Re: [PATCH v4 04/13] gpu: nova-core: Add a slice-buffer (sbuffer) datastructure

["Danilo Krummrich" <dakr@kernel.org>]

On Wed Oct 8, 2025 at 2:12 AM CEST, Alistair Popple wrote:
> diff --git a/drivers/gpu/nova-core/sbuffer.rs b/drivers/gpu/nova-core/sbuffer.rs
> new file mode 100644
> index 000000000000..e82f9d97ad21
> --- /dev/null
> +++ b/drivers/gpu/nova-core/sbuffer.rs
> @@ -0,0 +1,191 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +use core::ops::Deref;
> +
> +use kernel::alloc::KVec;
> +use kernel::error::code::*;
> +use kernel::prelude::*;
> +
> +/// A buffer abstraction for discontiguous byte slices.
> +///
> +/// This allows you to treat multiple non-contiguous `&mut [u8]` slices
> +/// as a single stream-like read/write buffer.
> +///
> +/// Example:
> +///
> +/// let mut buf1 = [0u8; 3];
> +/// let mut buf2 = [0u8; 5];
> +/// let mut sbuffer = SWriteBuffer::new([&buf1, &buf2]);
> +///
> +/// let data = b"hellowo";

Not that it matters, but "hellowo"? :)

> +/// let result = sbuffer.write_all(0, data);
> +///
> +/// A sliding window of slices to proceed.
> +///
> +/// Both read and write buffers are implemented in terms of operating on slices of a requested
> +/// size. This base class implements logic that can be shared between the two to support that.
> +///
> +/// `S` is a slice type, `I` is an iterator yielding `S`.
> +pub(crate) struct SBuffer<I: Iterator> {
> +    /// `Some` if we are not at the end of the data yet.
> +    cur_slice: Option<I::Item>,
> +    /// All the slices remaining after `cur_slice`.
> +    slices: I,
> +}

Does it make sense to split SBuffer into itself and a separate SBufferIter that
keeps a reference to the SBuffer? If not, I'd rename it to SBufferIter to make
it obvious to the user that it is an iterator type.

> +impl<'a, I> SBuffer<I>
> +where
> +    I: Iterator,
> +{
> +    #[expect(unused)]
> +    pub(crate) fn new_reader(slices: impl IntoIterator<IntoIter = I>) -> Self
> +    where
> +        I: Iterator<Item = &'a [u8]>,
> +    {
> +        Self::new(slices)
> +    }
> +
> +    #[expect(unused)]
> +    pub(crate) fn new_writer(slices: impl IntoIterator<IntoIter = I>) -> Self
> +    where
> +        I: Iterator<Item = &'a mut [u8]>,
> +    {
> +        Self::new(slices)
> +    }

Please add some documentation for the constructors.

> +    fn new(slices: impl IntoIterator<IntoIter = I>) -> Self
> +    where
> +        I::Item: Deref<Target = [u8]>,
> +    {
> +        let mut slices = slices.into_iter();
> +
> +        Self {
> +            // Skip empty slices to avoid trouble down the road.
> +            cur_slice: slices.find(|s| !s.deref().is_empty()),
> +            slices,
> +        }
> +    }
> +
> +    fn get_slice_internal(
> +        &mut self,
> +        len: usize,
> +        mut f: impl FnMut(I::Item, usize) -> (I::Item, I::Item),
> +    ) -> Option<I::Item>
> +    where
> +        I::Item: Deref<Target = [u8]>,
> +    {
> +        match self.cur_slice.take() {
> +            None => None,
> +            Some(cur_slice) => {
> +                if len >= cur_slice.len() {
> +                    // Caller requested more data than is in the current slice, return it entirely
> +                    // and prepare the following slice for being used. Skip empty slices to avoid
> +                    // trouble.
> +                    self.cur_slice = self.slices.find(|s| !s.is_empty());
> +
> +                    Some(cur_slice)
> +                } else {
> +                    // The current slice can satisfy the request, split it and return a slice of
> +                    // the requested size.
> +                    let (ret, next) = f(cur_slice, len);
> +                    self.cur_slice = Some(next);
> +
> +                    Some(ret)
> +                }
> +            }
> +        }
> +    }
> +}
> +
> +/// Provides a way to get non-mutable slices of data to read from.
> +impl<'a, I> SBuffer<I>
> +where
> +    I: Iterator<Item = &'a [u8]>,
> +{
> +    /// Returns a slice of at most `len` bytes, or `None` if we are at the end of the data.
> +    ///
> +    /// If a slice shorter than `len` bytes has been returned, the caller can call this method
> +    /// again until it returns `None` to try and obtain the remainder of the data.
> +    fn get_slice(&mut self, len: usize) -> Option<&'a [u8]> {
> +        self.get_slice_internal(len, |s, pos| s.split_at(pos))
> +    }
> +
> +    /// Ideally we would implement `Read`, but it is not available in `core`.
> +    /// So mimic `std::io::Read::read_exact`.
> +    #[expect(unused)]
> +    pub(crate) fn read_exact(&mut self, mut dst: &mut [u8]) -> Result {
> +        while !dst.is_empty() {
> +            match self.get_slice(dst.len()) {
> +                None => return Err(ETOOSMALL),

ETOOSMALL is an NFS error code (it should also never be exposed to userspace). I
suggest to implement a custom error type instead and make it resolve to ENOSPC
or probably just EINVAL instead.

> +                Some(src) => {
> +                    let dst_slice;
> +                    (dst_slice, dst) = dst.split_at_mut(src.len());
> +                    dst_slice.copy_from_slice(src);
> +                }
> +            }
> +        }
> +
> +        Ok(())
> +    }
> +
> +    /// Read all the remaining data into a `KVec`.
> +    ///
> +    /// `self` will be empty after this operation.
> +    #[expect(unused)]
> +    pub(crate) fn read_into_kvec(&mut self, flags: kernel::alloc::Flags) -> Result<KVec<u8>> {
> +        let mut buf = KVec::<u8>::new();
> +
> +        if let Some(slice) = core::mem::take(&mut self.cur_slice) {
> +            buf.extend_from_slice(slice, flags)?;
> +        }
> +        for slice in &mut self.slices {
> +            buf.extend_from_slice(slice, flags)?;
> +        }
> +
> +        Ok(buf)
> +    }
> +}
> +
> +/// Provides a way to get mutable slices of data to write into.
> +impl<'a, I> SBuffer<I>
> +where
> +    I: Iterator<Item = &'a mut [u8]>,
> +{
> +    /// Returns a mutable slice of at most `len` bytes, or `None` if we are at the end of the data.
> +    ///
> +    /// If a slice shorter than `len` bytes has been returned, the caller can call this method
> +    /// again until it returns `None` to try and obtain the remainder of the data.
> +    fn get_slice_mut(&mut self, len: usize) -> Option<&'a mut [u8]> {
> +        self.get_slice_internal(len, |s, pos| s.split_at_mut(pos))
> +    }
> +
> +    /// Ideally we would implement `Write`, but it is not available in `core`.
> +    /// So mimic `std::io::Write::write_all`.
> +    #[expect(unused)]
> +    pub(crate) fn write_all(&mut self, mut src: &[u8]) -> Result {
> +        while !src.is_empty() {
> +            match self.get_slice_mut(src.len()) {
> +                None => return Err(ETOOSMALL),
> +                Some(dst) => {
> +                    let src_slice;
> +                    (src_slice, src) = src.split_at(dst.len());
> +                    dst.copy_from_slice(src_slice);
> +                }
> +            }
> +        }
> +
> +        Ok(())
> +    }
> +}
> +
> +impl<'a, I> Iterator for SBuffer<I>
> +where
> +    I: Iterator<Item = &'a [u8]>,
> +{
> +    type Item = u8;
> +
> +    fn next(&mut self) -> Option<Self::Item> {
> +        // Returned slices are guaranteed to not be empty so we can safely index the first entry.
> +        self.get_slice(1).map(|s| s[0])
> +    }
> +}
> -- 
> 2.50.1