Re: [PATCH v2 03/11] rust: io: use pointer types instead of address

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

Gary Guo <gary@garyguo.net> writes:

> This carries the size information with the pointer type and metadata, makes
> it possible to use I/O projections and paves the way for IO view types.
>
> With this change, minimum size information becomes available through types;
> so `KnownSize::MIN_SIZE` can be used and `IoKnownSize` trait is no longer
> necessary. The trait is kept for compatibility and can be removed when
> users stop using it for bounds.
>
> PCI config space uses only offsets and not pointers like MMIO; for this
> null pointers (with proper size metadata) is used. This is okay as I/O
> trait impl and I/O projections can operate on invalid pointers, and for PCI
> config space we will only use address info and ignore the provenance.
>
> Signed-off-by: Gary Guo <gary@garyguo.net>
> ---
>  rust/kernel/devres.rs |   2 +-
>  rust/kernel/io.rs     | 123 +++++++++++++++++++++-----------------------------
>  rust/kernel/io/mem.rs |   2 +-
>  rust/kernel/pci/io.rs |  74 ++++++++++++++++++------------
>  4 files changed, 99 insertions(+), 102 deletions(-)
>
> diff --git a/rust/kernel/devres.rs b/rust/kernel/devres.rs
> index 3e22c63efb98..ea86e9c62cdf 100644
> --- a/rust/kernel/devres.rs
> +++ b/rust/kernel/devres.rs
> @@ -101,7 +101,7 @@ struct Inner<T> {
>  /// impl<const SIZE: usize> Drop for IoMem<SIZE> {
>  ///     fn drop(&mut self) {
>  ///         // SAFETY: `self.0.addr()` is guaranteed to be properly mapped by `Self::new`.
> -///         unsafe { bindings::iounmap(self.0.addr() as *mut c_void); };
> +///         unsafe { bindings::iounmap(self.0.as_ptr().cast()); };
>  ///     }
>  /// }
>  ///
> diff --git a/rust/kernel/io.rs b/rust/kernel/io.rs
> index 0b9c97c0a1d7..1682f2a0d20d 100644
> --- a/rust/kernel/io.rs
> +++ b/rust/kernel/io.rs
> @@ -105,8 +105,8 @@ pub fn new_region(addr: usize, size: usize) -> Result<Self> {
>  impl<T: ?Sized + KnownSize> MmioRaw<T> {
>      /// Returns the base address of the MMIO region.
>      #[inline]
> -    pub fn addr(&self) -> usize {
> -        self.addr.addr()
> +    pub fn as_ptr(&self) -> *mut T {
> +        self.addr
>      }
>  
>      /// Returns the size of the MMIO region.
> @@ -166,7 +166,7 @@ pub fn size(&self) -> usize {
>  /// impl<const SIZE: usize> Drop for IoMem<SIZE> {
>  ///     fn drop(&mut self) {
>  ///         // SAFETY: `self.0.addr()` is guaranteed to be properly mapped by `Self::new`.
> -///         unsafe { bindings::iounmap(self.0.addr() as *mut c_void); };
> +///         unsafe { bindings::iounmap(self.0.as_ptr().cast()); };
>  ///     }
>  /// }
>  ///
> @@ -217,14 +217,14 @@ pub trait IoCapable<T> {
>      /// # Safety
>      ///
>      /// The range `[address..address + size_of::<T>()]` must be within the bounds of `Self`.
> -    unsafe fn io_read(&self, address: usize) -> T;
> +    unsafe fn io_read(&self, address: *mut T) -> T;
>  
>      /// Performs an I/O write of `value` at `address`.
>      ///
>      /// # Safety
>      ///
>      /// The range `[address..address + size_of::<T>()]` must be within the bounds of `Self`.
> -    unsafe fn io_write(&self, value: T, address: usize);
> +    unsafe fn io_write(&self, value: T, address: *mut T);
>  }
>  
>  /// Describes a given I/O location: its offset, width, and type to convert the raw value from and
> @@ -291,23 +291,35 @@ fn offset(self) -> usize {
>  /// 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.
>  pub trait Io {
> -    /// Returns the base address of this mapping.
> -    fn addr(&self) -> usize;
> +    /// Type of this I/O region. For untyped I/O regions, [`Region`] type can be used.
> +    type Type: ?Sized + KnownSize;
> +
> +    /// Returns the base pointer of this mapping.
> +    ///
> +    /// This is a pointer to capture metadata. The specific meaning of the pointer depends on
> +    /// I/O backend and is not necessarily valid.
> +    fn as_ptr(&self) -> *mut Self::Type;
> +
> +    /// Returns the absolute I/O address for a given `offset`,
> +    /// performing compile-time bound checks.
> +    // Always inline to optimize out error path of `build_assert`.
> +    #[inline(always)]
> +    fn io_addr_assert<U>(&self, offset: usize) -> *mut U {
> +        build_assert!(offset_valid::<U>(offset, Self::Type::MIN_SIZE));

Consider renaming this function `io_addr_build_assert` for clarity.

At any rate:

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

Best regards,
Andreas Hindborg