Re: [PATCH v4 2/3] rust: io: mem: add a generic iomem abstraction

[Charalampos Mitrodimas <charmitro@posteo.net>]

Daniel Almeida <daniel.almeida@collabora.com> writes:

> Add a generic iomem abstraction to safely read and write ioremapped
> regions.
>
> The reads and writes are done through IoRaw, and are thus checked either
> at compile-time, if the size of the region is known at that point, or at
> runtime otherwise.
>
> Non-exclusive access to the underlying memory region is made possible to
> cater to cases where overlapped regions are unavoidable.
>
> Signed-off-by: Daniel Almeida <daniel.almeida@collabora.com>
> ---
>  rust/helpers/io.c     |  10 ++++
>  rust/kernel/io.rs     |   1 +
>  rust/kernel/io/mem.rs | 108 ++++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 119 insertions(+)
>  create mode 100644 rust/kernel/io/mem.rs
>
> diff --git a/rust/helpers/io.c b/rust/helpers/io.c
> index 3cb47bd01942..cb10060c08ae 100644
> --- a/rust/helpers/io.c
> +++ b/rust/helpers/io.c
> @@ -106,3 +106,13 @@ resource_size_t rust_helper_resource_size(struct resource *res)
>  	return resource_size(res);
>  }
>
> +struct resource *rust_helper_request_mem_region(resource_size_t start, resource_size_t n,
> +				    const char *name)
> +{
> +	return request_mem_region(start, n, name);
> +}
> +
> +void rust_helper_release_mem_region(resource_size_t start, resource_size_t n)
> +{
> +	release_mem_region(start, n);
> +}
> diff --git a/rust/kernel/io.rs b/rust/kernel/io.rs
> index 566d8b177e01..9ce3482b5ecd 100644
> --- a/rust/kernel/io.rs
> +++ b/rust/kernel/io.rs
> @@ -7,6 +7,7 @@
>  use crate::error::{code::EINVAL, Result};
>  use crate::{bindings, build_assert};
>
> +pub mod mem;
>  pub mod resource;
>
>  /// Raw representation of an MMIO region.
> diff --git a/rust/kernel/io/mem.rs b/rust/kernel/io/mem.rs
> new file mode 100644
> index 000000000000..f2147db715bf
> --- /dev/null
> +++ b/rust/kernel/io/mem.rs
> @@ -0,0 +1,108 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +//! Generic memory-mapped IO.
> +
> +use core::ops::Deref;
> +
> +use crate::io::resource::Resource;
> +use crate::io::Io;
> +use crate::io::IoRaw;
> +use crate::prelude::*;
> +
> +/// A generic memory-mapped IO region.
> +///
> +/// Accesses to the underlying region is checked either at compile time, if the
> +/// region's size is known at that point, or at runtime otherwise.
> +///
> +/// Whether `IoMem` represents an exclusive access to the underlying memory
> +/// region is determined by the caller at creation time, as overlapping access
> +/// may be needed in some cases.
> +///
> +/// # Invariants
> +///
> +/// `IoMem` always holds an `IoRaw` inststance that holds a valid pointer to the
> +/// start of the I/O memory mapped region and its size.
> +pub struct IoMem<const SIZE: usize = 0> {
> +    io: IoRaw<SIZE>,
> +    res_start: u64,
> +    exclusive: bool,
> +}
> +
> +impl<const SIZE: usize> IoMem<SIZE> {
> +    /// Creates a new `IoMem` instance.
> +    ///
> +    /// `exclusive` determines whether the memory region should be exclusively
> +    ///
> +    /// # Safety
> +    ///
> +    /// The caller must ensure that the underlying resource remains valid
> +    /// throughout the `IoMem`'s lifetime. This is usually done by wrapping the
> +    /// `IoMem` in a `Devres` instance, which will properly revoke the access
> +    /// when the device is unbound from the matched driver.
> +    pub(crate) unsafe fn new(resource: &Resource, exclusive: bool) -> Result<Self> {
> +        let size = resource.size();
> +        if size == 0 {
> +            return Err(ENOMEM);
> +        }

Shouldn't this be EINVAL instead? AFAIR, ENOMEM is typically used for
memory allocation failures or resource exhaustion, whereas EINVAL seems
more appropriate for signaling invalid resource configurations

> +
> +        let res_start = resource.start();
> +
> +        if exclusive {
> +            // SAFETY:
> +            // - `res_start` and `size` are read from a presumably valid `struct resource`.
> +            // - `size` is known not to be zero at this point.
> +            // - `resource.name()` returns a valid C string.
> +            let mem_region = unsafe {
> +                bindings::request_mem_region(res_start, size, resource.name().as_char_ptr())
> +            };
> +
> +            if mem_region.is_null() {
> +                return Err(EBUSY);
> +            }
> +        }
> +
> +        // SAFETY:
> +        // - `res_start` and `size` are read from a presumably valid `struct resource`.
> +        // - `size` is known not to be zero at this point.
> +        let addr = unsafe { bindings::ioremap(res_start, size as usize) };
> +        if addr.is_null() {
> +            if exclusive {
> +                // SAFETY:
> +                // - `res_start` and `size` are read from a presumably valid `struct resource`.
> +                // - `size` is the same as the one passed to `request_mem_region`.
> +                unsafe { bindings::release_mem_region(res_start, size) };
> +            }
> +            return Err(ENOMEM);
> +        }
> +
> +        let io = IoRaw::new(addr as usize, size as usize)?;
> +
> +        Ok(IoMem {
> +            io,
> +            res_start,
> +            exclusive,
> +        })
> +    }
> +}
> +
> +impl<const SIZE: usize> Drop for IoMem<SIZE> {
> +    fn drop(&mut self) {
> +        if self.exclusive {
> +            // SAFETY: `res_start` and `io.maxsize()` were the values passed to
> +            // `request_mem_region`.
> +            unsafe { bindings::release_mem_region(self.res_start, self.io.maxsize() as u64) }
> +        }
> +
> +        // SAFETY: Safe as by the invariant of `Io`.
> +        unsafe { bindings::iounmap(self.io.addr() as *mut core::ffi::c_void) }
> +    }
> +}
> +
> +impl<const SIZE: usize> Deref for IoMem<SIZE> {
> +    type Target = Io<SIZE>;
> +
> +    fn deref(&self) -> &Self::Target {
> +        // SAFETY: Safe as by the invariant of `IoMem`.
> +        unsafe { Io::from_raw(&self.io) }
> +    }
> +}