Re: [PATCH v6 2/3] rust: io: mem: add a generic iomem abstraction

[Alice Ryhl]

On Sun, Feb 2, 2025 at 11:45 PM Asahi Lina <lina@xxxxxxxxxxxxx> wrote:
>
>
>
> On 1/31/25 7:05 AM, Daniel Almeida wrote:
> > 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@xxxxxxxxxxxxx>
> > ---
> >  rust/kernel/io.rs     |   1 +
> >  rust/kernel/io/mem.rs | 125 ++++++++++++++++++++++++++++++++++++++++++
> >  2 files changed, 126 insertions(+)
> >  create mode 100644 rust/kernel/io/mem.rs
> >
> > 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..f87433ed858e
> > --- /dev/null
> > +++ b/rust/kernel/io/mem.rs
> > @@ -0,0 +1,125 @@
> > +// SPDX-License-Identifier: GPL-2.0
> > +
> > +//! Generic memory-mapped IO.
> > +
> > +use core::ops::Deref;
> > +
> > +use crate::device::Device;
> > +use crate::devres::Devres;
> > +use crate::io::resource::Region;
> > +use crate::io::resource::Resource;
> > +use crate::io::Io;
> > +use crate::io::IoRaw;
> > +use crate::prelude::*;
> > +
> > +/// An exclusive memory-mapped IO region.
> > +///
> > +/// # Invariants
> > +///
> > +/// - ExclusiveIoMem has exclusive access to the underlying `iomem`.
> > +pub struct ExclusiveIoMem<const SIZE: usize> {
> > +    /// The region abstraction. This represents exclusive access to the
> > +    /// range represented by the underlying `iomem`.
> > +    ///
> > +    /// It's placed first to ensure that the region is released before it is
> > +    /// unmapped as a result of the drop order.
> > +    #[allow(dead_code)]
> > +    region: Region,
> > +    /// The underlying `IoMem` instance.
> > +    iomem: IoMem<SIZE>,
> > +}
> > +
> > +impl<const SIZE: usize> ExclusiveIoMem<SIZE> {
> > +    /// Creates a new `ExclusiveIoMem` instance.
> > +    pub(crate) fn ioremap(resource: &Resource) -> Result<Self> {
> > +        let iomem = IoMem::ioremap(resource)?;
> > +
> > +        let start = resource.start();
> > +        let size = resource.size();
> > +        let name = resource.name();
> > +
> > +        let region = resource
> > +            .request_mem_region(start, size, name)
> > +            .ok_or(EBUSY)?;
> > +
> > +        let iomem = ExclusiveIoMem { iomem, region };
> > +
> > +        Ok(iomem)
> > +    }
> > +
> > +    pub(crate) fn new(resource: &Resource, device: &Device) -> Result<Devres<Self>> {
> > +        let iomem = Self::ioremap(resource)?;
> > +        let devres = Devres::new(device, iomem, GFP_KERNEL)?;
> > +
> > +        Ok(devres)
> > +    }
> > +}
> > +
> > +impl<const SIZE: usize> Deref for ExclusiveIoMem<SIZE> {
> > +    type Target = Io<SIZE>;
> > +
> > +    fn deref(&self) -> &Self::Target {
> > +        &*self.iomem
> > +    }
> > +}
> > +
> > +/// 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.
> > +///
> > +/// # Invariants
> > +///
> > +/// `IoMem` always holds an `IoRaw` inststance that holds a valid pointer to the
> > +/// start of the I/O memory mapped region.
> > +pub struct IoMem<const SIZE: usize = 0> {
> > +    io: IoRaw<SIZE>,
> > +}
> > +
> > +impl<const SIZE: usize> IoMem<SIZE> {
> > +    fn ioremap(resource: &Resource) -> Result<Self> {
> > +        let size = resource.size();
> > +        if size == 0 {
> > +            return Err(EINVAL);
> > +        }
> > +
> > +        let res_start = resource.start();
> > +
> > +        // 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 kernel::ffi::c_ulong) };
>
> This does not work for all platforms. Some, like Apple platforms,
> require ioremap_np(). You should check for the
> `IORESOURCE_MEM_NONPOSTED` resource flag and use the appropriate
> function, like this (from an older spin of this code):
>
> https://github.com/AsahiLinux/linux/blob/fce34c83f1dca5b10cc2c866fd8832a362de7974/rust/kernel/io_mem.rs#L166C36-L166C70

Hmm. Is detecting this based on the flag the right approach? On the C
side, they use different methods and I wonder if we should do the
same?

Alice