Nouveau - Oct 2025 - [PATCH v4 04/13] gpu: nova-core: Add a slice-buffer (sbuffer) datastructure

From: Joel Fernandes <joelagnelf at nvidia.com>

A data structure that can be used to write across multiple slices which
may be out of order in memory. This lets SBuffer user correctly and
safely write out of memory order, without error-prone tracking of
pointers/offsets.

 let mut buf1 = [0u8; 3];
 let mut buf2 = [0u8; 5];
 let mut sbuffer = SBuffer::new([&mut buf1[..], &mut buf2[..]]);

 let data = b"hello";
 let result = sbuffer.write(data);

An internal conversion of gsp.rs to use this resulted in a nice -ve delta:
gsp.rs: 37 insertions(+), 99 deletions(-)

Co-developed-by: Alistair Popple <apopple at nvidia.com>
Signed-off-by: Alistair Popple <apopple at nvidia.com>
Signed-off-by: Joel Fernandes <joelagnelf at nvidia.com>
Reviewed-by: Lyude Paul <lyude at redhat.com>

---

Changes for v3:
 - Addressed minor review comment from Lyude
---
 drivers/gpu/nova-core/nova_core.rs |   1 +
 drivers/gpu/nova-core/sbuffer.rs   | 191 +++++++++++++++++++++++++++++
 2 files changed, 192 insertions(+)
 create mode 100644 drivers/gpu/nova-core/sbuffer.rs

diff --git a/drivers/gpu/nova-core/nova_core.rs
b/drivers/gpu/nova-core/nova_core.rs
index fffcaee2249f..a6feeba6254c 100644
--- a/drivers/gpu/nova-core/nova_core.rs
+++ b/drivers/gpu/nova-core/nova_core.rs
@@ -11,6 +11,7 @@
 mod gpu;
 mod gsp;
 mod regs;
+mod sbuffer;
 mod util;
 mod vbios;
 
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";
+/// 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,
+}
+
+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)
+    }
+
+    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),
+                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

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