    ///
    /// ```no_run
    /// use kernel::io::{
    ///     Io,
    ///     Mmio,
    ///     Region,
    ///     RelaxedMmio,
    /// };
    ///
    /// fn do_io(io: Mmio<'_, Region<0x100>>) {
    ///     // The access is performed using `readl_relaxed` instead of `readl`.
    ///     let v = io.relaxed().read32(0x10);
    /// }
    ///
    /// ```
    #[inline]
    pub fn relaxed(self) -> RelaxedMmio<'a, T> {
        RelaxedMmio(self)
    }
}

// MMIO regions support 8, 16, and 32-bit accesses.
impl_mmio_io_capable!(RelaxedMmioBackend, u8, readb_relaxed, writeb_relaxed);
impl_mmio_io_capable!(RelaxedMmioBackend, u16, readw_relaxed, writew_relaxed);
impl_mmio_io_capable!(RelaxedMmioBackend, u32, readl_relaxed, writel_relaxed);
// MMIO regions on 64-bit systems also support 64-bit accesses.
#[cfg(CONFIG_64BIT)]
impl_mmio_io_capable!(RelaxedMmioBackend, u64, readq_relaxed, writeq_relaxed);